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Tuesday, October 11, 2011

Why are so many primitive stars observed in the Galaxy halo?

Why are so many primitive stars observed in the Galaxy halo?

Carl H. Gibsona1, Theo M. Nieuwenhuizenb and Rudolph E. Schildc
a University of California at San Diego, La Jolla, CA, 92093.0411, USA
b Institute for Theoretical Physics, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
c Harvard.Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA


Small values of lithium observed in a small, primitive, Galaxy-Halo star SDSS J102915 +
172927 cannot be explained using the standard cold dark matter CDM theory of star
formation, but are easily understood using the Gibson/Schild 1996 hydrogravitationaldynamics
(HGD) theory. From HGD, primordial H-4He gas fragments into Earth-mass
planets in trillion-planet proto-globular-star-cluster (PGC) clumps at the 300 Kyr time of
transition from the plasma epoch, soon after the big bang. The first HGD stars formed
from pristine, frictionally-merging, gas-planets within the gently stressed clumps of the
early universe, burning most available lithium in brown-dwarfs and hot-stars before
creating metals that permit cooler burning. The Caffau halo star is a present day
example. CDM first stars (Population III) were massive and promptly exploded, re-
ionizing the gas of the universe and seeding it with metals, thus making the observed star
unexplainable. From HGD, CDM and its massive first stars, and re-ionization by Pop III
supernovae, never happened. All stars are formed from planets in primordial clumps.
HGD first stars (Pop III) were small and long-lived, and the largest ones were hot. We
suggest such small HGD (Pop III) stars still form in the gently stressed Galaxy halo.

Keywords: Cosmology, star formation, planet formation, astrobiology.

The standard model of cosmology .CDMHC is in trouble on all sides. It fails to permit
life to form by natural causes (Gibson, Wickramasinghe, Schild 2011; Gibson, Schild,
Wickramasinghe 2011). It fails to include basic fluid mechanical processes crucial to
gravitational structure formation of the various astrophysical fluids; that is, the kinematic
viscosity, turbulence and stratified turbulent mixing. It requires non-physical entities
such as persistent anti-gravity dark energy .
and weakly interactive cold-dark-matter
CDM particles that clump rather than diffuse in ways that are repudiated by astronomical
measurements (Kroupa et al. 2011) and fluid mechanical theory (TMN declines to
criticize .). CDM clumps have never been convincingly observed, and neither are the
numerous small galaxies that are expected within CDM-clump gravitational potential
wells as precursors to normal galaxies by hierarchical clustering (HC). .CDMHC fails
most miserably in its predictions about star and planet formation. As predicted by hydrogravitational-
dynamics HGD cosmology (Gibson 1996), and as observed by quasar
microlensing (Schild 1996), for every star in a galaxy there should be 30 million planets,
not 8-10 as expected from .CDMHC. In particular, the primitive Galaxy-Halo stars and

1 Corresponding author: Depts. of MAE and SIO, CASS, cgibson@ucsd.edu, http://sdcc3.ucsd.edu/~ir118


their Lithium abundances observed by Caffau and her colleagues (Caffau et al. 2011) are
easily explained and expected from HGD cosmology, but are quite impossible to explain
from .CDMHC cosmology and its CDM-halo models of star and galaxy formation,
which are accepted and taken to be standard in astronomy. The “impossible” Caffau star,
as it has come to be known, is shown in Figure 1.

Figure 1. Spectroscopic studies using the Very Large Telescope (VLA) reveal distant Galaxy-Halo stars that cannot be
explained using the standard model of cosmology and the standard model of star formation (Caffau et al. 2011). From
HGD cosmology, all stars form in PGC clumps of gas planets, the dark matter of galaxies, and burn lithium and tritium
while the Pop III stars are small. Small Pop III stars are impossible according to .CDMHC cosmology (Bromm and
Loeb 2003).

As shown in Fig. 1, the Caffau star appears very ordinary until examined by the large
VLT telescope in Chile for its chemical composition using standard (.CDMHC) models
and the sensitive, precise, and extensive spectroscopic evidence the VLT produces.
Using the data and the standard .CDMHC model, the Caffau star is impossible.

The difficulty is with the lithium abundance, which is far below levels observed in the
Galaxy disk and the expected primordial levels, as shown in Figure 2 (adapted from Fig.
2a of Caffau et al. 2011). The small levels of lithium in halo stars, compared to disk
stars, is illustrated using the Spite plateau (Spite & Spite 1982), shown by the light
dashed line, which is less than primordial levels (dark dashed line) by a factor of 2-3.
Such large differences can no longer be attributed to errors in the primordial lithium
abundance. Experts in the field admit that the problem worsens (Cyburt et al. 2008).

A more likely explanation is that Galaxy disk stars have been seeded with metals
produced by supernovae that are more frequent for tidally-agitated disk-PGCs compared


to pristine gentle-halo-PGCs. This makes a difference from HGD cosmology, where
stars are formed from planets in clumps that isolate their supernova chemicals, but does
not for .CDMHC cosmology, where stars and galaxies condense from gas and dust in
>1021 m CDM halos that incubate and contaminate internal stars and galaxies, diameters
much larger than <1018 m PGCs. Galaxy-halo planet-clumps (PGCs) can remain
relatively uncontaminated by metals until their first supernova. All their stars burn
lithium, starting with brown dwarfs. From HGD, star brightness increases with tidal
agitation of the PGC, which increases the planet accretion rate. The more pristine the
planets the hotter the largest stars that they create (~0.8 solar in globular star clusters).

Figure 2. Spectroscopic studies using the Very Large Telescope VLA reveal distant Galaxy-Halo stars that cannot be
explained using the standard model of cosmology and the standard model of star formation (Fig. 2a Caffau et al. 2011).
The heavy dashed blue line is the primordial gas lithium abundance. The light red dashed Spite plateau (Spite & Spite
1982) is lower by a factor of two, but the Caffau and Schneider halo stars (green) are lower by orders of magnitude.
The HGD interpretation (asterisks) is that halo stars in their PGC dark matter clumps are less likely to be seeded by
supernovae metals than disk stars in their more tidally agitated, and therefore explosive, PGCs.

As shown in Fig. 2, HGD cosmology gives a straightforward explanation for the small
lithium abundances observed (Caffau et al. 2011, Schneider et al. 2008) in small halo
stars. Because all stars are formed by planet mergers within PGC clumps, and because
PGC clumps in the halo of galaxies are more likely to consist of pristine primordial gas
planets that those in the disk, the stars formed are likely to burn all of their lithium during
formation rather than the cooler population II stars of the disk that have been seeded with
supernovae metals. Because the first .CDMHC stars are so large and their supernovae
so violent, stars from uncontaminated primordial gas become rare or nonexistent. The
abundance of metal A compared to B is found from the expression [A/B] = log(NA/NB) .

Nieuwenhuizen (2011a,b) examines the effects of 3He concentrations on star formation
and death, as well as the formation of early galactic central black holes and bulges from


Jeans clusters (PGCs) of .BDs (micro-brown-dwarfs). In the following we will discuss
the star formation theories of .CDMHC versus HGD cosmologies, and make comparison
to observations. Conclusions are presented.

Gravitational structure formation in hydro-gravitational-dynamics HGD cosmology
depends on kinematic viscosity, turbulence, and molecular diffusivity, quantities that are
neglected entirely by .CDMHC cosmology. The two cosmologies give very different
predictions with respect to the formation of stars and the formation of planets. How do
these predictions affect the interpretation of the Caffau star? What is the evidence
supporting the two cosmologies?

Cold Dark Matter is in trouble for several reasons. The only known non-baryonic dark
matter material is neutrinos, and the most popular alternative candidate, the neutralino, is
failing all tests using the Large Hadron Collider, as shown in Figure 3.

Figure 3. Preliminary results from the Large Hadron Collider show departures of the Observed radiation measured by
the LCHb detector (blue) compared to that expected using the standard model of particle physics (red). The Figure is
taken from a slide presented (Raven 2011) in August at the Lepton-Photon particle physics meeting in Mombai, India.

Neutralinos are predicted by supersymmetry as cold dark matter particles because they
are massive, about a hundred times a proton mass, and weakly collisional, with expected
collision cross sections as small as 10-49 m2. The LCHb strategy is to look for collisions
of bottom quark related particles with massive supersymmetry CDM candidates. The
preliminary results reported by Gerhard Raven (LP meeting Mombai 2011) so far show
no evidence that such particles exist.

Star formation by the standard .CDMHC cosmology (Bromm and Loeb 2003) is very
different than HGD star formation. CDM seeds that form by the Jeans 1902 instability
theory somehow hierarchically cluster (HC) to form CDM halos of larger mass. Stars
form as the primordial gas falls into the resulting potential wells. A period of 400 Myr or
more called the “dark ages” is required before any stars appear. Gravity pulls all the gas


to the center of the CDM halo, so the first stars were enormous, metal free, Population III
stars that rapidly exploded to form metals. The supernova were so powerful they
completely re-ionized and contaminated all the gas of the universe. Attempts to detect
this “first starlight” have failed, probably because it never happened (Madau 2006).
.CDMHC assumes that a threshold mass fraction of metals Z > 10-7 is required to form
stars smaller than 0.8 solar mass (Schneider et al. 2003). HGD claims star brightness
shows the rate of planet accretion, not the star mass, and that Z is irrelevant to star
brightness. The first stars were small Population III stars of the old globular star clusters

All .CDMHC stars after re-ionization should be contaminated by metals from the
supernovae. They should burn cooler than the small Population III stars that are expected
in the Galaxy halo from HGD (Fig. 1). It is a mystery to .CDMHC how any small
Population III stars could be formed (Bromm & Loeb 2003), let alone the very large
numbers of small Population III halo stars observed, such as the Caffau star of Fig. 1 and
Fig. 2 (Frebel et al. 2008).

The mystery is easily solved by HGD cosmology (Gibson 1996, Schild 1996). Rather
than cold dark matter condensation during the plasma epoch, structure formation begins
when the viscous forces match the gravitational forces at the Schwarz viscous scale LSV =
(../.G)1/2, where .
is the rate-of-strain, .
is the kinematic viscosity, .
is the density and
G is Newton’s gravitational constant. Structure formation is prevented by the photon
viscosity of the plasma until time 1012 seconds, when LSV > LH first matches the
increasing scale of causal connection LH = ct, where c is the speed of light and t is the
time. The computed mass of the first structures found in this way closely matches the
observed mass of superclusters (Gibson 2000), about 1046 kg.

The last structures formed in the plasma are protogalaxies, with mass about 1043 kg and
linear morphology caused by fragmentation along vortex lines of weak turbulence
produced by expanding protosuperclustervoids (Gibson, Schild and Wickramasinghe
2011). Rather than condensing into CDM halos, the non-baryonic dark matter is super-
diffusive. It forms the halos of clusters and superclusters and a negligible part of galaxy
mass. A substantial portion of the non-baryonic dark matter appears to be neutrinos
(Nieuwenhuizen & Morandi 2011) from gravitational lensing by a galaxy cluster. A
permanent form of anti-gravity (dark energy) is not needed by HGD cosmology (TMN
reserves judgement), since big bang turbulence supplies the large anti-gravity negative
stresses (10113 Pa) required for mass-energy extraction at Planck scales, by vortex
stretching (Gibson 2010).

The phase transition from plasma to gas occurs at t = 1013 seconds. Because heat is
transferred at the speed of light and pressure at the speed of sound, the protogalaxies
fragment at two length scales: the Jeans scale LJ = VS.g and LSV, where VS is the speed of
sound and .g= (.G)-1/2 is the gravitational free fall time. Each protogalaxy fragments
into 1018 m Jeans mass clumps (PGCs) of primordial gas planets (now frozen-solidhydrogen
107 m .BD microBrownDwarfs) that have persisted as the dark matter
(Nieuwenhuizen, Schild and Gibson 2011). As the universe cooled, more and more


planets froze and their PGCs diffused from the central core to form the galaxy halo and
galaxy accretion disk. Most of the PGCs of the halo remain as clumps of a trillion,
pristine, primordial-gas-planets in metastable equilibrium, with no stars whatsoever.
Whatever stars form are most likely to be similar to the Caffau star of Fig. 1 and Fig. 2,
with the small Pop III stars of old globular clusters. There should be many such stars,
with very low lithium abundance, as observed (Sbordone et al. 2010).

In a future work we will show how the long-standing .CDMHC mystery of 7Li
abundance (Cyburt et al. 2008) can be addressed using HGD cosmology.

Large numbers of primitive halo stars revealed by the VLT (Very Large Telescope) and
its very sensitive spectrographs are easy to understand using HGD cosmology, but are
impossible to explain using standard .CDMHC cosmology. The reason is that the
standard cosmology is wrong about how stars are formed. Stars are not formed from gas
that falls into CDM gravitational potential wells, they are formed within proto-globularstar-
cluster (PGC) clumps of primordial gas planets, by mergers of the planets to form
larger planets and finally stars. PGCs that freeze and diffuse into the galaxy halo are
generally pristine, with no stars at all and no metals besides traces of lithium. Their
planets merge to form larger planets, brown dwarfs and hot small stars that burn all traces
of primordial lithium. Lithium abundances in Population II disk stars are only a factor of
three or less smaller than primordial gas values, compared to more than a factor of ten for
many halo stars observed.

No observations support the existence of CDM halos. The LHC experiments of Fig. 3
show no evidence that the leading CDM particle candidates exist. Even if they did, they
would be irrelevant because they are weakly collisional and would simply diffuse away
from the baryons, just as neutrinos do. A host of new observations, including those of the
Caffau star, suggest it is time to abandon .CDMHC cosmology in favor of HGD.


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What Can the Origin of Life on Earth Tell Us About the Cosmos?

Journal of Cosmology, 2010, Vol 10, 3408-3417.
JournalofCosmology.com, August, 2010

What Can the Origin of Life on Earth
Tell Us About the Cosmos?
Stephen Freeland, Ph.D., and Gayle K. Philip, Ph.D.,
NASA Astrobiology Institute, University of Hawaii, Honolulu, 96822, HI, USA
Though many have expressed opinions about the "life friendliness" of the universe, science still has a lot to learn about the nature of life, and its place within the cosmos. The challenge is to extrapolate from the biology that we can observe into what we might expect from life elsewhere. Although some interesting clues are emerging, even the narrower claim that life's fundamental biochemistry will turn out to be truly universal is an opinion that extends beyond current scientific knowledge. However, further research is both desirable and directly achievable through standard evolutionary investigations. Optimality theory can be applied systematically to investigate the relative contributions of random chance versus predictability in producing such fundamental evolutionary outcomes as the size and composition of the nucleic-acid and amino acid alphabets, and the evolution of a genetic code that links nucleic-acid genotype and protein phenotype. Answers here will form an important component of cosmology.
Keywords: evolution, cosmology; biochemistry; amino acids; natural selection; chance

1. Cosmology Must Include Astrobiology
Any thorough cosmology (i.e. a logical account of what we may find when we explore the universe) must include biology. At the simplest level, any explanation for the universe would be incomplete if it failed to reference the intelligence doing all this explaining (Joseph 2009; Stapp 2009). For example, if the physical world is created by the process of observing, measuring and knowing (Heisenberg, 1955) then the consciousness that generates cosmology affects the universe that it explains. Taking another approach, we may argue that biology is an important component of cosmology simply because life has been present on Earth for around one third of the existence of the universe (Mojzsis, 2005; Nemchin et al. 2008; O'Neil et al. 2008). But life’s cosmic significance extends far beyond these attributes of sentience and long-term persistence.
A thorough cosmology must incorporate an explanation of where everything came from (i.e. cosmogeny) and through this lens, biology has a deep and complex relevance to our understanding of the universe (Nitschke and Russell 2010). For example, scientists working at the interface of planetary and life sciences are finding that the one example of life we know has extensively reshaped the planet on which it occurs: it has transformed our atmosphere (e.g. see Joseph 2010; Kasting, 2001), it is responsible for the existence of more than half of the mineral species around us (Hazen et al., 2008). Biology has pulled our entire planet’s entropic equilibrium far away from those of our lifeless planetary neighbors (Lovelock, 1965). Life is an intrinsic feature of this planet. In other words, "Earth is no more a planet-sized chunk of rock inhabited with life than your body is a skeleton infested with cells" (Margulis and Sagan, 2000). More flippantly, we can say that Shakespeare was wrong to characterize life as "a poor player, that struts and frets his hour upon the stage and then is heard no more." Since life transforms the planet, it would be more accurate to say that life re-sets the stage as it acts, leaving behind a profound legacy by redesigning the environment to be inherited by future life. If life on our planet is a representative example of something broader, then biology is reshaping the cosmos. Thus a comprehensive cosmology must take account of how commonly life occurs and how predictably it influences the environments in which it resides.
The challenge of placing biology within a truly universal context (i.e. the study of the origin(s), evolution, distribution and future of life in the universe) is one that falls under the nascent, interdisciplinary science of astrobiology. Astrobiology exceeds the boundaries of traditional evolutionary biology which are limited, by definition, to describing the process of change in genetic material that connects today’s species back to the earliest life on our planet. Astrobiology seeks to connect this evolution into a bigger picture that extends back through life’s origin(s) and beyond, into chemistry and physics (desMarais 2008). Only from this big picture can we understand biology’s full impact upon the universe, its true relevance to cosmology. One day the questions of astrobiology may be answered empirically, through the results of space exploration (e.g. see Chela-Flores, 2007), but for the immediate future they require ingenious induction and inference from the one example of life that we know into the complementary insights of earth and space science.
2. Life’s "Fit" to the Physical Universe is Not Understood by Science
If life reshapes planets, then life’s impact upon the cosmos is tied to its ubiquity and thus -- barring a truly universal Panspermia (Gibson and Wickramasinghe, 2010; Joseph and Schild 2010) -- life’s cosmic significance is linked to the frequency with which it originates. This amounts to a question about the ease with which physics flows, via chemistry, into biology: the closer and more inescapable these connections, the more life-friendly (biocentric) our universe.
One early attempt to address this question concluded that "the fitness of the [physical] environment far precedes the existence of the living organisms" (Henderson, 1913). In other words, the physics of our universe appears finely-tuned to produce life. An intervening century of subsequent exploration for this important idea has occurred mostly within the physical sciences (e.g. see McMullin, 2008). In particular, proponents of the Anthropic Principle (reviewed in Barrow, 1986) have focused on the observation that due to the interaction of a small set of fundamental physical constants, our universe exhibits such life-friendly attributes as diverse chemical elements (including abundant carbon and water), and their accretion into stars that bathe their orbiting planets with electromagnetic radiation. Specifically, interest surrounds the observation that these constants appear to be independent of one another, yet each must take a value within a narrow range in order to define a universe in which life can exist. Thus, given that each of the constants does indeed fall within the appropriate range (such that we do indeed exist), some researchers have been led to the view: "The laws that characterize our actual universe, as opposed to an infinite number of alternative possible universes, seem almost contrived … so that life and consciousness may emerge" (Davies, 1995). If this is correct, then a sizeable fraction of the universe may be co-evolving cosmochemically, astrochemically, geochemically and biochemically.
However, many remain skeptical. For example, some assert that we observe a finely-tuned match between physics and biology only because our universe is part of a larger multiverse in which many different versions of physics co-exist (see Journal of Cosmology, 2010, Volume 4 for recent progress in this idea). Accordingly, our life may simply exist within the particular slice of physics particularly well-suited to contain us (e.g. see Coleman, 1988) and estimating the total cosmic frequency of life would require extensive knowledge of physical realities beyond that in which we find ourselves. Such understanding seems like a distant goal, particularly when placed alongside other criticisms of the Anthropic Principle that simply deny any fine-tuning exists within the physics we know. Specifically, there are those who maintain that from what we can perceive of the cosmos "the final laws of nature, the book of rules that govern all natural phenomena … are utterly impersonal and quite without any special role for life" (Weinberg, 1999). While further research within physics (e.g. Livio 2008) and chemistry (e.g. Barrow 2008; Lane 2010; Russell and Kanik 2010) can increase the scientific content of this particular debate, comparing these two critiques of the biocentric universe suggests a deeper underlying problem: that physics alone cannot measure the relationship between physics and biology. A clue to why this should be so lies in the very phrase "Anthropic Principle," which refers to the existence of humanity but was coined by physicists to discuss the ease of abiogenesis. To biologists, an enormous gap separates the emergence of life from the emergence of our sentient species - and this gap is filled by biological evolution. The potential importance of evolution for understanding life’s place within the cosmos is illustrated by yet another critique of the Anthropic Principle: that fundamental biology really does match closely the precise "settings" of the physical universe, but this fit is uninformative because life would have evolved itself to match any physics (Erwin, 2003).
This view usually comes from evolutionary biologists who are underwhelmed to hear that fundamental properties of biology match the physics of our universe, given experience in studying the exquisite adaptations by which organisms on Earth have evolved to fit their diverse environments. Now this assertion (that perceptions of fine-tuning within physics are a mere post-hoc fallacy) may oversimplify the findings of physics as much as any careless invocation of “anthropocentrism” oversimplifies the findings of biology. No clear empirical evidence demonstrates that life could evolve to fit a universe without carbon or water any more than it supports the assertion that this chemistry will produce sentient beings (though see Morris, 2008, Naganuma, and Sekine, 2010; Rampelotto, 2010; Schulze-Makuch 2010, for interesting ideas on each of these points). But taken together these ideas illustrate the true challenge for understanding the role and impact of abiogenesis within the cosmos as a whole: we need to better understand what life can look like.
Certainly, the past few decades have revolutionized biologists’ concepts of what constitutes a habitable environment (e.g. Pikuta et al. 2007) and one of NASA’s major goals for astrobiology is to ascertain the (as yet unknown) limits on the range of conditions to which life on our own planet has adapted itself (des Marais, 2008).
Meanwhile, exciting developments in abiogenesis are showing that life on Earth may have emerged with ease, perhaps even as an inescapable solution (necessity) to easing a tension of thermodynamics and entropy (e.g. Lane 2010; Russell and Kanik 2010; Nitschke and Russell, 2010). But in between this information about why life evolves, and the insights presented in this volume and elsewhere as to how the origin of life took place, a third and equally important challenge is to understand the relative contributions of chance versus necessity (or at least predictability) in shaping what properties we then expect to emerge through evolution. For example, even if we accept at face value that life will appear wherever rocky planets host groundwater containing carbon dioxide (Russell and Kanik, 2010), forced into existence to resolve the entropic disequilibrium (Nitschke and Russell, 2010), we remain far from knowing whether an independent origin of life would evolve into something similar to the particular version of biology that we know so well from our own planet.
3. What Features of Life Offer Clues to Chance Versus Predictability In Its Origin?
At first sight, it may seem strange to talk about "the particular life" of our planet: an obvious characteristic of biology to the casual observer is its impressive diversity. Some have gone so far as to assert that simply comparing organisms as different as "algae, beetles, sponges, jellyfish, snakes, condors, and giant sequoias" leaves the observer "hard-pressed to believe that they all came from the same universe, much less the same planet" (deGrasse Tyson, 2003). But since the middle of the 20th century, life scientists have come to see that this appearance is deceptive. Beneath the surface, all life on Earth shares a remarkably simple and unvarying nature. To be specific, the discovery of an inseparably elegant structure and function for DNA paved the way for Crick’s (1958) declaration of a "central dogma" to life’s molecular basis: information stored as genetic material (genotype) carries the complete and precise instructions for building all the components of an active metabolism (phenotype). This metabolism interacts with the environment to extract raw materials, energy and perform all functions necessary for successfully copying the genetic material into a new generation.
The simplicity and uniformity of the underlying biochemistry is notable: between the origin of life and the time of the “Last Universal Common Ancestor” for all of today’s biodiversity, a handful of biological constants evolved. For example, all organisms came to use the nucleic acid DNA to safely store genetic information in the form of a linear sequence of chemical letters. All came to spell out these genetic messages using an alphabet of exactly 4 such chemical letters (each letter comprises a ribose sugar joined to a phosphate and either a purine or a pyrimidine base). All came to use the similar nucleic acid, RNA, to transmit this genetic information for processing (by information-reading molecular machines) into networks of interacting proteins that constitute metabolism. All came to construct individual proteins by linking together amino acids drawn from an alphabet of exactly 20 different options. In order to transform genetic information into metabolism, all came to use an identical genetic code (the system of rules by which “letters” of a nucleic acid gene are translated into corresponding “letters” of a protein, just as the dots and dashes of Morse Code are translated into the letters of English). This fundamental biochemistry, this central dogma of molecular biology, became the framework for billions of years of subsequent evolution, from the emergence of E. coli to that of H. sapiens.
Researchers have now discovered some instances where these universal, biochemical constants have subsequently loosened a little. For example, some viruses have re-evolved an RNA genome, a disparate handful of lineages has evolved minor changes to the rules of their genetic code, and some have even added one of two amino acids to their protein alphabet. However, these observations merely emphasize that fundamental biochemistry is an evolved and evolvable phenomenon. This in turn highlights the question: which of these evolutionary moves were predictable and inevitable (i.e. might be expected to occur elsewhere), and which are part of a unique and idiosyncratic trajectory for Earth-biology?
Understanding whether the emergence of life on our planet represents a haphazard collection of accidents or the rapid evolution of predictable characteristics that we might anticipate elsewhere rivals the importance of physics' fundamental constants when it comes to interpreting our place within the cosmos.
4. Replacing Reasonable Debate with Science
Just as for the broader topic of a life-friendly universe, scientists have offered no shortage of confidently diverse views on the related question of what, if anything, is predictable about biological evolution. Some argue that "man knows at last that he is alone in the universe's unfeeling immensity, out of which he emerged only by chance" (Monod, 1972); or that "any replay … would lead evolution down a pathway radically different from the road actually taken" (Gould, 1989). Others assert more optimistic assessments: that "If the history of life on Earth could be rewound and replayed, many of the same innovations would reappear" (Than, 2006); or that "far from its myriad of products being fortuitous and accidental, evolution is remarkably predictable" (Conway Morris, 2010). When the question is reduced to the predictability of Earth’s fundamental biochemistry, optimism seems to predominate: we read that "the building blocks of life anywhere will be similar to our own" (Chela-Flores, 2007). These assertions can be applied generally, e.g. the only "Differences in evolutionary systems likely will lie at the higher-order levels" (Pace, 2001), or specifically, e.g. "If life were to arise on another planet, we would expect that … about 75% of the amino acids would be the same as on the earth" (Weber and Miller, 1981).
However, every quote presented in this essay up until this point has been carefully selected to illustrate an opinion of a scientist rather than science. In stating that, we do not mean to assert that these opinions are wrong, or even that they lack good reasoning. Indeed most represent the interpretation of an expert, offered within a formal or informal review of scientific data, and often consciously presented as an opinion). However, these assertions exist outside the framework of what constitutes science precisely because they present no clear test of what they assert. To be sure, some qualify as scientific hypotheses in the sense that they make explicit predictions (Weber and Miller, 1981; Chela-Flores, 2007) but until these tests are performed, our view is they may not be considered as fully-fledged science.
Previously and elsewhere we have used a rather less well-informed opinion to point out the problems that can emerge from muddling rational opinions with science (Freeland, 2010). Here, our message is more positive: each detail of life’s fundamental biochemistry suggests one or more adaptive hypotheses that are worthy of scientific investigation. Diverse opinions about the universality of biochemistry can be gathered and assessed objectively by simple application of appropriate scientific tests to yield a robust picture of the relative roles of chance and predictability in shaping life on Earth.
5. Current Scientific Knowledge for Chance Versus Necessity in Establishing Biochemistry
When considering life’s fundamental biochemistry, the key phenomena are of interest precisely because they evolved just once, early on. One possible explanation is that they represent an optimal solution to one or more fundamental challenges of life (replication, metabolism, adaptive evolution etc.), and thus we might expect them to occur wherever life emerges. However, the persistence through billions of years of evolution of a trait such as “a DNA genome using 4 nucleotides” is an unreliable indicator of its inevitability because characteristics that evolve early can become "locked in" by the emergence of new innovations that make adaptive sense only in relation to them.
However, Optimality Theory (Maynard Smith, 1978) is a well-established evolutionary research tool suitable for investigating just such situations. A researcher defines boundaries on what alternative outcomes were plausible, and then asks whether the particular outcome that became reality is one that distinguishes itself by virtue of being especially advantageous (i.e. optimal) such that it represents a predictable outcome of natural selection. The challenge here is to identify objectively measurable criteria that define the evolutionary fitness (value) of each possible outcome.
This approach has been widely used to study evolutionary outcomes that range from animal behavior (Charnov, 1976) to genome structure (Xia, 1995), and many such investigations have tackled aspects of life’s biochemistry. For example, when it comes to the nucleic acids in which genetic information is stored, we know that many nonbiological nucleotides are possible (Piccirilli et al., 1990; Bergstrom et al., 1997; Delaney et al., 2003). We also know that many are used outside of genetics within the metabolism of today’s biosphere (e.g. see Grosjean and Benne, 1998). From here, several focused optimality studies have offered detailed explanations for the size and composition of the genetic alphabet used by life (Gardner et al., 2003; Mukhopadhyay et al., 2003; Szathmary, 2003). Even the choice of ribose as the sugar component of nucleic acid has been proposed to be optimal for base-pairing strength relative to other prebiotically plausible alternatives (Egli et al., 2006). Assuming that some sort of nucleic acid available, then the details of life’s genetic biochemistry appear to be more or less predictable. Yet this assumption has a problem. The very existence of nucleic acids within early biochemistry remains one of the larger mysteries of life’s origin (e.g. see Shapiro, 2007).
Certainly, nucleic acid presents some highly attractive features as both genetic and catalytic material (Szathmary and Maynard Smith, 1995), and if life got under way using a less complex genetic system, then it is easier to understand how the "useful" properties of nucleotides could have been selected for (Bartel and Unrau, 1999; Eschenmoser, 1999; Pace, 2001; Gardner, et al., 2003; Szathmary, 2003). Indeed, there has been no shortage of suggestions as to possible forerunners of RNA. These include peptide-nucleic acid (Nelson et al., 2000), triose nucleic acid (Schoning et al., 2000), polyglyceric acid (Weber, 1989), and even inorganic crystals such as clay (Cairns-Smith, 1982). But so far, no one has proposed an unambiguous test to discriminate which of these suggestions is most relevant, and how they fit with broader insights into abiogenesis. Thus the current frontier for such investigations seems to be understanding exactly how nucleic acids entered early evolution. In effect, we are blocked from executing a broader optimality study because no clear model for the appropriate "possibility space" of alternatives is widely accepted. Indeed, even the simple suggestion that perhaps the primitive genetic system comprised fewer than four nucleotides has produced such varied and inconsistent ideas that the different hypotheses combine to undermine one another’s credibility (see Knight et al., 2004).
Meanwhile, when it comes to the amino acids, the situation described for nucleotides is almost precisely reversed: the preponderance of scientific effort thus far has gone to defining which ones were available to the origin of life, both on our planet and elsewhere in the galaxy (Martins and Sephton, 2009; Cleaves, 2010), and to exploring the historical process by which some of them became incorporated into biology (Higgs and Pudritz, 2009). All this work clearly indicates that biology has chosen only one small subset of what was chemically and biochemically possible (Lu and Freeland, 2006), but what has been largely lacking to date is an appropriate use of optimality theory to ask whether this sub-set shows any clear signs of being a predictable (high fitness) solution that we might expect in an independent abiogenesis. The first detailed attempt at such an optimality study asked whether life’s choice of amino acids showed unusually high statistical variance in size, charge and hydrophobicity: the three important properties that connect amino acids to the functional roles that they play within proteins (Lu and Freeland, 2008). Complex results brought us to the conclusion that "the question of whether early life selected a non-randomly diverse alphabet of amino acids remains far from clear … It is possible that this is because there is no clear pattern to be found … but … our purpose here is not so much to make a strong claim for or against the role of natural selection in choosing an amino acid alphabet; rather, it is to highlight the potential and demonstrate the plausibility of using a quantitative framework to think about the origin of proteinaceous amino acids". In other words, the point of replacing reasoned arguments with science is not that the reasoning of experts is inherently bad, or that science gives better answers immediately. Rather it is that science provides a clear and objective process for progressing knowledge. The repeatability of a given test allows a new round of research to choose exactly which assumption(s) to vary in order to see their effect on outcomes and interpretation. To take the specific example of the amino acids, one point of departure would be to question whether a better measurement of amino acid functional diversity would provide a different picture?
Meanwhile, other aspects of the amino acid alphabet remain almost entirely unexplored. For example the only attempts to address the related issue of whether the amino acid alphabet size of 20 is in some way optimal have been tucked in as afterthoughts within publications pursuing a fundamentally different theme (Wong, 1976; Szathmary, 1991). Here, as above, the challenge is to think through a logically defensible fitness function. One interesting insight is that the speed and efficiency of evolution is influenced by the pattern in which genetic code-words are assigned to the 20 standard amino acids (Zhu and Freeland, 2006). It thus seems likely that altering the size of the amino acid alphabet would have a similar or stronger effect: perhaps here we might find that the size of the amino acid alphabet complements that of the nucleotide alphabet as some sort of optimal match? Neither of these tests is likely to provide a final answer to the universality of life’s choice of amino acids, but by their very nature, both tests would sit in defined relationship to previous studies, sketching a coherent picture to the question of what we might expect from an independent origin for life, and thus what we might expect to find as a role for biology within the cosmos.

Acknowledgments: This material is based upon work supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement No. NNA09DA77A issued through the Office of Space Science.

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Chance or Necessity? Bioenergetics and the Probability of Life

Journal of Cosmology, 2010, Vol 10, 3286-3304.
JournalofCosmology.com, August, 2010

Chance or Necessity?
Bioenergetics and the Probability of Life
Nick Lane, Ph.D.,
Department of Genetics, Evolution and Environment, University College London, Gower Street, London, UK.

The emergence of life is probable on any wet, rocky planet. Serpentinization gives rise to alkaline hydrothermal vents that form: (i) simple organics; (ii) catalysts that direct primordial metabolism (iii) micropores with cell-like properties; and (iv) proton gradients equivalent to the proton-motive force. Thermodynamic constraints dictate that all anaerobic chemolithotrophic cells must depend on chemiosmotic coupling, explaining the near-universal use of proton gradients today. But proton gradients also limit the evolutionary potential of prokaryotes. Only a rare and stochastic event, an endosymbiosis between prokaryotes, permitted the evolution of morphologically complex life on Earth, as only such an endosymbiosis made it possible for chemiosmotic coupling to be controlled by multiple small genome outposts across a wide area of internal membranes. This leap in bioenergetic capacity in turn enabled the expansion in cell volume and genome size characteristic of eukaryotes. The origin of life and evolution of prokaryotes is therefore deterministic and probable (necessity), while the evolution of more complex eukaryotic life is stochastic and improbable (chance). These bioenergetic principles are likely to apply throughout the universe.

Forty years ago, Jacques Monod published his seminal work, Chance and Necessity (Monod, 1971). Monod's bleak perspective, that the origin of life was a freak accident, and that we are almost certainly alone in an empty universe, was opposed by Christian de Duve, who has argued that life is a "cosmic imperative", and as such will arise throughout the universe (de Duve, 1995). It is emblematic of the state of play that either view could still be right. A similar dialectic exists between evolutionists. Steven Jay Gould advocated the contingency of evolution: wind back the clock, and life will evolve again along radically different lines (Gould, 1989). Simon Conway Morris argues that convergence is the stronger force – the principles of engineering dictate that the same, or similar, solutions will emerge repeatedly (Conway Morris, 2003). Wind back the clock and humanity will enjoy a second coming.
In the 40 years since Chance and Necessity, we have certainly learned a great deal about the origin of life. But Monod would perhaps be surprised to discover that many of the same concepts are still bandied around. The Miller-Urey experiment is still regularly cited, rightly – it is the cornerstone of all experimental research into the origin of life (Miller, 1953). Primordial soup is equally venerable, dating back to Haldane and Oparin in the 1920s (Haldane, 1929; Oparin, 1924); and soup still stains the pages of modern text books. As one of the pioneers of molecular biology, notably the discovery of messenger RNA (Jacob and Monod, 1961), Monod would surely have been delighted by the RNA world. Promoted mostly by molecular biologists, with no love of geochemistry, the RNA world is usually taken to play out in soup, if only for simplicity (Orgel, 2008). There is little that Monod would find unfamiliar with this world. No wonder he saw life as an improbable freak. Apart from the absence of evidence for soup, or the absence of heterotrophs that might have fermented soup from anywhere near the base of any phylogenetic tree, the lack of thermodynamic driving force makes the emergence of life from soup improbable in the extreme (Lane, Allen and Martin, 2010). If any doubts remain, Say and Fuchs (2010) have shown that the enzymes involved in synthesizing sugars predate those that break sugars down.
But Monod knew nothing about vents. Black smokers were not discovered until 1977, seven years after his book, and one year after Monod died. There's nothing of the equilibrium about smokers, but they suffer equally intractable problems (Bada, 2004). Black smokers are short-lived and unstable (Kelley et al., 2002), intensely hot, and lend meaning to the word 'vent': a chimney pumping effluent straight out into the ocean. Today, life in black smokers depends on oxygen, and therefore indirectly photosynthesis: not heat, not pyrites, not surface chemistry, but the reaction of H2S with O2.
Four billion years ago there was little, if any, oxygen. Wächtershäuser's conception of surface chemistry pulled by pyrites formation (Wächtershäuser, 1988; 2006) would have surprised Monod, as it did everyone else, but ingenious as the idea is, it bears no relationship to life as we know it today. If the failing of soup is to ignore thermodynamics, the failing of surface catalysis is to ignore microbiology, the chemistry of real life. Another big failing of surface catalysis is that, as soon as a product is released from the surface, it is flushed out of the vent to disperse in the ocean. Again, the sheer improbability of life seems little short of a miracle.
But there is an alternative, pioneered over the last 20 years by Michael Russell and colleagues, which turns the probability of life's origins on its head (Russell et al., 1993; 1994; Russell and Hall, 1997; Martin and Russell, 2003; Martin and Russell, 2007; Russell and Kanik, 2010). It lacks only for a colourful name to distinguish it from black smokers. White non-smoker will not do; but "alkaline hydrothermal vent" is rather less memorable than "black smoker", and not sufficiently distinct. Every other article on alkaline hydrothermal vents seems to be illustrated with pictures of black smokers (see Figure 1). That will not do either.

Figure 1. A black smoker compared with an alkaline hydrothermal vent. A: Nature Tower, a 30-metre tall active alkaline vent at Lost City, rising from the serpentine bedrock. The actively venting areas are brighter white. The marker is 1 m in height. Reproduced with permission from Deborah S. Kelley and the Oceanography Society (Oceanography vol 18 no 3, September 2005). B: A volcanically driven black smoker, venting at 350°C, on the Juan de Fuca Ridge, Northeast Pacific Ocean. The marker is 1m in height. Reproduced with permission from Deborah S. Kelley and the Oceanography Society (Oceanography vol 18 no 3, September 2005). C: Microscopic structure of an alkaline vent, showing interconnecting compartments that provide an ideal hatchery for the origin of life. The section is about 1 cm across and 30 microns thick. Reproduced with permission from Deborah S. Kelley and the Oceanography Society (Oceanography vol 20 no 4, December 2007). Alkaline hydrothermal vents are formed not by volcanism but by the process of serpentinization, in which water reacts with ultramafic rocks from the Earth's upper mantle, like peridotite, containing magnesium and iron-rich minerals, in particular olivine (Sleep et al., 2004; Bach et al., 2006). Such minerals are common across the oceanic crust, and serpentinization is a global process, likely to occur on any wet rocky planet (Fyfe et al., 1994; Russell and Kanik, 2010). The water reacts: it physically binds to olivine, hydroxylating the mineral, thereby metamorphosing it into serpentinite, named after its mottled green colouration and sinuous appearance. The reduction of water by olivine releases hydrogen gas and hydroxide anions, via exothermic reactions, the heat of which drives the alkaline fluids back to the surface, where they emerge into the ocean as warm alkaline hydrothermal vents, with a pH of around 9 to 11 (Martin et al., 2008). Four billion years ago, such vents would have been the ideal hatcheries for life, as they provide essentially all the requirements needed for sustained abiotic chemistry and the beginnings of natural selection.
Life is notoriously difficult to define, but much easier to describe. All life today is made of reduced carbon compounds, typically formed by reducing CO2; all life uses enzymes to catalyse metabolism; all life replicates using DNA as a hereditary template; all life is composed of cells which concentrate molecules and discriminate between inside and outside; and all life is powered by proton gradients (occasionally sodium gradients) which are tapped by chemiosmotic coupling across a membrane.
Plainly life needs a thermodynamic driving force, which today is always redox chemistry: the transfer of electrons from donors to acceptors, glaringly absent in soup. The simplest such transfer is from H2 to CO2, to form organics such as methane and acetate. There are only five known primary pathways of carbon assimilation across all of life (Thauer, 2007). The simplest and cheapest in energetic terms is the acetyl CoA pathway in acetogens and methanogens, which runs all the way to pyruvate (Fuchs, 1989; Berg et al., 2010).
Everything else requires more ATP, or some other form of energy such as sunlight, to reduce CO2, and is therefore less suited as a primordial pathway (Fuchs and Stupperich, 1985; Shock et al., 1998). Alkaline hydrothermal vents are the only major source of hydrogen on the planet (Sleep et al., 2004). As in the acetyl CoA pathway today, the energy released by the reaction of H2 with CO2 can drive the formation of thioesters (simpler acetyl thioesters like acetyl methyl sulfide perhaps substituting for acetyl CoA in the primordial pathways) and acetyl phosphate (Martin and Russell, 2007). In principle such energy-rich compounds could drive intermediary metabolism, such as the reverse Krebs cycle and amino acid formation; and indeed acetyl phosphate is still used alongside ATP in many prokaryotes today (Wolfe, 2010).
Then life requires catalysis. In the strongest version of the RNA world, the first catalysts were RNA molecules, which raises the serious question of where all the RNA comes from. A weaker version of the RNA world allows for RNA catalysis, but not as the only catalyst. Minerals containing transition metals, such as Fe-S clusters, are found even today at the active sites of critical enzymes, including respiratory complexes and hydrogenase enzymes (Russell et al., 2008). Many of these are identical to mineral structures found in alkaline vents, including almost all the enzymes of the acetyl CoA pathway in methanogens and acetogens, giving these pathways "rocky roots" (Russell and Martin, 2004). These minerals, by catalysing the formation of organics that enter into intermediary metabolism (driven by phosphorylation), give rise to a richer catalytic spectrum of amino acids, polypeptides, nucleotides and simple cofactors, many of which are nucleotides (Copley et al., 2007; Yarus, 2010). Again, the continuous flow of hydrogen and reduced carbon compounds should drive these reactions forward, with the catalysts providing direction (the beginnings of later metabolic pathways) as much as speed (Copley et al., 2007).
So, alkaline vents provide the thermodynamic driving force, abundant raw materials, sustained over scores of millennia, and the catalysts needed for each step in the same form as modern enzymes, if initially without their protein matrix. (Proteins speed up reactions enormously; but it is the cofactor or mineral cluster that determines the nature of the reaction, and therefore the primordial metabolic pathways; Copley et al., 2007.) They also provide reduced nitrogen, such as ammonia (Martin and Russell, 2007). Phosphate is stable in alkaline conditions, likely bound to Mg2+M (Mellersh and Smith, 2010). Much pioneering research on abiotic chemistry, envisaged to occur beneath a hypothetical reducing atmosphere (Saladino et al., 2008; Powner et al., 2009; Costanzo et al., 2009) applies equally to alkaline vents, except that the steady supply of reduced precursors is real and measurable, even in modern vent systems like Lost City (Proskurowski et al, 2008; Bradley et al., 2009). The details are beyond the scope of this paper, but are discussed in other articles in this edition of the Journal of Cosmology.
Life is cellular. Without restricted, enclosed spaces, it is not possible to concentrate the monomers needed for protein and nucleic acid polymerization; they simply disperse into the oceans at far too low a concentration to be any use. Alkaline vents are a honeycomb of interconnected micropores, on roughly a cellular scale, through which reduced fluids percolate, their slow circulation driven by convection currents and thermal diffusion (Russell and Hall, 1997; Martin and Russell, 2003; Baaske et al., 2007). In modern vent systems such as Lost City, the pores are enclosed by delicate aragonite walls (Kelley et al., 2001), but four billion years ago the walls would have been catalytic, composed of bubbly membranous iron-sulfur minerals, perhaps mingling with aragonite (Russell and Kanik, 2010). Today, the oceans are oxygenated, so iron rapidly precipitates out as rusty sediments. In the past, this process occurred on a vast scale to form banded iron formations, or BIFs (Lane, 2002). But four billion years ago, BIFs were all in the future, and the oceans were saturated with ferrous iron (hence iron was available at those primordial vents delivered from the Hadean Ocean, but it is not today). Sulfide could have been carried to the vents in the alkaline solution (Mielke et al., 2010). Fischer-Tropsch syntheses take place in modern vents (McCollom et al., 1999; Konn et al., 2009) and would certainly have been more common if catalysed by transition metals like iron. Such reactions are likely to have coated the pore walls with hydrophobic hydrocarbons, giving rise to the first rudimentary, and probably fragmentary, membranes, which did not hinder flow, but did foster the beginnings of membrane chemistry (Lane et al., 2010).
Such microporous honeycombs have been shown to concentrate single nucleotides and oligomers of RNA and DNA up to thousands of times their starting concentration (Baaske et al., 2007; Budin et al., 2009; Mast and Braun, 2010), a unique system making an RNA world plausible for the first time (or specifically, a weak form in which RNA functions as both catalyst and replicator, but not alone). Cyclic nucleotides have been shown to spontaneously polymerise into RNA in aqueous solution, when at high concentration and temperatures of around 80oC (Costanzo et al., 2009) - conditions that practically define alkaline vents, but no other setting. Interestingly, modern RNA polymerase is an Mg2+ enzyme (Zaychikov et al, 1997), which again brings to mind relatively cool vent fluids, bearing Mg2+ dissolved from olivine. It is not implausible that Mg2+, chelated by self-selecting amino acids, might exhibit some RNA polymerase activity under vent conditions. The catalytic action of many ribozymes is also dependent on Mg2+ (Uchimaru et al., 1993).
Replication is the next step. Thermal currents in hydrothermal vents simulate thermal PCR (Baaske et al., 2007): double-stranded DNA melts at higher temperatures, and anneals at lower temperatures. In the presence of a polymerase enzyme, around 1000 cycles of replication take place in an experimental vent system in a single day (Mast and Braun, 2010).
Equally important, the honeycomb cells in alkaline vents provide an ideal setting for the beginnings of natural selection (Koonin and Martin, 2006). Selection for replication speed of RNA invariably generates small RNAs capable of replicating at high speed, as Spiegelman showed with his "monster" (Oehlenschläger and Eigen, 1997). The theoretical question of how selection begins to act on encoded properties, such as suites of metabolic traits, rather than on replication speed alone, is perhaps the first major transition in evolution. Most models suggest that selection for metabolic traits, as in Eigen's hypercycle model (Eigen and Schuster, 1977), and Szathmáry's metabolic replicator model (Száthmary and Demeter, 1987) is quickly overwhelmed by faster-replicating parasites, and the system swiftly collapses (Branciamore et al., 2009). The only way that selection can act at the level of metabolism is through selection for a higher entity like a cell, which excludes parasitic RNA. The magical appearance of cells, of course, brings its own well-known problems; but Szathmáry has recently shown that compartmentalization per se – specifically the honeycomb pores of alkaline vents – provides a way of temporarily excluding the parasitic fast-replicating RNA, so enabling the group selection of metabolic traits (Branciamore et al., 2009). The origin of genomes in alkaline vents has been addressed in detail by Koonin and Martin (2006). Thus, alkaline vents potentially solve the origin of selection for cells and metabolism, as well as the origin of the replicators themselves.
For these reasons alone, alkaline hydrothermal vent systems should be seen as not merely the most promising setting for the origin of life, but as the only model that makes the emergence of life look like a probable and deterministic outcome of geology, geochemistry and thermodynamics (Table 1). Alkaline vents are electrochemical reactors that provide integrity, concentration, catalysis, replication and a suitable environment for selection. It is hard to imagine life not emerging in such a system, especially if one pictures the oceanic crust as host to practically contiguous vent systems, as might indeed have been the case (Russell and Arndt, 2005). One could even imagine a natural selection of vents, in which life emerges from the system with the best balance of H2, flow and catalysis, perhaps going on to infect nearby systems. But the most compelling evidence for the singular importance of alkaline vents is the role of chemiosmotic coupling in life today.
Table 1. Comparison of models for the origin of life. Comparative overall likelihood of primordial conditions giving rise to a prokaryotic cell. Only alkaline hydrothermal vents give a seamless transition from geochemistry to biochemistry, providing a thermodynamic driving force, stable and continuous flux of reduced precursors, mineral catalysis, phosphate activation, cell-like structures, proton gradients, replicators and a suitable environment for selection. See text for discussion of parameters.

All life is chemiosmotic. This is shocking and highly significant. The term "chemiosmotic coupling" refers to the mechanism of energy transduction in cells. Electrons are passed from a donor to an acceptor, by way of electron-transport chains that act, in effect, like electrical wires. The flow of electrons powers the transfer of protons across a membrane, to form a proton gradient over the membrane. The flow of protons through the ATPase – an amazing protein turbine – powers the synthesis of ATP. Thus the oxidation of an electron donor is coupled to ATP synthesis by way of a proton gradient. The concept was pioneered over 30 years by Peter Mitchell, against widespread resistance, for which he won the Nobel Prize in 1978 (Lane, 2005; Prebble and Webber, 2003). Since then, the exact mechanisms by which electron flow is coupled to proton transfer, and proton-motive force to ATP synthesis (Abrahams, et al., 1994) have been elucidated at atomic resolution, most recently in the remarkable piston action of complex I (Efremov et al., 2010). But the wider questions that originally motivated Mitchell himself have received surprisingly little attention.
Proton gradients are nearly universal, only occasionally being replaced by sodium gradients, which requisition the same apparatus (Lane et al., 2010). All three domains of life – bacteria, archaea and eukaryotes – use analogous systems (Lane, 2005). Both respiration and photosynthesis form ATP using proton gradients. The bacterial flagellum is proton powered, as is the movement of many solutes and metabolites in and out of the cell (Harold, 1986). Bacteriorhodopsin-based ATP synthesis in prokaryotes uses sunlight to translocate protons, via conformational changes in rhodopsin (Luecke et al., 1998). Even obligate fermenters, which do not use the proton gradient for energy transduction, still generate a proton gradient over the cell membrane to power membrane transporters (antiporters and symporters). ATP from fermentation drives the ATP synthase in reverse, to pump protons over the membrane (Harold, 1986). The fact that all cells use the same machinery, right down to the ATP synthase itself (which is homologous, if not identical, in archaea and bacteria; Mulkidjanian et al., 2007) means that chemiosmotic coupling is as universal as the universal code itself, a fundamental property of life on earth. But why?
Most models for the origin of life tack on chemiosmotic coupling as a peculiarity that just happens to need explaining somewhere down the line, rather than as a fundamental, perhaps even necessary, feature of life. Russell has long argued the significance of proton gradients for life, explaining their existence as a natural outcome of the origin of life in alkaline vents (Russell et al., 1993, 1994; Russell and Hall, 1997; Martin and Russell, 2007; Nitschke and Russell, 2009). The reason is simple: alkaline fluids, bubbling into a mildly acidic ocean (probably around pH 5 to 6, as CO2 levels were as much as 1000 times higher four billion years ago than today) give rise to a natural proton gradient of 4 to 5 pH units, equating to a difference in proton concentration of about 10,000-fold across the vent walls. Such a proton gradient is of a similar magnitude (giving a membrane potential of around 200 mV) and polarity (outside acidic) to the proton gradients used by cells today (Russell et al., 1993, 1994; Lane et al., 2010; Figure 2). Plainly it is easier to tap a natural, pre-existing gradient than to generate one from scratch; indeed it is hard to imagine how, or why, life would have 'invented' such a profoundly counter-intuitive mechanism of energy generation, had proton gradients not been a pervasive aspect of the environment.

Figure 2. Chemiosmotic properties of microporous vents and modern cells. A: The proton motive force across the boundary of a microporous cell in an alkaline hydrothermal vent.   The proton motive force is a gradient of proton concentration and electrical potential that stores energy and makes it available for synthesis and transport.  The proton gradient is produced by alkaline (high pH hydrothermal fluids derived from serpentinization, and an acidic (low pH) external environment of carbonic acid solution (primordial seawater). B: The proton motive force of living cells.  The proton-motive force is a gradient of proton concentration and electrical potential that stores energy and makes it available for synthesis and transport. The proton gradient is produced by an alkaline (high pH) cytoplasm and by an acidic (low pH) extracellular environment. The gradient is continuously replenished by electrons flowing across the membrane from donors to acceptors. P-phase: positive; N phase: negative. The details of how proton gradients might be tapped in a vent system are beyond the scope of this article, and have been discussed elsewhere (Nitschke and Russell, 2009; Lane et al., 2010). They are amenable to experiment. In terms of the probability of life, the question is this: did chemiosmotic coupling arise just because it could (a fluke aided by the environment) or because it must? Martin and Russell (2007) have demonstrated, on the basis of bioenergetics, that chemiosmotic coupling is in fact strictly necessary. Despite being the only pathway able to drive carbon and energy metabolism without an input of energy from anywhere else, the acetyl CoA pathway nonetheless depends on chemiosmotic coupling (Thauer et al., 2008). Substrate-level phosphorylation (the direct transfer of phosphate from one moiety to another) does not provide enough energy for growth in either methanogens or acetogens (Martin and Russell, 2007; Thauer et al., 2008). The reaction of H2 with CO2, despite being exergonic overall, requires surmounting a low kinetic barrier to form a methyl group (Martin and Russell, 2007; Nitschke and Russell, 2009; Lane et al., 2010). In vents, the critical intermediates are provided free of charge. Outside the vents, however, cells must pay themselves. Using substrate-level phosphorylation, cells must spend one ATP to gain one ATP, which precludes the possibility of growth. This, in fact, is a fundamental problem with chemistry, and boils down to a startling proposition: life cannot evolve by chemistry alone. The secret of chemiosmotic coupling is that it is not chemistry (Lane, 2005). Just as the DNA code enables information to replace chemistry, so too chemiosmotic coupling enables metabolism to escape the bounds of chemistry.
Chemistry is the reaction of molecules with each other, and can be defined by stoichiometry. It is not possible for one molecule to react with half a molecule, so chemical equations must be balanced to give whole numbers, to the chagrin of students. One of the molecular peculiarities of respiration, which foreshadowed the chemiosmotic hypothesis in the first place, is that it is not stoichiometric: the number of electrons transferred to oxygen does not correspond in a simple way to the amount of ATP synthesised. The interpolation of Mitchell's proton gradient explained all that. A gradient, by its nature, is composed of gradations, and so is not stoichiometric. A gradient, in effect, disconnects the exergonic reaction (which can be repeated indefinitely to generate a proton gradient) from ATP synthesis. Thus a reaction that releases enough energy to synthesise 1.5 ATPs is worthless in terms of stoichiometric chemistry, but pays off with a gradient. The 'small change' can be saved as protons towards the next ATP. Net growth is now possible. The remarkable conclusion is that life is obliged to be chemiosmotic. The fact that alkaline hydrothermal vents are naturally chemiosmotic is the defining reason to view them as the ideal incubator of life. One is tempted to say that life could not have arisen elsewhere, because no other environment provides dynamic proton (or Na+) gradients for 'free'.
This should be qualified a little. Photosynthesis overcomes these problems, as it taps the energy of the sun. Chemiosmotic coupling is not strictly necessary, even if still useful. But photosynthesis is altogether more complex, and in terms of how life works today, sunlight could not have been the primordial source of energy. Likewise, aerobic reactions (such as H2S with O2) do produce enough energy for growth; but as already noted, there was little or no O2 before oxygenic photosynthesis. Fermentation of primordial soup could also, in principle, provide sufficient energy, as the organic molecules in soup come 'free', formed, in theory, by UV radiation acting on methane and ammonia. But free soup is not a bargain if the pot is empty; and the absence of a thermodynamic driving force means that life did not arise by fermentation, as discussed earlier. So within this more restricted ambit, the origin of life by chemolithotrophy in anaerobic oceans is only possible with chemiosmotic coupling. The first prokaryotes that emerged from vents were necessarily chemiosmotic. But this mechanism, while freeing life from vents, also conceals hidden costs. These costs explain why life runs into a big problem getting past bacteria.
Complex life arose only once on Earth in four billion years. This statement requires defending before continuing. The argument is based on three points: (i) all complex life is eukaryotic; (ii) prokaryotes show no tendency to evolve greater complexity; and (iii) intermediate forms exist, proving that the niche is viable (refuting the argument from extinction); but all these intermediate forms evolved by reductive evolution from more complex ancestors, rather than accruing greater complexity by point mutations or endosymbioses in prokaryotic populations (Lane and Martin, 2010).
All morphologically complex life on Earth is composed of eukaryotic cells; and all eukaryotes share a common ancestor. The nature of this common ancestor is inferred from the traits shared by all the major supergroups of eukaryotes. All, by definition, share the nucleus with its characteristic double membrane and large nuclear pore complexes. All have straight chromosomes with centromeres and telomeres; all share detailed chromatin and chromosomal structures, right down to the same introns in the same place in the same genes; all share dynamic cytoskeletons, endoplasmic reticulum, Golgi bodies, lysosomes, mitochondria (all those that do not have mitochondria today, had them in the past; see discussion below) and so on. All share dynamic processes like mitosis and meiotic sex. On average eukaryotic cells are much larger than prokaryotes (4 to 5 orders of magnitude) and have much larger genomes (on average 2 to 3 orders of magnitude, but larger eukaryotic genomes range up to 100,000 Mb, compared with just 13 Mb in cyanobacteria). In short, the last common ancestor of all eukaryotes was very different to any known prokaryote, whether bacterium or archaeon (Lane and Martin, 2010).
An interesting theoretical question is how the common ancestor of eukaryotes came to share so many traits (assuming that they cannot all have evolved simultaneously). It is likely that only meiotic sex in a small population of mating cells could have given rise to the last eukaryotic ancestor. While lateral gene transfer is common in bacteria and archaea, forming a network rather than a pure tree of life (Dagan and Martin, 2007, 2009), lateral gene transfer is non-reciprocal and so leads to greater variation, rather than less variation, in a population. Being reciprocal, sex does the opposite: it concentrates all shared properties within a single interbreeding population, restricting variance within the population, but increasing variance between populations (speciation). If the eukaryotic common ancestor had evolved by binary fission and lateral gene transfer, in bacterial fashion, it is likely that some eukaryotes would have a nucleus and others not; some would have an endoplasmic reticulum or mitochondria, others not, and so on. The very fact that all eukaryotes share so many fundamental traits almost certainly means that they arose in a small and rapidly evolving sexual population (Lane, 2009). That population had to be small, because a larger, geographically more structured population would inevitably diverge into different species, with different traits, as happened soon afterwards in the eukaryotic supergroups. The population had to evolve rapidly for much the same reasons: a leisurely pace of evolution would tend to promote branching and diversification through the dispersal and geographical structuring of stable populations. A high mutation rate in the earliest eukaryotes (caused by an early bombardment of mitochondrial genes and retrotransposons; Martin and Koonin, 2006) explains both the requirement for sex and the speed of evolution (Lane, 2009). No other mode of evolution could account easily for the large number of shared traits in the last eukaryotic ancestor.
In contrast, bacteria show no tendency to evolve greater complexity. Or rather, more perplexingly, they show a little tendency to evolve greater complexity, but not much. In essence, bacteria have made a start up every avenue of eukaryotic complexity, but then stopped short. So, for example, prokaryotes have evolved structures resembling a nucleus, recombination, straight chromosomes, internal membranes, multiple replicons, giant size, extreme polyploidy, dynamic cytoskeletons, predation, parasitism, introns and exons, intercellular signalling (quorum sensing) and even endosymbionts (Lane, 2007; Lane and Martin, 2010). What stopped them evolving true complexity? There is a serious question here about the nature of natural selection itself. In what way is the nucleus or phagocytosis different from an eye? In the case of the eye, the evolution of a fully functional image-forming eye can be modelled as a succession of steps from a light-sensitive spot, each step offering a small selective advantage over the last (Nilsson and Pelger, 1994; Figure 3). Not surprisingly, eyes have evolved essentially independently on scores of occasions in metazoans from a light-sensitive spot, albeit under the control of a small group of highly conserved regulatory genes such as Pax-6, which nonetheless independently recruited completely different sets of genes to construct a wide range of morphologically disparate eyes, from the mirror eyes of scallops to the convergent camera eyes of molluscs and mammals (Lane, 2009, Vopalensky and Kozmik, 2009)

Figure 3. The steps to evolve an eye The succession of steps needed to evolve a functional eye from a light-sensitive spot, whereby each step offers a small advantage over the previous step, as modelled by Dan-Eric Nilsson and Suzanne Pelger. The numbers give an approximate number of generations for each change, assuming that each step involves a change of less than 1% (i.e. slightly more lens; slightly greater curve, etc). Note that if lifespan is 1 year for small marine creatures, the total time required to evolve an eye is less than 0.5 million years. As each step offers an advantage favoured by natural selection, it is not surprising that eyes evolved essentially independently on at least 65 occasions. If the evolution of phagocytosis or the nucleus also involved a succession of small steps, each one favoured by selection, the over-riding question is why did these traits not evolve repeatedly in vast populations of prokaryotes, over vast aeons of time? Reproduced with permission from Dan-Eric Nilsson: Michael Land and Dan-Eric Nilsson, Animal eyes. OUP, Oxford, 2002. But if each step to evolve the nucleus or phagocytosis also offers a slight selective advantage, then why don't such traits evolve repeatedly in prokaryotes? The trilobite eye probably evolved in a few million years; bacteria have had four billion years to evolve phagocytosis, and yet have apparently never done so, despite having a dynamic cytoskeleton (Vats et al., 2009), frequently losing the cell wall (Allan et al. 2009), and evolving internal compartments (for example in planctomycetes; Lindsay et al., 2001). The only reasonable conclusion is that natural selection does not tend to drive the evolution of complexity in prokaryotes, presumably because selection for fast replication and genomic streamlining invariably favours small and morphologically simply cells (Lane, 2005, 2007).
One alternative, frequently cited possibility, is that more complex prokaryotes, or proto-eukaryotes, arise regularly but are out-competed by the eukaryotes that already exist, and so are always driven to extinction, leaving no trace in the (imperfect) fossil record. According to this view, the eukaryotes emerged from some kind of environmental bottleneck (such as rising oxygen levels) and then drove any later arrivals, and for that matter their own more primitive ancestors, to extinction (de Duve, 2005, 2007; Kurland et al, 2006; Poole and Penny, 2007). Extinction is taken to be the fate of virtually every species anyway, so what could be more natural? In this reading it is true that all complex life is eukaryotic, and that eukaryotes arose only once; but in principle that says nothing about probability, because complex life could emerge repeatedly, with high probability, but when it does it is always outcompeted by the highly-adapted eukaryotes that already occupy the niche. The fact that mass extinctions often lead to evolutionary radiations, at least among plants and animals, seems to justify this view.
But this argument will not do. First, it is dubious whether any microbial group has ever fallen extinct. Methanogens and acetogens, energetically the most tenuous forms of life, have survived for nearly four billion years (Ueno et al., 2006). The continuity of the global geochemical cycles over that time – the sulfur cycle, nitrogen cycle, methanogenesis, and so on – attest to the fact that these microbial niches have been continuously occupied for nearly three billion years (Canfield et al., 2006). Thus, there is no evidence for extinction in the geochemical record, and the onus ought to be on those who invoke extinction to provide evidence. Second, niches are frequently shared by microbes, including eukaryotes. The tiny pico-eukaryotes, for example, discovered a decade ago (Moreira and Lopez-Garcia, 2002), compete on essentially prokaryotic terms with prokaryotes, yet are abundant (Piganeau, et al., 2008), not extinct as would surely be predicted by the bottleneck hypothesis. Likewise, the oceans are filled with mesophilic archaea that live alongside bacteria in the same niche; neither group shows any sign of extinction (Prosser and Nicol, 2008). Supposedly mesophilic eukaryotes are often found in extreme environments, once thought to be the sole preserve of extremophile archaea and bacteria, such as the Rio Tinto in Spain, with extreme acidity and heavy metal levels (Amaral Zetler et al., 2002), and the abysmal Mediterranean anoxic hypersaline lakes, such as L'Atlante basin (Danovaro et al., 2010). The biodiversity of life itself points to the ability of species, especially microbes, to coexist within ecosystems.
Third, and more specifically, there are more than 1000 known species of protist that do not have mitochondria: they occupy the intermediate complexity niche between prokaryotes and eukaryotes, a group dubbed 'archezoa' by Cavalier-Smith in the 1980s and taken to be exactly that, primitive eukaryotes that had not yet acquired mitochondria, or much of the rest of the eukaryotic paraphernalia, beyond the nucleus (Cavalier-Smith, 1987). Over the last 15 years or so, all of these groups turned out to have possessed mitochondria in the past, which later became specialised to anaerobic environments as ‘relict’ organelles such as hydrogenosomes or mitosomes (Tielens et al., 2002; Embley and Martin, 2006; van der Giezen, 2009). The general consensus is that, if there ever was a primitive amitochondriate eukaryote, an 'authentic' archezoon, then it has fallen extinct. But in light of the arguments above, why should it have fallen extinct? Because it was outcompeted to extinction in anaerobic environments by eukaryotic cells that are exactly the same (which is to say they are archezoa, if not primitively so)? If that were the case, the archezoa must have been outcompeted in anaerobic environments by eukaryotes with mitochondria that (according to the archezoa hypothesis) had previously been adapted to aerobic environments (Cavalier-Smith, 2002; 2009), hence were presumably at a disadvantage in these ancient and stable anaerobic environments.
None of this sounds plausible. Because anaerobic eukaryotes are widely distributed across all six eukaryotic supergroups (Mentel and Martin, 2010), they most likely descend from the last eukaryotic common ancestor, which probably arose in the anaerobic 'Canfield' oceans 1 to 2 billion years ago (Knoll et al., 2006). Anaerobic environments have been continuously occupied by bacteria and presumably eukaryotes throughout that period (Lyons et al., 2009; Falkowski et al., 2008), so there has been plenty of time for genuinely primitive amitochondriate eukaryotes – real archezoa –to evolve repeatedly. Yet every single example evolved by reductive evolution from more complex ancestors that once had mitochondria, whereas not one single example evolved greater complexity by either point mutation or other primary endosymbiotic events in large populations of prokaryotes already well adapted to anaerobic environments. It is said to be unreasonable to conduct a statistical analysis of eukaryotic origins, because they only evolved once; but if chance were the only factor, from a purely statistical point of view the probability of all 1000+ species of archezoa being derived by reductive evolution from more complex ancestors, rather than simply evolving just a little more complexity, is about 1 in 10300 against. That constitutes good evidence that eukaryotic origins were genuinely improbable – a singular event – but once that threshold had been crossed, the niche was invaded repeatedly by reductive evolution from more complex eukaryotic ancestors. Chemiosmotic constraints explain why.
Prokaryotes respire over their plasma membrane, which is to say they generate a proton gradient over the plasma membrane. This means that prokaryotes face constraints in their surface-area-to-volume ratio. Double the dimensions of a bacterium, and the surface-area-to-volume ratio halves. Thus, the capacity for ATP synthesis relative to protein synthesis falls with increasing cell volume, placing large bacteria at a bioenergetic disadvantage. Most prokaryotes are indeed small, on average 4 or 5 orders of magnitude smaller than eukaryotic cells. If a spherical prokaryote were scaled up to the same size as an average eukaryotic cell, while continuing to respire over its plasma membrane, its energetic efficiency would fall off by around 40 to 50-fold. The actual difference is in fact several orders of magnitude, as discussed in detail elsewhere (Lane and Martin, 2010), because eukaryotes do not respire over their plasma membrane, but internally in mitochondria. Plainly, unless prokaryotes can find some way to adapt, larger cells will be penalised by selection.
To a point, prokaryotes do adapt to offset the disadvantages of size. For example, rods (bacilli) have a larger surface-area-to-volume ratio than spheres (cocci) so the larger the bacterium, the less likely it is to be spherical. Likewise, the density of respiratory complexes could be increased, and the rate-constants of respiratory enzymes increased. To a point this happens: on average, the metabolic rate per gram is faster in bacteria than in eukaryotes (Lane and Martin, 2010; Makarieva et al., 2005; Fenchal and Finlay, 1983). But the fact that the gap is not great (less than an order of magnitude) implies that other constraints, such as protein crowding and loss of membrane fluidity, offset the respiratory advantages. A third possibility, by far the most revealing, is simply to internalise respiration on intracellular membranes. This gets around the problem in essentially the same way as eukaryotes. Some bacteria do exactly this, including cyanobacteria and many nitrifying bacteria: their bioenergetic membranes are extensively invaginated within circumscribed areas of the cell (Pinevich, 1997). Nonetheless, even this approach appears to be limited in scale. Most prokaryotes remain small, even those with extensive invaginations of bioenergetic membranes. There would appear to be some other limiting factor.
This limitation is not to be found in something that prokaryotes do not have, but in something that eukaryotes do have: mitochondrial DNA. Mitochondrial respiration is bacterial, not surprisingly, as mitochondria were derived from bacteria (Sagan, 1967). Mitochondria almost always retain a small genome, the sequence and characteristics of which were instrumental in establishing the bacterial ancestry of mitochondria (Gray et al., 1999). Specifically, all eukaryotes that retain the ability to generate ATP by chemiosmotic coupling have retained the same small group of integral membrane proteins involved in coupling electron flow to proton pumping (Gray et al., 1999; 2004). Each eukaryotic supergroup has independently lost the vast majority of its ancestral genome, leaving each with an identical core set of genes encoding respiratory subunits (as well as a variable group of other genes in some cases). Conversely, if the entire genome has gone – usually the case in hydrogenosomes and mitosomes – the cell correspondingly loses the power to generate ATP by chemiosmotic coupling (van der Giezen, 2009). A simple and powerful hypothesis, long propounded by John F. Allen, is that mitochondrial genes are needed for the control of respiration – specifically for redox regulation of gene expression adjacent to the bioenergetic membranes: Co-localization for Redox Regulation, known as the CORR hypothesis (Allen, 1993; 2003). But regardless of the exact reasons for their retention, the same core group of respiratory genes would hardly be retained in all eukaryotes capable of chemiosmotic coupling by chance alone. It is a fair bet that they are necessary for respiration. Conversely, without these core genes, a large cell would lose control of respiration.
There is strong evidence that the rate of cell respiration does indeed depend on the copy number of mitochondrial DNA, hence the respiratory efficiency of eukaryotes depends on mitochondrial genes (Rocher et al., 2008). There is also strong evidence, as discussed already, that all eukaryotes either have mitochondria, or had them in the past and later lost them by reductive evolution (van der Giezen, 2009). Thus, the last common ancestor of all eukaryotes had mitochondria, making the origin of mitochondria and the origin of the eukaryotic cell plausibly one and the same event. Several rigorous, large-scale full genome studies support this position (Cox et al., 2008; Yutin et al., 2008; Pissani et al., 2007; Rivera and Lake, 2004) – eukaryotes are genetically chimaeras, with most informational genes (those that control gene expression) deriving from archaea, while operational genes (those that control metabolic flux) mostly derive from bacteria, the majority, presumably, from the ancestor of the mitochondria. The simplest explanation consistent with all of these findings is that the eukaryotic cell originated in an endosymbiosis between an archeaon and a bacterium, as proposed by William Martin and Miklos Müller in the hydrogen hypothesis (Martin and Müller, 1998), which has recently gained support from several studies (Atteia et al., 2009; Fritz-Laylin et al., 2010; Danovaro et al., 2010; Lane, 2010). The acquisition of mitochondria transformed the selection pressures acting on the host cell, enabling it to expand in cell size, and significantly in genome size, without a bioenergetic penalty (Lane, 2005; 2007; Lane and Martin, 2010). Prokaryotes are unable to co-localize the correct core contingent of genes alongside their internal bioenergetic membranes, and therefore cannot meet the large energy costs of any significant expansion in genome size. The detailed reasons for this are discussed elsewhere (Lane and Martin, 2010).
An endosymbiosis in prokaryotes reconciles virtually all the conflicting data about the origin of the eukaryotic cell, and more generally, complex life on Earth. It explains why the origin of eukaryotes was a singular event – endosymbioses in prokaryotes, which cannot phagocytose, are vanishingly rare occurrences, albeit one or two cases are known (Wujek, 1979; von Dohlen et al., 2001). These remain as endosymbionts rather than organelles, so plainly the conversion of an endosymbiont into an organelle is also problematic. If the evolution of complexity depends on a rare and stochastic event, that explains why natural selection does not continuously give rise to eukaryotic complexity in prokaryotic populations. The unique acquisition of mitochondria by a prokaryote transformed the course of evolution. By permitting cells to expand their cell volume and genome size without a crippling energetic penalty, eukaryotes could, and did, accumulate genes, ultimately the basis of complexity (Lane and Martin, 2010). It makes sense that prokaryotes are almost universally small and genetically streamlined, showing little tendency to evolve eukaryotic complexity (Lane, 2005, 2007). It makes sense that multicellular eukaryotes only arose once in 4 billion years of evolution. And it makes sense that the acquisition of mitochondria released the first eukaryotes from the energetic constraints that limit all prokaryotes, enabling the evolution of genetically and morphologically complex life on Earth (Lane and Martin, 2010).
This bioenergetic perspective provides an insight into the evolution of life, not just on Earth but also elsewhere in the universe. The origin of life is theoretically probable on any wet rocky planet, as the common geological process of serpentinization is predicted to give rise to alkaline vents on a global scale. Alkaline vents provide an ideal incubator for life, meeting all the conditions needed for life to evolve – specifically a thermodynamic driving force, stability, a steady supply of hydrogen, CO2 and reduced nitrogen, the catalysts needed to drive primordial metabolism, a means of concentration in cell-like micropores, and natural proton gradients enabling the evolution of chemiosmotic coupling, the only means of powering free-living chemolithotrophic cells in anaerobic oceans. But even if the origin of prokaryotes is probable on any wet rocky planet, the very fact that prokaryotes necessarily depend on chemiosmotic coupling constrains evolution. Only a rare and stochastic endosymbiosis can free prokaryotes from the energetic constraints of respiring across the plasma membrane. On Earth this was apparently a singular event, and prokaryotes otherwise show little tendency to evolve greater complexity. The same constraints must face life on other planets too, because chemistry, and the principles of thermodynamics and energetics, are universal. The unavoidable conclusion is that the universe should be full of bacteria, but more complex life will be rare – and perhaps surprisingly similar to us. Aliens will have mitochondria too.

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