I'm glad you wrote this review, now I don't have to :-)
(I agree that it's an excellent book and heartily endorse it.)
Nick Lane thinks the reason is because genes have to be close to a cell membrane in order to make ATP. The square-cube law means that a cell's membrane grows slower than its volume. A big archaeon is limited in how much ATP it can create which limits how many genes it can have which limits its complexity.
Among the three domains of life, eukaryotes are the most complex. By offloading ATP production to mitochondria, they get around the square-cube law limiting ATP production. They can have more genes which enables more complexity.
The way I remember his argument is a bit different, and not involving the square-cube law: (1) if some little section of a membrane is producing ATP, it needs a nearby copy of all the genes for ATP production; (2) if there's a big complicated cell doing lots of stuff, it needs a lot of surface area for producing ATP; (3) It follows from (1) + (2) that if there's a big complicated cell doing lots of stuff, it needs a gazillion copies of its genome per cell; (4) if the genome is really huge, then it's prohibitively expensive to make a gazillion copies of it per cell; (5) therefore genomes can't be huge; (6) therefore cells can't be too complicated.
The reason eukaryotes avoid this dilemma is that they have two genomes. One genome is tiny, and has just the genes for ATP production, and there are gazillion copies of that genome per cell. The other genome is huge, with massive numbers of genes, but it's OK because there's only one copy of that genome per cell.
So the way I remember the story, it doesn't involve the square-cube law. Can't a bacteria increase the effective surface area by, say, putting ATP production machinery onto a vesicle? It's been a while since I read the book, I could be misremembering. :-)
I'm glad you didn't write this review, now I got to instead :-)
I could be wrong and I would like to be corrected if I am. From your writings and from having met you, I would not be surprised if you know far more biology than I do.
(3) It follows from (1) + (2) that if there's a big complicated cell doing lots of stuff, it needs a gazillion copies of its genome per cell;
Why can't bacterium/archaeon just have many copies of the same ATP gene on a single genome? Duplicating a DNA segment is not an uncommon mutation.
On the other hand, packaging ATP genes in mitochondria lets a cell create more or less of them on demand, something it can't do with a repeated sequence in DNA.
Can't a bacteria increase the effective surface area by, say, putting ATP production machinery onto a vesicle?
If I understand correctly, mitochondria use cristae for this purpose instead of vesicles. Cristae are what gives mitochondria membranes their characteristic shape.
I think the "nearby" part is important ... I thought his claim was basically that if I'm an ATP production machine, I need the genome to be really close to me distance-wise (like within X nanometers, for some number X that I don't know), so that if I have a broken part then a replacement can be quickly manufactured on demand in the right location. So having 100 copies of the gene that are physically attached to each other doesn't help at all towards solving the problem, and in fact makes the problem worse.
I really don't know much biology, I'm just going off my memory of the book :-)
He definitely did say something to that effect and it definitely is easier to have the genome near the cell wall of a small cell than a large cell.
You say "the genome" but note that one bacterium (i.e. one cell) can have more than one copy of its entire genome inside it, e.g. "many bacteria harbor multiple copies of their genome per cell", "Enormous bacterium uses thousands of genome copies to its advantage", etc. That's what I was (implicitly) referring to. :-)
I was also halfway through a review of this book. Since I've only met one other person who'd read it I thought it was unlikely anyone else would! I guess LWers have more similar interests than I would have predicted.
I suppose I'll review another book instead!
Me too. I'd hate to find out Jemist's work has gone to waste. A second review could add to what I have here. I skimmed at least half the book including sex, biochemistry and why eukaryotes appeared exactly once. I came to this book with my own perspective. Someone with a different perspective could draw different main ideas from it. Besides, I like the idea of more biological content on LW.
I'm aware of several stabs at The Biogenesis Question and I'm wondering how to reconcile the Nick Lane's book's claims with some of them.
The best take I know, in terms of a beautifully reasoned presentation about the idea that maybe biogenesis wasn't an accident but rather might have been necessary. Here's a roughly 60 minute talk (that works at 2X speed) from Eric Smith, of the Santa Fe Institute.
Maybe the biggest point of disagreement is here?
Life that isn't harvesting negentropy isn't alive. For life to exist it needs a barrier between its ordered self and its chaotic environment. Life on Earth uses a cell wall composed of a lipid membrane. Since the inside of the cell is ordered and the outside is chaotic, there is an entropy gradient across the cell wall. Here's where the primordial soup theory becomes implausible. A primordial soup is at chemical equilibrium. A cell is at chemical disequilibrium.
In contrast with this, Eric Smith's model also has life "requiring" a gradient, however, from his perspective initially there was a geochemical disequilibrium (with no "selfs", plausibly no "cells", and thus no "evolution as such") but instead something similar to: (1) the electrical disequilibrium based on electrons that makes lightning necessary, or (2) the tornado that releases the stress of high dry cold air ABOVE low wet warm air.
The geophysical stress he points to is the production of excess CO2 (from white smokers) and the combination of this with some source of hydrogen, and the construction of lower energy O2 and CH4... which is a giant jump that has no particularly obvious catalysts.
On this reading, "some aqueous chemistry" that self-catalyzes, to turn energy dense molecules into waste products HAD to autocatalyze SOMEHOW in order to release the stress.
In this theory metabolism came first. Maybe a "metabolic biome" came first? Then the leakage from the "stress releasing vortex in chemistry" was mixed and remixed to build up larger biomolecules (maybe more than one) and some "best catalyst of itself" fell out of the process.
Then the fact that the modern catalytic facilitators all have a subunit which is the "A" of ACTG... turns out not to be that surprising. So maybe the "A" (adenine) was already around, and then its re-use in RNA and DNA (and the A of "ATP" and so on) is like... yeah, that just makes sense?
But anything that facilitated that metabolic hurricane would probably have been OK... It probably didn't have to be specifically adenine. Any enzymes (or before that ribozymes) or even just any specific nucleotides and so on... are just the director and the stagehands, not the irreplaceable "story" and its core players.
So I guess a first question I have is whether The Vital Question talked about metabolism and autocatalysis or Eric Smith or the reverse Kreb cycle or adenine or any of this stuff?
It seems like it is very focused on the formation of membranes into cells which I had previously always just sort of assumed (maybe naively) would happen automatically through the self-assembly of micelles. Maybe micelles make the autocatalytic processes go faster, in which case they could show up just from thermodynamics?
It could be that my imagination is too free here, but I could imagine that there might be micelles that form as little "processing bubbles" and the faster they go inside the bubble the faster they can pop open and seed the water with their kind of chemistry... that is pre-dispossed to form more micelles?
By contrast:
Nick Lane proposes that LUCA was a membrane capped pore in an olavine rock.
It seems like Nick Lane thinks the hard part is... membranes? Really?
Thus my second question is what Lane's evidence was for why membrane formation is so hard, and are such a big part of the story (in comparison to other factors).
I feel like the most likely answer will have has something to do with Sodium pumps?
An antiporter is like a turnstile to a building which lets exactly one person into a building for every person which leaves except instead of people it operates on ions. A Na⁺/H⁺ antiporter lets in one sodium ion for every hydrogen ion it lets out and one hydrogen ion in for every sodium ion it lets in. If there is more hydrogen on one side of the membrane, a Na⁺/H⁺ antiporter acts as a hydrogen-powered sodium pump
But, like... maybe the early waters were just more potassium rich than modern sodium rich seawater? This isn't my area of expertise, (I'm just curious about everything and have big gaps in my knowledge about... basically everything), but I'm curious for more details in this possible version of the story.
I've read Nick Lane's book, but I'm not familiar with Eric Smith's stuff (yet).
Lane agrees that life started from a geochemical disequilibrium, in Lane's model it's serpentinization.
Lane's book spends a lot of time talking about the Acetyl-CoA cycle, which is a key part of the Citric Acid cycle, as well as many other metabolic cycles. But he doesn't mention the reverse Krebs cycle or adenine specifically, AFAICT. He cites Eric Smith favorably as going into more details on the specific biochemistry involved.
Regarding membranes, his model is that the first membranes were very leaky with respect to ions. Which is a good thing in the disequilibrium environment, since otherwise the cell would get too full of ions, and die! But there are two big problem for cells with such membranes: 1. their gradient will quickly equilibriate if they leave the sea vent, thus killing them. 2. Evolving a less leaky membrane will reduce their ability to use the sea-vent gradient, and thus by itself will not be selected for. A proton pump will not give an advantage in this case (the argument is a bit complex). The key is that a Sodium antiporter pump needed to come after the leaky membrane. This by itself gives an advantage, by also creating an Na+ gradient across the membrane, and this advantage is strengthened by making the membrane incrementally less leaky. (Lane explicitly claims that the oceans were high in Sodium ions, and low in Potassium ions 4 billion years ago, FWIW.)
But just the Sodium pump wasn't enough to allow the cells to survive autonomously, since it trades positive ions for positive ions one to one. However, it now makes it advantageous to evolve a proton pump. And that was what finally facilitated the ability for cells to become fully autonomous. He goes on to claim that the direction the proton pump was "installed" across the membrane creates different problems, and that Bacteria and Archea are the result of this happenstance.
Thanks! Does he explain "why Olivine"?
If I had a hunch it would route through the idea of metal atoms that fit into larger enzymes like rock knives wielded by squishy caveman hands.
As wikipedia says of inorganic biochemical co-factors:
In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list commonly includes iron, magnesium, manganese, cobalt, copper, zinc, and molybdenum.
So the thing I would naively do, to find candidate minerals, would be to try to sift conserved co-factors, especially with a focus on ribozymes that do the same trick, and then these "metalloribozymes" (some of which do exist in evolved systems still) might suggest typical mineral environments where things first started and never stopped being essential...
...but I don't seem much mention of Olivine playing well with copper or zinc, and [manganese olivine] goes to its own name: Tephroite. But there was no mention of Tephroite in the review.
So this is where my own naive priors would send me looking: for minerals full of elemental cofactors common to ancient metalloribozymes. Lots of data sifting seems likely to be useful, over ribozyme and enzyme databases, and mineral databases, which I have not done.
Did Nick Lane do this kind of data sifting to find the name "Olivine", or did he use some other method?
One idea is that he got the name from the literature, and perhaps cites whoever proposed Olivine? When I search [olivine biogenesis] I get Serpentinization results which echo the link you offered (and youtube on [serpentization] gives a fun experimental video that is interesting but sheds no particular light for me).
Synthesizing this... maybe the practical upshot is just "most normal mantle material reacting with seawater" could be loosely called "something-like-olivine turning to something-like-serpentine" and since there is so much mantle, and so much seawater, priors say that "this general reaction" is the key reaction at a high and fuzzy level of description. Maybe?
It looks like he got it from Mike Russell, though he disagrees with Russell on the details.
Regarding cofactors, the important thing (in Lane's version of this model) about serpentinization is that it is alkaline. This would cause iron to precipitate out (as iron hydroxides and iron sulphates), and dissolve minerals such as nickel and molybdenum.
He points out nickel-doped greigite () as a mineral with a crystal structure similar to several ancient metalloribozymes, and which he thinks was likely to have deposited on the walls of pores in the rock (serpentine?).
One side of the membrane is exposed to water in contact with the alkali ovaline rock. ... Nick Lane proposes that LUCA was a membrane capped pore in an olavine rock.
Correction: Nick Lane never says the vent itself (in which the proto-cell membranes arose) was made of olivine. The olivine is far underground. Sinking seawater reacts with it (serpentinization) and bubbles back up. It precipitates out into the vents (mineralized sponge "chimneys") which itself is not olivine.
Here's what Wikipedia says about Lost City, which Lane gives as an example of a modern alkaline hydrothermal vent:
Once pore waters have permeated the surface and return to the surface, aragonite, brucite, and calcite chimneys are formed as calcium carbonates precipitate out of solution. Younger chimneys are primarily brucite and aragonite, being white and flaky in appearance.
Thanks for this review. I particularly appreciated the explanation of why the transition from primordial soup to cell is hard to explain. Do you know how Lane's book has been received by other biochemists?
There would be quite a long period of time between the initial formation of these micro-pore membranes and the point in time where they could form free floating cells. In that time, they would need to evolve all the machinery to exist unsupported: better membranes, RNA, maybe even DNA, and some method of generating their own proton gradient. And to evolve at all, they would need to reproduce.
The problem is that these creatures are stuck in their respective pores. In order to evolve beyond a stage where they could barely even be considered alive at all, these things would need a way of colonizing other pores. I'm not sure if Lane discusses the question at all, but it seems like a difficult problem. The process would initially have to be so simple it could happen naturally without any molecular machinery.
EDIT: I've now actually read this book, and looked at a figure depicting the situation, and found out that rather than the membranes being stretched across the pore like the skin of a drum, they would form a bubble the sits in the pore. I can easily imagine a bubble growing over time and then dividing when it gets too large, with one of the bubbles wandering off until it gets stuck in another pore. This resolves my confusion.
I think that might have been the deeper reason to talk about this mechanism:
Thermal currents through microporous labrynths have a remarkable capacity to concentrate organic molecules (including amino acids, fatty acids and nucleotides) to extreme levels, thousands or even millions of times the starting concentration, by way of a process known as thermophoresis. This is a little like the tendency of small items of laundry to accumulate inside a duvet cover in the washing machine.
Under this model, maybe during an exogenous(?) cooling cycle each pore gets ffillyed with "whatever it gets filled with" and then chemistry happens, and the reaction somehow becomes exothermic and so the fastest chemistry sprays back out into the medium, and so there is more of the faster/hotter chemistry for the next time "chemicals are being caught in pores like socks in a duvet cover"?
I am not any sort of chemist, let alone a biochemist, so this is all fanciful speculation.
I also haven't read the book, but I don't read the review as suggesting "creatures" at all. Just collections of compounds near membranes, in a positive feedback loop where the presence of those compounds makes it more likely that those compounds are produced from the other stuff in the environment.
In that sort of picture, it seems somewhat plausible that initially such feedback loops and diffusion could stand in for reproduction within the environment of a single smoker. Greater or lesser variations in the environment might provide selection effects toward robustness of feedback loops.
I can imagine some intermediate stage between free-floating chemicals near membranes and a fully formed cell: forming an extra membrane when conditions change in some direction, such as increased temperature. This could prevent diffusion and mixing of any more harmful environment with the mixture. When the conditions return to normal, the extra membrane dissolves and the chemicals start diffusing and catalysing their own production again.
Here's where the primordial soup theory becomes implausible. A primordial soup is at chemical equilibrium. A cell is at chemical disequilibrium. Could a primordial soup form a cell? Sure, in the same sense that all the atoms in my body could quantum teleport themselves to the Restaurant at the End of the Universe. "Things randomly happened to violate thermodynamics" is so statistically unlikely it should be the last resort we fall back upon after every conceivable hypotheses fails.
Not if we already believe the world is Big. (And indeed one conclusion we could draw from all this is that the world is probably Big.) In a big world -- say, one of infinite spatial or temporal extent -- everything that can happen does happen infinitely often, no matter how unlikely it is to happen in any particular place. Indeed, given that we don't see any aliens in the sky, there must be some sort of Great Filter, and so why don't we just conclude that this is probably it? Solve two mysteries at once! (why we don't see aliens & the Vital Question).
Relatedly, for a project I'm working on, it would be great to know precisely how unlikely it would be for a given puddle of primordial soup to spontaneously originate life in a given second. Are we talking 1/2^100, or 1/2^10000, or what?
One thing is: we're in a universe where there are in fact geochemical features that concentrate reactive precursors. For example, the white smokers (a.k.a. alkaline hydrothermal vents) discussed here are expected to exist on lots of planets (IIRC, at least according to Nick Lane—I think he said white smoker = water + olivine, and those are both super common on planets).
So, if geochemical features that concentrate reactive precursors do in fact exist all over the place, and if such features make it × more likely (or whatever) for life to arise, then the question of whether life could arise even in the absence of such features would appear to be an irrelevant question, right?
Anyway, if you want the emergence of life to be the great filter (as opposed to the emergence of eukaryotes, for example, which did in fact take longer on earth), you can still have that in an alkaline hydrothermal vent. For example, maybe it has to be just the right sub-sub-type of alkaline hydrothermal vent, a sub-sub-type which can only form on certain types of very young planets, or whatever. Or maybe you have to win the lottery with the reactive precursors coming together into just the right configuration (although I guess the speed that it happened on earth would vote against that one).
BTW the book has a good discussion of why the emergence of eukaryotes would be plausibly "hard"—like all the things that needed to go right in the first few generations after the merge. It seems to have happened only once in earth's history, despite there being a gazillion coexisting archaea and bacteria, all around the planet, all the time.
I'm confused by this - even if everything happens infinitely many times, there are still different sizes of infinity and we select the biggest as the most likely. How does a big world shift this perspective?
What do you mean by different sizes of infinity here? Your ultimate claim is that a hypothesis in which life appears on every 10^40th planet is more probable than a hypothesis in which life appears on every 10^400th planet, even if both hypotheses have infinitely many instances of life appearing. (Perhaps you are using SIA, instead of SSA? SIA rewards hypotheses that have larger population sizes.)
If this claim is true, wouldn't it also be true that a hypothesis in which life appears on every planet is more probable than a hypothesis in which life appears on every 10^40th planet? Shouldn't we be looking hard to find an explanation for why aliens might be hiding from us, since it's so improbable that aliens just don't exist?
I was speaking to the probability of life appearing in different parts of the (finite) ocean, so possibly I misunderstood what you were addressing. But since the reasoning should generalize:
I mean different sizes of infinity like these:
I'm not familiar enough with either SSA or SIA to apply them, and my grasp of anthropic reasoning is shaky in the extreme, but the idea of not rewarding population sizes baffles me. Do you have a preferred breakdown for this point I should check out, or will google serve me well enough?
If this claim is true, wouldn't it also be true that a hypothesis in which life appears on every planet is more probable than a hypothesis in which life appears on every 10^40th planet?
If planets and stars lasted infinitely long, and were sufficiently constrained in their composition, then I would say yes. But this chain of reasoning ignores the local information we have about the problem. Returning to the primordial soup quote you were responding to, the argument is that what we know of thermodynamics doesn't allow a causal mechanism to work. By contrast, the white-smoker vent hypothesis does allow a causal mechanism to work; therefore we should prefer it as the explanation for the origin of life (as we know it).
When I try running the intuition of your example of different frequencies of planetary genesis in reverse, and on primordial soup:
We should keep primordial soup on the table because a big world predicts this will still work an infinite number of times, then surely we must also keep a less-complex primordial soup (say, a primordial cocktail) on the table for the same reason; and then bare rock, with no soup at all; and then a complex cell springing into existence with no causal history; etc. This may be true, but doesn't really seem helpful in terms of what to expect.
An alternative framing: would it be fair to say that under a big world, every prediction happens an infinite number of times? If I accept that all infinities should be treated the same, that still leaves us with the ability to compare the number of infinities, which should lead us to favor the hypotheses according to how many predictions they allow us to make.
Regarding the appearance of aliens, have you checked out the grabby aliens post yet? I recommend it.
If density/population size is so important, you may not like the conclusions you end up with! In particular, you probably end up concluding that this whole world is a simulation run on a different substrate, and that different substrate is infinite in extent and also extremely densely packed with life. I think...
SSA vs. SIA: I don't have a favorite link, sorry. It's been a while since I thought about this stuff. I remember liking Katja Grace's posts on the subject, and Bostrom's Anthropic Bias. Plus googling and the philosophy literature should work pretty well. Also some Stuart Armstrong posts but that's more advanced.
Yeah, thinking about grabby aliens is the thing I need to know the probability number for. :)
Before seeing any evidence, we should indeed expect that life has high density in the universe. We just have enough data to rule that out. More generally I think UDASSA is probably the best framework for approaching problems like this, and it would hold that, in situations where our existence is contingent on an anthropically-selected unlikely event, we should still expect that this event is as likely as possible while being consistent with the evidence. So 10^-40 likelihood origination events more probably than 10^-400 likelihood events.
Also, primordial soups don't necessarily have to be in chemical equilibrium. If the soup is sitting in sunlight, then it could very easily be out of chemical equilibrium, and probably would be. (Soups could be out of equilibrium for a variety of other reasons too, like a consistent influx of chemicals coming out of the ground.) Sunlight comes from a surface with a temperature of 5700K. Compared to a puddle with a temperature of 300K sitting on the Earth's surface, those photons are absurdly energetic, and could easily bump up the prevalence of some molecules that would otherwise have too much free-energy to exist.
Once you have excess free energy, copying information becomes possible, and it becomes conceivable that you could have self-copying information, aka life.
There is a singularity at the beginning of biology. We know what happened before biology―that's just physics. We know what happened after the first cells: evolution. But we don't know how the first cells formed out of nonliving matter.
The most popular theory is called "primordial soup". The basic idea is that organic molecules (which just means chemical compounds with carbon-hydrogen bones) formed from natural physical processes and then some of those organic molecules randomly organized themselves into RNA and then that RNA built itself a cell. Bam.
The primordial soup theory is implausible from a physics perspective due to thermodynamics.
Living cells are at disequilibrium with their environment. They must harvest negentropy from an external source in order to stay alive. This isn't a property life happens to have. It's the essence of what life is. Life that isn't harvesting negentropy isn't alive.
For life to exist it needs a barrier between its ordered self and its chaotic environment. Life on Earth uses a cell wall composed of a lipid membrane. Since the inside of the cell is ordered and the outside is chaotic, there is an entropy gradient across the cell wall.
Here's where the primordial soup theory becomes implausible. A primordial soup is at chemical equilibrium. A cell is at chemical disequilibrium. Could a primordial soup form a cell? Sure, in the same sense that all the atoms in my body could quantum teleport themselves to the Restaurant at the End of the Universe. "Things randomly happened to violate thermodynamics" is so statistically unlikely it should be the last resort we fall back upon after every conceivable hypotheses fails.
The Vital Question by Nick Lane provide a theory of the origin of life that is physically plausible.
Proton Gradients
Cellular biology textbooks spend a lot of time on:
I can imagine life without DNA. I can imagine life without mitochondria and without chloroplasts. (Most organisms don't have them.) I cannot imagine life without metabolism. Sustained metabolism requires a cell to pump chemicals in and out of an organism.
The most important chemical reaction is the synthesis of ATP. It would not be an exaggeration to say ATP powers life. ATP is manufactured by harvesting energy from a proton gradient across a membrane.
Membrane Bioenergetics
There are three domains of life: bacteria, archaea and eukaryotes. The first eukaryote formed when an archaea formed a symbiotic relationship with a bacterium. If the primordial soup theory is correct then there is must be a single origin of life. Bacteria or archaea must have a common ancestor. We call it LUCA (Last Universal Common Ancestor).
But there are some mysteries. Membranes are made of lipids. Part of a lipid is made of glycerol. All archaea/bacteria use their respective lipids and their respective glycerol. Evolution is messy. It never replaces something that thoroughly. If one was replaced by the other then we should see some intermediate organisms. The fact that we don't is is evidence archaea/bacteria lipids and glycerol were never replaced. If the membranes they were never replaced then bacteria membranes arose independently from archaea membranes.
This chicken and egg problem is the core of Nick Lane's argument. Either proton gradients arose first or proton pumps arose first. Proton gradients can form by ordinary nonliving physical processes. Proton pumps cannot. Nick Lane concludes proton gradients must have come first.
Starting with proton gradients solves the chemical disequilibrium problem too.
Geothermal Vents
You know those black smokers with the anemones, tube worms and the giant clams? They rely on photosynthesis. Not directly, of course. No sunlight makes it down to the bottom of the ocean. Instead, they react hydrogen sulphide (H₂S) with oxygen (O₂). That oxygen comes from photosynthesis.
Nick Lane proposes instead that life originated a different kind of ocean vent called a white smoker.
So that's why my laundry accumulates in the duvet cover!
Lipids self-assemble into membranes. These membranes naturally form inside the tiny pores like soapy water on a bubble blower. One side of the membrane is exposed to water in contact with the alkali ovaline rock. The other side is exposed to the water. This creates a proton gradient behind which there are concentrated organic molecules.
There's no oxygen yet. Instead of reacting hydrogen sulfide with oxygen, proto-life could react hydrogen (H₂) with carbon dioxide (CO₂) to form methane (CH₄). At any given pH, it's impossible to for life reduce CO₂ with H₂. But a membrane doesn't have just a single pH. It has a different one on each side of the membrane.
Nick Lane proposes that LUCA was a membrane capped pore in an olavine vents. Nick Lane even makes specific predictions about which biological machinery LUCA did and din't have.
Early Evolution
Nick Lane practices epistemic rigor by not speculating beyond his area of expertise.
We now have LUCA established over a proton gradient. But we still have the chicken-and-egg problem of proton-impermeable membranes and hydrogen pumps. Which came first? Nick Lane guesses it was a Na⁺/H⁺ antiporter.
An antiporter is like a turnstile to a building which lets exactly one person into a building for every person which leaves except instead of people it operates on ions. A Na⁺/H⁺ antiporter lets in one sodium ion for every hydrogen ion it lets out and one hydrogen ion in for every sodium ion it lets in. If there is more hydrogen on one side of the membrane, a Na⁺/H⁺ antiporter acts as a hydrogen-powered sodium pump[1]. Sodium is much bigger than hydrogen. It's easy to evolve a sodium-impermeable membrane. The sodium gradient can drive metabolic reactions. Once you have all this machinery set up, a proton-impermeable membrane evolving naturally.
The Na⁺/H⁺ antiporter theory predicts that our oldest enzymes (those found in both bacteria and archaea which make DNA and RNA work) will be optimized to work at low sodium concentration even though the oceans have been high in sodium since life originated―and this is exactly how modern biochemistry works.
LUCA still replies on an inorganic proton gradient. It can't pop free of the rock until it has proton pumps. But we finally have proton-impermeable membranes! Proton-impermeable membranes let us evolve proton pumps. Proton pumps are an alternative to geologic proton gradients. With proton pumps and proton-impermeable membranes, a vesicle can separate from LUCA to form a proper cell.
Bacteria and Archaea
Methanogens are archaea. Acetogens are bacteria. They have different acetyl CoA pathways going all the way back to LUCA. Nick Lane proposes they evolved proton pumps independently. To cut to the chase, one of them pumps H⁺ into the cell for one biochemical reason; the other pumps H⁺ out of the cell for another biochemical reason[2]. This has various downstream effects―some random (like the handedness of glycerol), others logical.
We end up with two kinds of cells descended from LUCA: bacteria and archaea. They both rely on DNA but their membranes, cell walls and DNA replication are very different.
Eukaryotes
The book goes downhill[3] from there. The rest of the book isn't bad. In fact, it's a pretty great introduction to cellular biochemistry approachable to laypeople even though it's full of of technical detail. This post-eukaryote evolutionary theory is neither new nor controversial so I'll just give a quick summary.
Eukaryotes formed when an archaeon consumed a bacterium, forming a symbiotic relationship. The symbiotic organism is called an eukaryote and the bacteria inside it are is called mitochondria.
Mitochondria provide a eukaryote's ATP. Why shouldn't a eukaryote create its own ATP? Nick Lane thinks the reason is because genes have to be close to a cell membrane in order to make ATP. The square-cube law means that a cell's membrane grows slower than its volume. A big archaeon is limited in how much ATP it can create which limits how many genes it can have which limits its complexity.
Among the three domains of life, eukaryotes are the most complex. By offloading ATP production to mitochondria, they get around the square-cube law limiting ATP production. They can have more genes which enables more complexity.
Once you have eukaryotes, sex becomes very important. Nick Lane has a chapter on sex too. It's interesting and if it was written in any other book I'd say "this is top-tier science writing" but Nick Lane sets the bar of so impossibly high by proposing a fascinating new theory to one of the most important questions in all science that merely "top-tier science writing" is a step down. He gives a testable hypothesis about "what causes ageing" backed up by diagrams and evolutionary theory. For most science writers, that would be the entire book. To Nick Lane, it's ancillary.
The coolest thing about The Vital Question is it makes precise predictions about what life on other planets might look like. We may not ever get to test this theory ourselves but―perhaps―eons from now, some descent of humankind will stumble across infant alien life, load this book from its Most Ancient Archives and find out whether Nick Lane was right.
Credits
This post was funded by Less Wrong. Thank you!
Photos
A Na⁺/H⁺ antiporter is mechanically possible because H⁺ rarely exists in isolation. It binds to Hydrogen forming H₃O⁺ which has a radius similar to Na⁺. ↩︎
Nick Lane does explain the reasons. I'm skipping them . ↩︎
This is a physics joke. Entropy and energy always flow downhill. Life is the harnessing of energy flowing downhill. Life without energy flowing downhill is not alive. In the context of biophysics, saying Nick Lane's argument "goes downhill" means the argument flows logically from one idea to the next. ↩︎