The Burgess Shale and “Wonderful Life”

507 – 480 million years ago

The Burgess Shale (about 510 Mya) is not the oldest Cambrian deposit known. There are deposits from China (Chengjiang) closer to the beginning of the era. But it is particularly rich and well studied. It also featured in debates about some Big Questions: How important are evolutionary laws versus historical accidents? Has the living world become more or less diverse over time?

Stephen Jay Gould (1941-2002) was one of the most widely known evolutionary biologists of his time. In 1989 he wrote a book about the Burgess Shale, called “Wonderful Life.” The title alluded to Frank Capra’s movie, “It’s a Wonderful Life,”* starring Jimmy Stewart. In the movie, the Stewart character, who thinks he’s wasted his life stuck in his home town, has a chance to see how things would have turned out if he had died young in an accident. He learns that his existence made a huge difference to his town.

Similarly, Gould argued that accidents of which Cambrian species survived and which went extinct made a huge difference to the later evolution of life. To make his case, he developed a subsidiary argument: that the Cambrian fauna displays a radical diversity of body plans in comparison with later eras. Gould made an analogy with the early development of automobiles, which featured diesel engines, steam engines, and electrical engines, before settling down on mostly just gasoline engines. (He was writing before Priuses and Teslas, of course.)

This subsidiary argument has not fared well. Most of the supposedly radically different forms from the Burgess Shale turn out to be not that radically different from one another, or from modern forms. Most notorious was the case of a specimen called Hallucigenia. Paleontologists thought that this creature was like nothing that ever lived before. But later more complete finds of related forms made it clear they were looking at it upside down; its “legs” are actually defensive spikes, the “tentacles” on top are actually legs. It’s probably related to the ancestor of velvet worms, a group related to vertebrates and still living in Australia. A different perspective comes from Simon Conway Morris one of the experts on the Burgess Shale, in in his book “Crucible of Creation.”

And here, from a nice review of more recent work on contingency and determinism in evolution:

Gould would be pleased that his thought experiment of replaying life’s tape has been transformed into an empirical research program that explores the roles of historical contingency and natural selection at multiple levels. However, his view of historical influences as the central feature of evolution remains debatable. Laboratory replay experiments show that repeatable outcomes are common, at least when defined broadly (e.g., at the level of genes, not mutations). Moreover, convergence in nature is more common than many biologists would have wagered not long ago. On the other hand, as evolving lineages accumulate more differences, both experimental and comparative approaches suggest that the power of selection to drive convergence is reduced, and the contingent effects of history are amplified.

*Not to be confused with Jerome Bixby’s short story “It’s a Good Life,” about a very different small town.

On the Origin of Seafood by Means of Natural Selection,

Or the Preparation of Favorite Dishes in the Struggle for Dinner.

567 – 537 million years ago

The Cambrian explosion — shells and skeletons, and all the major animal phyla of today — is one of the major events in the history of life. It’s hard to miss – Darwin was well aware of it – because for the first time you have abundant well-preserved fossils of animals with hard parts. From now on, if I miss a tweet one day or another, it’s because I didn’t get to it, not because the evidence isn’t there.

Genetic evidence seemingly clashes with the fossil evidence. A “molecular clock” based on rates of gene divergence suggests that major animal phyla had begun diverging from one another long before the Cambrian explosion. But maybe the genetic evidence is wrong, and the “molecular clock” was running faster in the past than more recently. Or maybe complex organisms evolved long before the Cambrian, but left little or no fossil evidence. Or maybe the ancestors of today’s animals really did diverge early, but didn’t get complex until the Cambrian.

And why the explosion happened when it did is unresolved. Here’s a recent review. The early Snowball Earth episodes probably contributed in some way, and the rise of oxygen in the atmosphere must have been important. Or maybe there was some dramatic biological event that triggered the explosion: the origin of eyes, and/or the beginning of predation, setting off an arms race between predators and prey that’s been going on ever since.

If this last possibility is correct, then the transition from an Edenic, predator-free Ediacaran world to the Cambrian is a form of “symmetry breaking.” There is maybe an analogy here in human social evolution to the transition from an egalitarian society to a world of inequality, of rulers and ruled  (which also followed a – much milder – glacial episode).

Speaking of predation: the Logarithmic History blog is partly about commemorating great events in the past. It seems fitting to celebrate the Cambrian as the origin of seafood. If you take your time machine back to any time before the Cambrian, pickings will be slim – algae mostly, although we don’t really know what the Ediacara would have tasted like. The time-traveller’s menu gets a lot better with the Cambrian (although wood for a fire is still a problem). Nowadays you can’t hope to dine on trilobite, alas. (Check out March 13 last year for more of this sad story.) But sometime in the next few days why not have some mussels for dinner? (The recipe below has some non-Cambrian ingredients. It will be a few more days, incidentally, before the evolution of anything kosher.)

Steamed mussels, 4 servings

Wash and debeard:
4 to 6 pounds mussels
Place them in a large pot and add:
½ cup dry white wine
½ cup minced fresh parsley or other herbs
2 tablespoons chopped garlic
Cover the pot, place it over high heat, and cook, shaking the pot occasionally, until most of the mussels are opened, about 10 minutes. Use a slotted spoon to remove mussels to a serving bowl, then strain the cooking liquid over them. Drizzle over the mussels:
1 tablespoon extra-virgin olive oil
Juice of 1 lemon
Serve with:
Plenty of crusty bread (invented< 15,000 years ago, but who’s counting?)

Snow time

Before 720 million years ago, we find thick limestone deposits left by decaying algae. These were sequestering carbon, taking carbon dioxide out of the atmosphere, and cooling the Earth. At some point a positive feedback cycle kicked in, as polar seas froze and reflected more sunlight, cooling the planet further. The result was a succession of extreme Ice Ages. The Ice Age of the last two million years, which merely covered high latitudes with glaciers, off and on, were nothing compared to the Snowball Earth of the Cryogenian: at a minimum, polar seas were frozen, and tropical seas were slushy with icebergs. It’s possible that things were even more extreme: the entire sea may have been covered by a thick layer of ice, with a few photosynthetic algae surviving in the ice, and other organisms hanging on around deep sea hot water vents. A limited amount of oxygenated meltwater seeping into the ocean from under the glaciers may have kept early oxygen-breathers alive. For a hundred million years, climate oscillated abruptly between two steady states, frozen and warm.

It’s only in the last two decades we’ve begun to figure out this amazing story. If there’s a lesson here, it’s that Earth over the long run is far from a stable system. We will see again and again that the history of life, like human history, has been punctuated by catastrophes.

Above, a rock dropped from an iceberg or glacier into the middle of a tropical ocean

Duxelles

787 – 745 million years ago

Recently researchers announced the discovery of fossil mushrooms (or mushroom ancestors) dating back between 810 and 715 million years ago, 300 million years earlier than previously thought. The fossils came from what is now the Democratic Republic of the Congo. (And here is a report of even earlier fungi from Canada, going back 1 billion years.)

And below is a recipe to celebrate the occasion. Duxelles would go well with bread and wine, two more foods for which we can thank fungi.

Duxelles

Chop very fine or pulse in a food processor until they resemble coarse sand:

8 ounces of mushrooms, wiped clean

Squeeze about ¼ cup of mushrooms at a time in dampened cheesecloth or thin cotton or paper towel and wring them hard to extract their bitter juices. Heat in a medium skillet until the foam subsides:

1 ½ tablespoons butter

1 teaspoon vegetable oil

Add and cook briefly over medium heat:

2 tablespoons very finely minced shallots or scallions (white part only)

Add the mushrooms and cook, stirring often, over medium-high heat until they have begun to brown and there is little liquid, 5 to 6 minutes. Stir in:

1 tablespoon dry sherry of Madeira

Cook until completely evaporated. Add:

¼ cup heavy cream (optional)

Salt and ground black pepper to taste

Pinch of dried thyme or grated nutmeg

Let cool.

Thanks to The Joy of Cooking (1997)

Reuben, Reuben

Reuben, Reuben, I’ve been thinking
What a fine world this would be
If the men were all transported
Far beyond the northern sea.

Oh, my goodness, gracious, Rachel,
What a strange world this would be
If the men were all transported
Far beyond the northern sea.

Reuben, Reuben, I’ve been thinking
What a great life girls would lead
If they had no men about them
None to tease them, none to heed.

Rachel, Rachel, I’ve been thinking
Life would be so easy then
What a lovely world this would be
If you’d leave it to the men.

Reuben, Reuben, stop your teasing
If you’ve any love for me
I was only just a-fooling
As I thought, of course, you’d see.

Rachel, if you’ll not transport us
I will take you for my wife
And I’ll split with you my money
Every pay day of my life!

(Traditional song, many versions)

I once heard William Hamilton, the evolutionary theorist, give a talk where he suggested, among other things, that human beings might give up sexual reproduction in the future. He was building on a familiar evolutionary puzzle: sex, or more exactly the division between female (large, expensive gametes) and male (small, cheap, mobile/motile gametes) is a very expensive proposition. Imagine a sexually reproducing population in which females average two offspring each, one female, one male. On average, each female is replacing herself in the next generation with one daughter. Now imagine a mutant asexual female. She also gives birth to two offspring, but the offspring are both daughters, clones of their mother. The numbers of this mutant lineage will initially double in every generation, and they will eventually replace the sexually reproducing type entirely. 

Unless … There must be some advantage to sexual reproduction to make up for this huge disadvantage. One possibility is that sex somehow makes it easier to avoid the accumulation of deleterious mutations. Another possibility, which Hamilton was pushing, is that sex gives an advantage to host species in their evolutionary arms races with parasites. A species consisting of lots of genetically uniform clones is more vulnerable to pathogens. 

However … (and this is where Hamilton was going) a species that has developed medical technology to the point that it no longer has to worry so much about infectious disease might be able to dispense with sexual reproduction. So Hamilton pondered a germ-free hygienic future in which the clonal offspring of one or more fit, fecund, philoprogenitive females replaced the rest of the human race.

And … minus the evolutionary theorizing, some science fiction writers have imagined futures without a division between men and women. John Wyndham, writing back in the 1950s imagined a world in which a plague had wiped out men, and the surviving women carried on high tech reproduction without men in an ant-like caste society. Wyndham thought this was a bad thing, but for some feminist science fiction writers in the 1970s – Joanna Russ, James Tiptree, Jr. – a world without men was imagined as a utopia.

The most sustained piece of world-building of a society without “men” and “women” comes from Ursula Le Guin. The aliens on the planet Winter are much like humans on Earth, but they are hermaphrodites. Most of the time they are neuter, with underdeveloped male and female organs. But during kemmer (estrus, the breeding season), two individuals will pair off and develop complementary sexual characters and sexual desires. One member of the pair will temporarily develop as a female, the other as a male. They will copulate much as we do, and the female member of the pair will conceive and eventually give birth. The next time kemmer rolls around, the former female may develop as a male, and her former (and formerly male) partner (or another) may develop as a female.

Le Guin, the daughter of two anthropologists, gives us a richly developed thick description of the culture of Winter, not a utopia, but just another world, different from our own in some ways, similar in others. But what’s missing from her story is much consideration of the evolutionary dynamics underlying the exotic (to us) reproductive biology. In Le Guin’s scheme, the female member of a pair is still putting more energy than the male into producing a child. We have to expect that there will be considerable jockeying, both physiological and psychological, over who, in each reproductive episode, takes on the more burdensome role. On Winter, the Battle of the Sexes will still be fought, albeit on different terrain.

Meanwhile, back on Earth, Reuben, Reuben is a comic novel about the Battle of the Sexes in 1950s suburban Connecticut. The novel centers on the priapic poet Gowan MacGland, (obviously modeled on Dylan Thomas), who takes advantage of an American tour to enjoy the opportunities available to an alpha small-gamete-producer. Up to a point, anyway: the terrible lesson he learns is never, ever, ever cuckold your dentist.

When you were a tadpole and I was a fish

1.17 – 1.12 billion years ago

And here’s a post (with a poem) that works for Valentine’s Day.

The Boring Billion rolls on. The atmosphere is one percent oxygen or so thanks to photosynthetic bacteria and algae. The ocean still largely anoxic and thick with sulfates and sulfate-eating bacteria. Eukaryotes have been around for a while, and are diversified, although still all single celled (as far as we know).

Sexual reproduction begins with eukaryotes, and by now some groups are presumably differentiated into male and female. And so here’s a poem for Valentine’s Day, by the biologist Langdon Smith. Martin Gardner has a nice account of the poem, in his book “When you were a tadpole and I was a fish,” and here’s a great video of the poem spoken by Jean Shepherd.

Evolution
By Langdon Smith (1858-1908)

When you were a tadpole and I was a fish
In the Paleozoic time,
And side by side on the ebbing tide
We sprawled through the ooze and slime,
Or skittered with many a caudal flip
Through the depths of the Cambrian fen,
My heart was rife with the joy of life,
For I loved you even then.

Mindless we lived and mindless we loved
And mindless at last we died;
And deep in the rift of the Caradoc drift
We slumbered side by side.
The world turned on in the lathe of time,
The hot lands heaved amain,
Till we caught our breath from the womb of death
And crept into life again.

We were amphibians, scaled and tailed,
And drab as a dead man’s hand;
We coiled at ease ‘neath the dripping trees
Or trailed through the mud and sand.
Croaking and blind, with our three-clawed feet
Writing a language dumb,
With never a spark in the empty dark
To hint at a life to come.

Yet happy we lived and happy we loved,
And happy we died once more;
Our forms were rolled in the clinging mold
Of a Neocomian shore.
The eons came and the eons fled
And the sleep that wrapped us fast
Was riven away in a newer day
And the night of death was passed.

Then light and swift through the jungle trees
We swung in our airy flights,
Or breathed in the balms of the fronded palms
In the hush of the moonless nights;
And oh! what beautiful years were there
When our hearts clung each to each;
When life was filled and our senses thrilled
In the first faint dawn of speech.

Thus life by life and love by love
We passed through the cycles strange,
And breath by breath and death by death
We followed the chain of change.
Till there came a time in the law of life
When over the nursing sod
The shadows broke and the soul awoke
In a strange, dim dream of God.

I was thewed like an Auroch bull
And tusked like the great cave bear;
And you, my sweet, from head to feet
Were gowned in your glorious hair.
Deep in the gloom of a fireless cave,
When the night fell o’er the plain
And the moon hung red o’er the river bed
We mumbled the bones of the slain.

I flaked a flint to a cutting edge
And shaped it with brutish craft;
I broke a shank from the woodland lank
And fitted it, head and haft;
Then I hid me close to the reedy tarn,
Where the mammoth came to drink;
Through the brawn and bone I drove the stone
And slew him upon the brink.

Loud I howled through the moonlit wastes,
Loud answered our kith and kin;
From west to east to the crimson feast
The clan came tramping in.
O’er joint and gristle and padded hoof
We fought and clawed and tore,
And cheek by jowl with many a growl
We talked the marvel o’er.

I carved that fight on a reindeer bone
With rude and hairy hand;
I pictured his fall on the cavern wall
That men might understand.
For we lived by blood and the right of might
Ere human laws were drawn,
And the age of sin did not begin
Til our brutal tush was gone.

And that was a million years ago
In a time that no man knows;
Yet here tonight in the mellow light
We sit at Delmonico’s.
Your eyes are deep as the Devon springs,
Your hair is dark as jet,
Your years are few, your life is new,
Your soul untried, and yet –

Our trail is on the Kimmeridge clay
And the scarp of the Purbeck flags;
We have left our bones in the Bagshot stones
And deep in the Coralline crags;
Our love is old, our lives are old,
And death shall come amain;
Should it come today, what man may say
We shall not live again?

God wrought our souls from the Tremadoc beds
And furnish’d them wings to fly;
He sowed our spawn in the world’s dim dawn,
And I know that it shall not die,
Though cities have sprung above the graves
Where the crook-bone men made war
And the ox-wain creaks o’er the buried caves
Where the mummied mammoths are.

Then as we linger at luncheon here
O’er many a dainty dish,
Let us drink anew to the time when you
Were a tadpole and I was a fish.

Between Darwin’s and Saint Valentine’s Day

1.23 – 1.18 billion years ago

Yesterday was Darwin’s birthday (and Lincoln’s). Tomorrow is Valentine’s Day. Here’s a post appropriate for either day.

Imagine sex worked like this:

You’ve been feeling bad lately, getting sick a lot. You’re not at your best. You find someone who seems to be in better shape. One thing leads to another and you wind up acquiring body fluids from the other party – and picking up some new genes from them. The new genes help a lot in fighting off infection. You’re feeling better now.

Reproduction? That’s another matter, nothing directly to do with sex. When you reproduce, your offspring will carry all the genes you happen to have at the moment. (Here’s one drawback, according to The Onion.)

Also, I forgot to mention that you’re neither male or female – the gene exchange could have gone in the other direction if you’d both been in the mood. And your partner in the adventure above might not even have been the same species as you. (Just what counts as a species here isn’t well-defined.)

This is more or less how bacteria work out sex. (Joshua Lederberg got the Nobel Prize for figuring this out.) Eukaryotes (you’re one of them) mostly do it differently, combining sex and reproduction. It’s the story you learned in high school about passing on half your genes to a gamete (sex cell), which joins with another gamete to make a new organism.

Most eukaryotes also have two sexes. One theory we have about why that got started goes like this: Most of the DNA in a eukaryote cell is in the nucleus. But a small fraction is in the mitochondria, little powerhouses outside the nucleus that started out as bacteria, and got domesticated. Imagine that two gametes join together, and combine two sets of mitochondria. There’s a potential conflict here. Suppose your mitochondria have a mutation that lets them clobber your partner’s mitochondria. This is good (evolutionarily speaking) for the winning mitochondria, but very likely to be bad for the cell as a whole. Better for the cell as a whole is if one gamete, acting on instructions from the nucleus, preemptively clobbers all their own mitochondria, so that all the mitochondria come from just the other gamete. This is the beginning of what will eventually lead to a distinction between sperm and eggs, pollen and ovules, male and female. Which means you got all your mitochondrial DNA from your mom, something that will turn out to be important when we look later in the year at geneticists unraveling human prehistory. This is also an example of how selection at one level (within cells) can conflict with selection at another level (between cells). We’ll see such multilevel selection again and again, for example in the evolution of complex human societies.

Sex has to be highly advantageous, although we’re not sure exactly what the advantage is. The general answer is probably that an asexually reproducing organism almost never produces any offspring who have fewer harmful mutations than she has. But a sexually reproducing organism, passing on a random half of her genes to each of her offspring, can have some offspring with fewer harmful mutations, at the cost of having other offspring with more. There are various reasons (Muller’s ratchet, Kondrashov’s hatchet) why this could be evolutionarily advantageous.

In other words, with sexually reproduction, at least some of mum and dad’s kids can be less messed up than their parents; it’s asexually reproducing organisms that really embody Larkin’s dour verse … 

Man hands on misery to man,

It deepens like a coastal shelf

Get out as early as you can,

And don’t have any kids yourself.

Philip Larkin. This Be the Verse

… insofar as, when eukaryote species give up sex, they don’t seem to last long. Dandelions reproduce asexually: based on what we see in other organisms, they probably won’t be around for long, evolutionarily speaking. There’s one mysterious exception, tiny animals called bdelloid rotifers which have been reproducing asexually for tens of millions of years . For readers who are not bdelloid rotifers: Happy Valentine’s Day tomorrow! We’ll have an appropriate evolutionary post up tomorrow

The boring billion

1.83-1.74 billion years ago

We’re now doing the history of the universe at the rate of 100 million years per day.

The Boring Billion* is a billion or so years, from maybe 1.85 to .85 billion years ago, in which Earth’s climate, ecology, and geography were relatively stable. For most of this period, life and Earth seem to have been locked into a very different set of chemical cycles than what we’re used to today.

Today we have a planetary fuel cell that keeps electron-hungry oxygen in the atmosphere and ocean separate from reduced carbon in minerals deep underground. Animals exploit this fuel cell by consuming and oxidizing organic matter. And a few centuries ago, human beings found another way of tapping Earth’s fuel cell: unearthing underground carbon and hydrocarbon stores, and oxidizing (burning) them to fuel the Industrial Revolution.

During the Boring Billion, however, a different, lower-energy, planetary fuel cell operated. There was some oxygen in the atmosphere – a few percent versus 21 percent today, not enough to make it breathable to us. But it’s likely that much of the ocean below a thin surface layer was anoxic, without free oxygen. Atmospheric oxygen still reached the ocean, but indirectly. On land, oxygen combined with sulfur compounds to yield sulfates. When these washed into the ocean, bacteria in the anoxic zone used them to produce hydrogen sulfide, the chemical that gives rotten eggs their bad smell.

The term of art for this combination of no oxygen and lots of sulfides is euxinia, named after the Euxine, or Black Sea. When the Black Sea flooded 7500 years ago, the decay of organic matter used up all the oxygen below the top 150 meters or so, creating the world’s largest marine Dead Zone. But during the Boring Billion, it looks as if the whole ocean was largely euxinic, a Canfield Ocean.

And it may be that the dominant mode of photosynthesis was different back then too, with purple and green sulfur bacteria exploiting hydrogen sulfide and releasing relatively little oxygen in the process. There are bacteria today that can switch between aerobic and anaerobic photosynthesis depending on the supply of hydrogen sulfide. These photosynthetic bacteria are distinct from true algae, which are not bacteria but eukaryotes with chloroplasts. On the latest evidence true algae evolved at least 1.2 billion years ago, maybe 1.6 billion. They and their multi-cellular descendants – green plants – would eventually (after some extreme Ice Ages) make Earth a very different place.

* The correct, boring name for the Boring Billion is the Middle Proterozoic.

Life goes nuclear

2.06 – 1.95 Gya

Eukaryotic cells (Domain Eucaryota, which includes multicellular life, like plants, animals, and fungi) are, on average, much larger and more complex than the earlier evolved prokaryote cells (Domains Bacteria and Archaea*). They have organelles, including mitochondria that power them and chloroplasts (at least among plants) that carry out photosynthesis. Their DNA is stored in a nucleus, and consists not just of genes (as in prokaryotes), but of large stretches of non-coding DNA (most of their genome), separating pieces of genes. The ancestor of present-day eukaryotes reproduced sexually, although some eukaryotes have since given up sex.

There are different ways that life increases in complexity. The origin of the Eucarya has something in common with a much later event, the origin of agriculture (check out September 11 on logarithmichistory). Starting 10,000 years ago, we Homo sapiens brought other animals and plants under our control, managing their reproduction, and selecting them (first unintentionally, then intentionally) to suit our purposes, until now most domesticated creatures couldn’t survive in the wild. Our own numbers and social scale increased enormously with the rise of agriculture.

At least 2 billion years ago, an archaeon cell gobbled up one or more bacterial cells (or was parasitized by them). The bacteria ended up surviving inside it, and after many generations became a kind of domesticate inside their host. Eukaryotes do domestication one better than humans: they carry their livestock inside their bodies. Eventually this domesticate evolved into mitochondria, the little power packs that pump out ATP for the rest of the cell to use as as an energy source. Over the course of time all but a small fraction of the original bacterial genome was moved into the nucleus.

In the last few years we have come closer to understanding how this momentous step occurred: we have discovered a new branch of the Archaean tree, the Asgard archaea. The Asgard archaeans carry some genes otherwise found only in eukaryotes, and it looks likely the first eukaryote to start hosting the bacterial ancestors of mitochondria was either an Asgarder, or close branch. Just recently we finally succeeded in cultivating these creatures in the lab. (It was hard to do.) They are tiny little spheres with long filaments protruding from them. The partnership between Archaeans and bacteria may have begun with bacteria nestling in these protrusions.

Humans developed agriculture multiple times independently around the world. As far as we know, eukaryotes evolved only once, long after the origin of simpler forms. The evolution of eukaryotes might be very unlikely to occur during the habitable lifespan of a planet. The observable universe may be full of bacteria, but harbor more complex cells only sparsely.

* Domain Archaea, a billions-of-years-old group of single-celled organisms looking like bacteria but biochemically different, should not be confused with the Archaean Eon, a billions-of-years-long stretch of Earth history.

Compost

2.17 – 2.07 billion years ago

I’ve got a pre-capitalist exchange relationship with the cafe down the street – they give me coffee grounds for my compost pile, I bring them flowers later. 

For a long time, before I set things up with the coffee shop, I just let chopped up leaves sit in a pile in a corner of the yard. Nothing much happened. Eventually I wised up and did some research: proper composting requires the right balance of carbon and nitrogen. The nitrogen and some of the carbon go into building organic molecules. Most of the carbon, though, combines with oxygen to make carbon dioxide, releasing energy in the process for the bacteria that make the compost. The amount of energy is appreciable: on a mild winter day, with air temperature just above freezing, the inside of my compost heap was the temperature of a lukewarm bath.

But composting goes back way longer than my compost heap, and way longer than Homo sapiens has been around. In fact, composting may have gotten its start more than 2 billion years ago. Here’s the story:

Back when I was in college, a guy I knew was taking a course in organic chemistry. He despaired of mastering the material in time for the next exam. Another student who’d already taken the course advised him, “Just remember, electrons go where they ain’t. Everything else is details” This is a pretty good starting point for thinking about how life and Earth have coevolved. Carbon, sitting in the middle of the periodic table, with four electrons to share, and four empty slots available for other atoms’ electrons, is exceptionally versatile. It can form super-oxidized carbon dioxide, CO₂, where carbon’s extra electrons fill in oxygen’s empty slots. Or it can form super-reduced methane, CH₄, where carbon’s empty slots are filled by hydrogen’s extra electrons. Or it can form carbohydrates, basic formula CH₂O, neither super-oxidized nor super-reduced, where the oxygen atom gives the carbon atom some extra empty slots and the hydrogen atoms give the carbon atom some extra electrons.

Chemically, life is pretty much about reducing and oxidizing carbon compounds. Nowadays, this means, on the one hand, photosynthesis – using the sun’s energy to go from carbon dioxide to carbohydrates – and, on the other hand, aerobic respiration – running the same reaction in reverse to release energy. But this modern arrangement took eons to develop. It depended on geochemistry, specifically on the evolution of what has been called a planetary fuel cell, with an oxidizing atmosphere and ocean physically separated from buried reduced minerals. Given a breach in this separation, the oxygen and the reduced minerals will react and release energy. In future posts we’ll see how such short-circuits in the planetary fuel cell, a result of major lava flows, are probably the cause of most major mass extinctions.

The development of the planetary fuel cell was erratic. One puzzling episode is the Lomagundi Event, where oxygen levels went up 2.2 billion years ago and then back down 2.08 billion years ago. One theory is that the Event began with photosynthetic cyanobacteria pumping oxygen into the atmosphere. When these bacteria died, they just piled up, leaving a growing accumulation of organic matter in the world’s oceans. According to this theory, the end of the Event happened when other bacteria discovered they could oxidize the accumulated organic matter, and used up a good part of the atmosphere’s oxygen. In other words, the end of the Lomagundi Event marks the invention of composting.

The return to higher oxygen levels in the atmosphere would take some time, and deposition of reduced sediments.