The worst of times

260 million years ago: the Capitanian mass extinction

A capsule summary of the evolution of life on Earth goes like this: There is steady progress in adaptation, driven especially by arms races, sometimes involving competitors, sometimes predators and prey. But this progress is interrupted from time to time by catastrophes – mass extinctions resulting from extrinsic causes, sometimes astronomical, but more often geological. (We’ll see much later in the year that a similar summary of human history goes like this: steady progress in the scale of cooperation driven by arms races, with occasional catastrophic interruptions, often associated with the spread of epidemic diseases.)

The geological causes of mass extinctions have been coming into focus lately. Many mass extinctions co-occur with the formation of Large Igneous Provinces (LIPS), regions where vast amounts of lava have flowed out of the earth, triggering a whole cascade of changes: the destruction of the ozone layer by halogen gases, global warming induced by CO2 and methane, and anoxic seas.

Large Igneous Provinces aren’t always associated with mass extinctions. What makes some episodes of massive lava flow particularly destructive is that they produce short circuits in the “planetary fuel cell.” The development of complex life has depended on the separation between an oxygen-rich, electron-hungry atmosphere and a reducing, electron-stuffed planetary interior. Some of the biggest setbacks to complex life have happened when  lava flows from deep in the Earth’s interior punch through carbon deposits on their way up, and bridge this chemical gap between surface and interior.

The mass extinction 260 million years ago, the Capitanian, is not one of the classic five greatest mass extinctions, and has been overshadowed by the mother of all mass extinctions, the end-Permian, which happened just 8 million years later. But it took a major toll on living things, from marine organisms to dinocephalians. (The dinocephalians – more closely related to mammals than to dinosaurs, ranging up to hippo sized, and including both herbivores and carnivores – went entirely extinct with the Capitanian. See picture.) The Capitanian extinctions coincide with, and were probably caused by, the formation of the Emeishan LIP, now in southwest China.

A book published recently, The Worst of Times, pulls together the latest evidence that the Capitanian was the beginning of an 80 million year period in which mass extinctions were exceptionally common. Apparently the formation of the supercontinent of Pangaea and the Panthalassic superocean made living things particularly vulnerable to volcanically induced extinctions. Once Pangaea breaks up, mass extinctions are less frequent, and generally have different causes.  The death of the dinosaurs had an extra-terrestrial cause, and the mass extinction we’re in the middle of results from the activities of one very unusual species.

Devonian days

405 – 384 million years ago

(posted a day late)

Lots going on in the Devonian.

Forests are spreading. These early trees, genus Wattieza, are kin to ferns and horsetails. They stand 10 meters tall. No leaves yet, just fronds. The first forests will absorb carbon dioxide, and cool the planet.

Life is moving onto land. Tiktaaklik roseae, the “fishapod” discovered in 2006, is as nice a link between fish and amphibians as one could hope for, with both lungs and gills. Here’s a book by Neil Shubin, the co-discoverer.

You can see a lot further in the air than underwater. This may be one of the early selective pressures for evolving a proper neck and sticking one’s head out of water. Eventually, for some, the rest of the body would follow.

And evolution seems to be generally speeded up on land. It’s not just that animals and plants develop adaptations for life on land (obviously). But there is also a more general acceleration in the pace of evolution. Major innovations come at a faster pace among terrestrial organisms.

We are upside-down bugs

480 – 454 million years ago

“We are upside-down bugs” is not as catchy a song lyric as “We are stardust.” But the story may be just as interesting.

The proto-evolutionist anatomist Etienne Geoffroy Saint-Hilaire (1772-1844) proposed long ago that all animals – insects to vertebrates — share a “unity of composition.” He was opposed by his sometime friend and sometime rival, anti-evolutionist anatomist Georges Cuvier (1769-1832), who argued that the animal world is organized in four great “embranchements,” with nothing in common in their body plans. Geoffroy Saint-Hilaire noted that insects have their nervous systems running ventrally (through their bellies) and their digestive systems dorsally (through their backs), the opposite of vertebrates. So he proposed the daring hypothesis that, from head to tail, vertebrates and insects have the same body plan, but belly to back they are flipped around.

Remarkably enough, a modernized version of this hypothesis has been vindicated by developmental genetics. Vertebrates have a series of genes, the Hox genes, that control development. They are laid out in order, with the genes switching on the development of the head followed by genes for the upper body, etc. It turns out that much the same genes in the same order control development in insects (not exactly the same, but clearly related), even though the actual structure of insect bodies is very different. On the other hand, the gene that turns on ventral development in the fruit fly Drosophila is related to the gene that turns on dorsal development in the toad Xenopus, while the gene that turns on dorsal development in Drosophila is related to the gene that controls ventral development in Xenopus.

The hypothesis that seems to account for this is that back in the day –- before the Cambrian explosion – there was a small wormy bilaterally symmetrical organism, ancestor to almost all animals (except sponges and jellyfish and the Ediacaran Petalonamae). Some of the descendants of that primordial animal gave rise to protostomes (where the first opening in the embryo becomes a mouth) including arthropods (spiders, insects, etc.), molluscs (including clams, crustaceans, octopuses), and annelids (earthworms).

But somewhere along the pathway leading to the deuterostomes (where the first opening in the embryo becomes the anus, the second becomes the mouth), including the chordates, the vertebrates, and us, another set of descendants started swimming upside down. And the rest is (pre)history: this initial minor quirk of evolutionary history was well-entrenched by today’s date.

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?)

In memoriam, Paleozoic

256 – 243 million years ago

Alfred, Lord Tennyson, wrote his poem “In Memoriam AHH,” in response to the death of his friend Arthur Henry Hallam. Several cantos consider the bleak lessons of paleontology – not just the myriads of deaths, but the specter of species extinction. Tennyson finished the poem in 1849, a decade before “The Origin of Species,” when the possibility of non-divinely-directed evolution and the reality of mass extinctions like the end-Permian were becoming part of general awareness.

LV

Are God and Nature then at strife,
That Nature lends such evil dreams?
So careful of the type she seems,
So careless of the single life;

That I, considering everywhere
Her secret meaning in her deeds,
And finding that of fifty seeds
She often brings but one to bear,

I falter where I firmly trod,
And falling with my weight of cares
Upon the great world’s altar-stairs
That slope thro’ darkness up to God,

I stretch lame hands of faith, and grope,
And gather dust and chaff, and call
To what I feel is Lord of all,
And faintly trust the larger hope.

LVI

‘So careful of the type?’ but no.
From scarped cliff and quarried stone
She cries, ‘A thousand types are gone:
I care for nothing, all shall go.

‘Thou makest thine appeal to me:
I bring to life, I bring to death:
The spirit does but mean the breath:
I know no more.’ And he, shall he,

Man, her last work, who seem’d so fair,
Such splendid purpose in his eyes,
Who roll’d the psalm to wintry skies,
Who built him fanes of fruitless prayer,

Who trusted God was love indeed
And love Creation’s final law—
Tho’ Nature, red in tooth and claw
With ravine, shriek’d against his creed—

Who loved, who suffer’d countless ills,
Who battled for the True, the Just,
Be blown about the desert dust,
Or seal’d within the iron hills?

For one answer to Tennyson’s anguished question about human extinction, there’s an argument that says we can estimate how much longer humanity has got from just basic probability theory. It comes from astrophysicist Richard Gott, and goes like this: Homo sapiens has been around about 200,000 years. It’s not very likely that we’re living at the very beginning or very end of our species’ history, just like it’s not very likely that a name chosen at random from the phone book will come at the very beginning or the very end. Specifically, there’s only a 2.5% chance that we’re living in the first 2.5% of our species’ life span, and only a 2.5% chance we’re living in the last 2.5% of our species’ life span. So do the math, and there’s a 95% probability that our species will last somewhere between .2 million and 8 million years.

For more on Bayes’ Rule, and the future of humanity, here’s a recent book, The Doomsday Calculation.

The worst of times

260 million years ago: the Capitanian mass extinction

A capsule summary of the evolution of life on Earth goes like this: There is steady progress in adaptation, driven especially by arms races, sometimes involving competitors, sometimes predators and prey. But this progress is interrupted from time to time by catastrophes – mass extinctions resulting from extrinsic causes, sometimes astronomical, but more often geological. (We’ll see much later in the year that a similar summary of human history goes like this: steady progress in the scale of cooperation driven by arms races, with occasional catastrophic interruptions, often associated with the spread of epidemic diseases.)

The geological causes of mass extinctions have been coming into focus lately. Many mass extinctions co-occur with the formation of Large Igneous Provinces (LIPS), regions where vast amounts of lava have flowed out of the earth, triggering a whole cascade of changes: the destruction of the ozone layer by halogen gases, global warming induced by CO2 and methane, and anoxic seas.

Large Igneous Provinces aren’t always associated with mass extinctions. What makes some episodes of massive lava flow particularly destructive is that they produce short circuits in the “planetary fuel cell.” The development of complex life has depended on the separation between an oxygen-rich, electron-hungry atmosphere and a reducing, electron-stuffed planetary interior. Some of the biggest setbacks to complex life have happened when  lava flows from deep in the Earth’s interior punch through carbon deposits on their way up, and bridge this chemical gap between surface and interior.

The mass extinction 260 million years ago, the Capitanian, is not one of the classic five greatest mass extinctions, and has been overshadowed by the mother of all mass extinctions, the end-Permian, which happened just 8 million years later. But it took a major toll on living things, from marine organisms to dinocephalians. (The dinocephalians – more closely related to mammals than to dinosaurs, ranging up to hippo sized, and including both herbivores and carnivores – went entirely extinct with the Capitanian. See picture.) The Capitanian extinctions coincide with, and were probably caused by, the formation of the Emeishan LIP, now in southwest China.

A book published recently, The Worst of Times, pulls together the latest evidence that the Capitanian was the beginning of an 80 million year period in which mass extinctions were exceptionally common. Apparently the formation of the supercontinent of Pangaea and the Panthalassic superocean made living things particularly vulnerable to volcanically induced extinctions. Once Pangaea breaks up, mass extinctions are less frequent, and generally have different causes.  The death of the dinosaurs had an extra-terrestrial cause, and the mass extinction we’re in the middle of results from the activities of one very unusual species.

Coals to Newcastle

287 – 272 million years ago

It seems like Gaia really went on a bender in the late Carboniferous, getting drunk on oxygen. By some estimates, the atmosphere was over 30% oxygen back then, compared to 21% today. Living things took advantage of the opportunity. Insects apparently face an upper limit in size because they rely on diffusion through tracheas instead of forced respiration through lungs to get oxygen into their bodies. With more oxygen in the air, this limit was raised. The Carboniferous saw dragonflies with a wingspan up to 70 centimeters, and body lengths up to 30 centimeters, comparable to a seagull.

This happened because plants were turning carbon dioxide into organic matter and free oxygen, and the organic matter was accumulating. With carbon dioxide being removed from the atmosphere, the late Carboniferous and subsequent early Permian saw a reduced greenhouse effect, and global cooling. This was another Ice Age, with ice caps around the southern pole.

A lot of organic carbon ended up being buried. Much of the world’s coal, especially high quality anthracite, has its origin in Carboniferous tropical forests. Western Europe and eastern North America lay in the tropics at the time, and got a particularly generous allotment of coal. Three hundred million years later this bounty would fuel the early Industrial Revolution. (Thanks partly to some of my Welsh ancestors, who helped dig it up back in the day.)

Enemies

339- 322 million years ago

You’re trying to live without enemies. That’s all you think about, not having enemies.

Isaac Babel, Red Cavalry

Enemies are the most important agencies of selection.

Geerat Vermeij, Evolution and Escalation

Much of what we’ve been seeing since the onset of the Cambrian, Monday, February 27, is the outcome of evolutionary arms races, leading to steady improvements in teeth, claws, armor, and mobility. It may well be that the onset of predation is what triggered the Cambrian explosion in the first place. The paleontologist Geerat Vermeij argues that arms races and escalation – not adaptation to the physical environment – are the greatest cause of progressive evolution.

We’ll see when we start getting into human evolution, biological and social, that enemies – other people especially – and arms races go on being a major motor of change. But arms races and escalation are going to look different in human evolution than they do in most non-human evolution. People are super-cooperators, and violent competition in humans tends to involve more group-against-group competition, with rival groups monopolizing and competing over territory. And in the human analog of predation – the formation of stratified societies, where elites live off the mass of the population – the human “predators” commonly band together under the aegis of the state to regulate their competition. At their best, human elites are less like wolves and more like sheepdogs.

Arms races operate with greater intensity in some environments than others. Races are more intense on large landmasses than small. Hence the common pattern in both biological evolution and human social evolution that isolated small continents and islands are especially vulnerable to invasion when their isolation ends. And arms races may be more intense, and the pace of evolution correspondingly greater, in the (more or less) 2-D terrestrial environment compared to the 3-D oceans.

Yet there may be something else involved in the initial move onto land – it’s sometimes among refugees from arms races that the greatest evolutionary advances arise. Fish moving onto land may have been doing it partly to get to someplace where enemies were weak or scarce. Human analogs might be the early Ionian Greeks fleeing the Dorian invasions, the settlers of Polynesia lighting out for the territories to escape a lowly position in a social order of ranked lineages, or the New England Pilgrims fleeing an un-Godly England. Or Vermeij himself – he is competitively handicapped, having lost his sight at three years old, but has made a distinguished career studying shelled invertebrates by touch.

Devonian days

401 – 380 million years ago

Lots going on in the Devonian.

Forests are spreading. These early trees, genus Wattieza, are kin to ferns and horsetails. They stand 10 meters tall. No leaves yet, just fronds. The first forests will absorb carbon dioxide, and cool the planet.

Life is moving onto land. Tiktaaklik roseae, the “fishapod” discovered in 2006, is as nice a link between fish and amphibians as one could hope for, with both lungs and gills. Here’s a book by Neil Shubin, the co-discoverer.

You can see a lot further in the air than underwater. This may be one of the early selective pressures for evolving a proper neck and sticking one’s head out of water. Eventually, for some, the rest of the body would follow.

And evolution seems to be generally speeded up on land. It’s not just that animals and plants develop adaptations for life on land (obviously). But there is also a more general acceleration in the pace of evolution. Major innovations come at a faster pace among terrestrial organisms.

On the Origin of Seafood by Means of Natural Selection

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

562-532 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?)