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.

My name is LUCA, I live on the ocean floor

4.01 – 3.81 billion years ago

The rapid appearance of life on Earth suggests that life is common in the universe (just as the late appearance of human-level intelligence suggests that such intelligence is rare). Here’s a recent review of the current state of play on theories of early Earth and early Earth life. Just about as soon as the planet could support life we find chemical evidence for it, from Isua, Greenland (but no fossils yet). And there’s some more tentative evidence for fossils formed around hydrothermal vents all the way back at 4.28 billion years ago. This suggests that the origin of life is pretty easy (unless we want to go with panspermia). Mars may have been a more habitable place early in its history, and perhaps Mars exploration will one day solve the mystery of the origin of life in our Solar System

One approach to understanding the origin of life on Earth is to work backward from living organisms, to reconstruct the biochemistry of LUCA, the Last Universal Common Ancestor (not quite the same as the first living thing). Recent research on these lines implies that LUCA was a heat-loving microbe that relied on hydrogen as its energy source, suggesting an undersea volcano as a habitat.

History became legend, legend became myth

7.86 – 7.44 thousand years ago

This has proven my most popular post! For more on the general topic, here’s Patrick Nunn with an article and a book, The Edge of Memory: Ancient Stories, Oral Tradition, and the Post-Glacial World making the case that “some ancient narratives contain remarkably reliable records of real events.” And here’s an article on the topic from Smithsonian magazine.

Here’s a Klamath Indian story, recorded in 1865 (much abbreviated here).

One time when the Chief of the Below World was on the earth he saw Loha, the daughter of the tribal chief. Loha was a beautiful maiden, tall and straight as the arrowwood. The Chief of the Below World saw her and fell in love with her. He told her of his love and asked her to return with him to his lodge inside the mountain. But Loha refused to go with him. The Chief of the Below World was very angry. He swore he would have revenge on the people of Loha, that he would destroy them with the Curse of Fire. Raging and thundering on the top of his mountain, he saw the face of the Chief of the Above World on the top of Mount Shasta. From their mountaintops the two spirit chiefs began a furious battle. Mountains shook and crumbled. Red-hot rocks as large as the hills hurtled through the skies. Burning ashes fell like rain. The Chief of the Below World spewed fire from his mouth. Like an ocean of flame it devoured the forests on the mountains and the valleys. The Curse of Fire reached the homes of the people. Fleeing in terror before it, they found refuge in Klamath Lake. [Eventually the Chief Below the World] was driven into his home [by the Chief above the World], and the mountain fell upon him. When the morning sun rose, the high mountain was gone. The mountain which the Chief Below the World had called his own no longer towered near Mount Shasta. For many years the rain fell in torrents and filled the great hole that was made when the mountain fell upon the Chief of the Below World. Now you understand why my people do not visit the lake. From father to son has come the warning “Do not look upon this place.”

Almost 7,700 years ago, a volcanic eruption destroyed most of what had been the towering Mount Mazama, leaving behind a 4,000 foot deep crater that became Oregon’s Crater Lake. The Klamath Indian story about the origin of the lake preserves a clear memory of this event, from long before the invention of writing.

The story, and its connection with real geological events, is presented in When They Severed Earth From Sky. The book demonstrates in this and many other cases how myths and legends can preserve detailed information about the past. If you’re interested in how earlier generations remembered history and conceived of the past – a big topic on Logarithmic History – the book is strongly recommended. It’s intellectually worthy, setting forth general principles that govern the formation of legends, myths, and other oral history. And it’s also a fun read, casting new light on familiar figures like Prometheus (related to current Caucasian myths, and tied to volcanic Mount Elbrus in the Caucasus) and the Golden Calf.

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.

My name is LUCA. I live on the ocean floor.

4.01 – 3.81 billion years ago

How life began on Earth is still not well understood. The “RNA world” is one popular theory. In modern organisms, nucleic acids, DNA and RNA, store and transfer information, but proteins do the actual work of catalyzing chemical reactions. But RNA can act as a catalyst, so maybe the first replicating systems involved RNA catalyzing its own replication. However, RNA doesn’t spontaneously form very easily, so it’s not clear how the RNA world would have gotten started. Borate minerals might help but it’s not clear they were around that early.

A different approach to the topic is to work backward from living organisms, to reconstruct the biochemistry of LUCA, the Last Universal Common Ancestor (not quite the same as the first living thing). Recent research on these lines implies that LUCA was a heat-loving microbe that relied on hydrogen as its energy source, suggesting an undersea volcano as a habitat.

However the first organisms got established on Earth, it happened very quickly. Here’s a recent review of the current state of play on theories of early Earth and early Earth life. Just about as soon as the planet could support life we find chemical evidence for it, from Isua, Greenland (but no fossils yet). And there’s some more tentative evidence for fossils formed around hydrothermal vents all the way back at 4.28 billion years ago. This suggests that the origin of life is pretty easy (unless we want to go with panspermia). Mars may have been a more habitable place early in its history, and perhaps Mars exploration will one day solve the mystery of the origin of life in our Solar System

Bring out your dead

577 – 658

We’re now taking history less than one century per day.

Something major happened to Earth’s atmosphere in 535. We have reports from around the world of the sun being darkened or blotted out for more than a year, and evidence from tree rings and ice cores of an extreme cold spell. The culprit might have been dust thrown into the atmosphere by volcano or a comet. This on its own must have been bad news for the world’s population. But even more consequential was what happened starting seven years later. In 542, bubonic plague made an appearance in the Egyptian port of Pelusium, and rapidly spread around the Mediterranean, eventually reaching much of western Europe and Persia. (China seems to have gotten off more lightly.) It’s possible the epidemic had its origin among rodents in the east African Great Lakes region: disturbances to these populations after 535 may have contributed to the spread of plague, either up the Nile valley, or to trading towns on the Indian Ocean. Recent genetic evidence has confirmed that plague bacteria from this period are almost identical to those from the later, better known Black Death in the late Middle Ages. The plague struck repeatedly around west Eurasia for the next 200 years, before disappearing. The death toll must have been many tens of millions.755

Major movements of peoples would follow the plague in the sixth and seventh centuries. The Byzantine reconquest of most of the western Roman Empire, under Justinian, came undone as a new wave of Germanic barbarians, the Lombards, occupied Italy. The Anglo-Saxons expanded from the east of England to occupy most of present-day England. Slavs moved south to occupy most of the Balkans. And, most consequentially, Arabs under the banner of Islam occupied most of the Middle East and North Africa.

Exodus

1628 BCE, and later.

There are two great stories in the Western tradition that stand somewhere between legend and history: The Flight from Egypt and the Trojan War. Both have been scholarly battlegrounds, dismissed as pure invention by some, accepted as at least partly historical by others. In the case of the exodus story, a great many archeologists nowadays are strong skeptics. Here I summarize what I think is the best argument for the other side.

Barbara Sivertsen, in her book The Parting of the Sea, argues that the exodus story combines oral traditions arising from two different flights from Egypt. First, she suggests that some of the story reflects events around the time of a huge volcanic explosion, the largest in the last five thousand years, which destroyed most of the island of Thera (= Santorini) in 1628 BCE. Most of the Biblical plagues fit what would have been expected in northern Egypt at the time (and in the right time sequence). A tsunami reaching the Nile delta would have contaminated water, and caused fish to die off. Frogs would have been driven from the water. Caustic ash would have stung human skin (in later recountings, “stinging like gnats” was remembered as “stinging gnats”). Insects affected by ash would have sought shelter in people’s houses. Livestock outdoors would have died from breathing ash, and humans and livestock would have developed blisters. Eventually dust in the atmosphere would have precipitated hailstorms. The arrival of the heaviest part of the dust cloud would have shrouded the land in darkness. (Locusts, however, don’t fit the volcano story, and may be an embellishment or a coincidental plague.) All these developments would have precipitated a panicked flight from Egypt on the part of Israelites, led by Moses. (Lower Egypt at the time was ruled by charioteer Hyksos invaders.) According to the archeological evidence, the Wadi Tumilat, an oasis/caravanserai east of the Nile commonly identified as the Biblical Land of Goshen, is abandoned at this time and left uninhabited for centuries.

Other authors have suggested that the Thera eruption had some role in the exodus, but Sivertsen thinks there was also a later flight. In the mid-1400s, Egypt had a significant population of prisoners of war employed as slaves at Tell el-Da’ba, a naval base on the Mediterranean. In Sivertsen’s account, a wave of deaths of Egyptian children led Pharaoh Tuthmose III, frightened of the Israelite god, to expel a group of Israelite slaves. The pharaoh changed his mind, however, and sent an army in pursuit of the slaves along the northern shore of the Sinai. We know that in the mid-1400s, another volcanic eruption, on the Aegean island of Yalli, sent a tsunami around the shores of the eastern Mediterranean. This tsumani caught up with the Egyptian army, but missed Israelites camped further inland. The event was spectacular enough to be melded with the earlier exodus story.

A major reason for skepticism about the exodus story is that it has been very hard to find evidence for the Israelite conquest of Canaan in the fourteenth or thirteenth century BCE, which is when many accounts place the exodus. But if we follow Sivertsen in putting the first exodus much earlier, and allow that the “forty years” in the wilderness was really eighty years, then there is plenty of evidence for massive invasion and destruction of cities in Canaan in the mid 1500s, at the end of the Middle Bronze Age. Israelites could have been among the invaders of Canaan. Around 1550 BCE, the city of Jericho suffered an earthquake that knocked down some of the city walls. The city then burned to the ground, and was largely abandoned subsequently.

We saw earlier on Logarithmic History that oral history can preserve detailed memories of natural catastrophes for long periods of time. At the same time information about numbers and absolute dates mostly gets lost. It will be interesting to see how Sivertsen’s work holds up in the face of further discoveries.

Toba? or the sperm whale effect?

74 thousand years ago, a big chunk of the island of Sumatra blew up. It was the biggest volcanic explosion in the past two million years, expelling 2800 times as much debris as the Mount Saint Helens eruption in Washington State in 1980. Ash from the super-eruption is found all the way from Lake Malawi to the South China Sea. The resulting Toba caldera measures about 20 by 60 miles.

The Toba eruption coincides with a shift back to glacial conditions, and it may be that there’s a connection, that Earth went through a long volcanic winter after the eruption, which shifted climate to a colder equilibrium.

Did Toba have an effect on human evolution? Somewhere between 100 and 50 thousand years ago, human populations went through a bottleneck: modern humans are descended from just 1,000 to 10,000 breeding pairs from that period. It’s been argued that Toba wiped out the majority of Homo sapiens around at the time, leaving only a small group of survivors.

But the evidence that Toba is responsible for the bottleneck is equivocal. In some places humans seem to have passed through the period of the eruption without major disruptions. Also, there’s a point that gets missed in a lot of popular reporting: just because a species went through a bottleneck doesn’t necessarily mean that the population of the whole species ever shrank to that size (upper panel below). In the case of Homo sapiens it could be that the total population was always many times larger than 1,000-10,000. It’s just that the other tens or hundreds of thousands got replaced (lower panel below). In other words, we may not be looking at an external catastrophe wiping out most of humanity, and a few groups of survivors recovering. Instead, we may be looking at a small population of our eventual ancestors expanding and outcompeting other populations, so that it was our ancestors, not a volcano, who made sure that most human beings alive 74,000 years ago didn’t leave descendants.

This may reflect something special about human evolution: human beings typically belong to tribes and ethnic groups defined by distinctive cultures, and cultural boundaries (including language boundaries) often act as barriers to interbreeding. Several authors have suggested that this may make human beings unusually susceptible to population replacement via “cultural group selection,” and that this might account for humans having unusually low effective population size, as genes “hitchhike” along with expanding cultures. Interestingly, sperm whales, which live in populations defined by different song dialects (and other cultural differences) may show the same genetic pattern. And here’s me on kin selection and ethnic group selection, related.

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.