Tag Archives: Ice Age

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. 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. 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.

In May 2015, the Toba volcano grew more active than usual, producing large emissions of steam and foul gases. Locals were reported to be concerned.

Ice Age gear shift

833-788 thousand years ago

Around today’s date, there was a shift in the nature of glacial cycles.

But let’s back up a bit. Earth’s climate took a turn toward cool in the transition from Eocene to Oligocene, 35 million years ago (although with some warming in the Miocene). It was probably back then that much of Antarctica started being covered by ice. The establishment of open water all the way around Antarctica may have helped isolate and freeze the continent. And declining carbon dioxide levels, partly a result of weathering of rocks in the Himalayas, probably also made a difference. But it was back at the beginning of the Pleistocene, now dated to 2.5 million years ago, that the current Ice Age truly began, with glaciers covering large parts of northern North America and northern Europe.

Current Ice Age? Glaciers covering large parts of northern North America and northern Europe? This isn’t what the climate has been like for the past 10,0000 years. Within the current long Ice Age there have been long glacial periods and shorter interglacials, and we’re currently in an interglacial. Our own activities may have done something to prolong the interglacial, and stave off the return of the ice; more on this another day.

Three astronomical cycles govern the rhythm of glacial and interglacial. There’s a 100,000 year cycle as Earth’s orbit changes from somewhat more elliptical to somewhat more circular. There’s a 40,000 year cycle as Earth’s axis shifts from slightly more tilted (24.5 degrees off vertical) to slightly less (22.1 degrees). It’s currently tilted at 23.5 degrees. And there’s a 21,000 year cycle generated as the Earth precesses – wobbles like a top. Right now the North Pole is pointed at Polaris, and the Sun very recently started rising in the constellation Aquarius at the Spring equinox: hence the Age of Aquarius.

Between 2.5 million and 800,000 years ago, the glacial/interglacial alternation was dominated by the 40,000 year cycle. But beginning about 800,000 years, there has been a gear shift: the 100,000 year cycle has been dominant and swings in climate have been more extreme. (In Africa however the 21,000 year cycle is more important for alternations between rainy and dry. Africa is in a dry state now.)

One of the startling findings to come out of the last few decades of work on ice cores from Greenland and Antarctica is that not only have there have been huge long-term changes in climate, but there have also been extreme short term shifts, probably connected with changes in ocean currents. There have been a number of occasions over the last hundreds of thousands of years during which average temperatures shifted by 10-20 degrees Fahrenheit (5-10 degrees Celsius) for a millennium, or even for a century or less! (During the last 10,000 years, however, the climate has been unusually stable.)

This is bound to have had strong effects on human beings. Two anthropologists, Robert Boyd and Peter Richerson, who work on mathematical models of cultural evolution, have a general theory of how this pattern of oscillations might have affected human evolution. They argue that human adaptation takes place on multiple time scales. On very long time scales, human beings adapt to changes in the environment genetically. On very short time scale, human beings adapt to change through individual learning. But when change happens on intermediate time scales, adaptation takes place through social learning. With changes on intermediate time scales, your ancestors may not have enough time to adapt genetically to the current climate, but things may be stable for long enough that your culture and the wisdom of the elders have a lot to teach you about how to cope. So one of the really distinctive features of human beings – we are, more than any other creature, a cultural animal – may have been shaped by the nature of climate change especially over the last 800,000 years.

Coals to Newcastle

340-320 Mya

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.)

coal age

Snow time

744-704 million years ago

Tweeting was sparse for the past week, because for a billion years Earth was fairly stable. Any biological evolution towards greater complexity that was going on left little fossil evidence.

Then things changed dramatically. 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 few 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. 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.


14 kya. The last glacial phase looks like it’s coming to an end, and people in the Natufian culture of the Levant look like they’re gearing up to invent agriculture. They’ve settled in villages, and are harvesting and storing grain, but not yet sowing it. This turns out to be a false start though. In a thousand years or so the glaciers will come back for a final hurrah (the Younger Dryas event), and only after this will farming actually get going.

If you plug different frequencies of different genes from a bunch of populations into a computer and ask it to generate a tree where genetically similar populations share closer branches, you get something like this:

cavalli-sforza tree

This is from the pioneering synthesis of genetics by Cavali-Sforza and co-workers, back in 1994. This looks like a nice diagram of humans spreading out of Africa, maybe some taking a southern route (the Southeast Asian branch), and others a northern route (North Eurasian), and I used to teach it this way in anthropology classes. But as we look at ancient DNA, we’re finding that things are more complicated. Even 14,000 years ago, the structure of populations is different from what we’re used to today. We’ve already mentioned the Ancestral North Eurasians earlier, who just maybe could have spoken a language ancestral to Greenberg’s Eurasiatic family.

In the Near East, too, things were complicated. A paper out just this August shows that there were three very different hunting and gathering populations in Anatolia, Western Iran, and the Levant. Folks in Iran and the Levant were as genetically distinct as modern Europeans and Chinese! Either the Near East during this period had just been settled by migrants from widely separated places, or there had been strong barriers to gene flow in place for some time. Since then people in the area have mixed a lot.

Each of these populations of hunters and gatherers will give rise to its own set of farmers. The Natufians will contribute a lot to the ancestry of later farmers in the Levant. And apparently each set of farmers will send migrants off in a different direction: the Anatolians to Europe, the Iranians (or some Caucasian relatives) to the Eurasian steppe, and the Levantines to East Africa. It’s possible that the Natufians were speakers of a language ancestral to the Afro-Asiatic family, one of the oldest widely accepted language families, including Arabic and Hebrew, Somali and Oromo.

And here’s Razib Khan on our new understanding of how different ancient population structure was.

Reindeer Moon

18 kya. Glacial periods within the present Ice Age start gradually and end rapidly. The last glacial period began around 90 thousand years ago, and reached its peak, the Last Glacial Maximum (LGM), around 20 thousand years ago. During the LGM most of Northern Europe and northern North America were under ice. Sea levels were more than 300 feet lower than today. Human beings in Europe were confined to refugia in the south, notably Spain and southern France.

In the period after the LGM, temperatures rise, the ice retreats, and humans recolonize the North. Not just Europe: people will gradually make their way into Siberia, and some of these Siberians will eventually make their way to the New World.

For a flavor of life during these times, Elizabeth Marshall Thomas is worth checking out. She is the author of two beautifully written novels set in this period in Siberia (Reindeer Moon, and The Animal Wife). As a child, Thomas was part of an expedition to the Kalahari, as recounted in her book The Harmless People. Her experiences, along with northern archeology, color her imaginative reconstruction of Ice Age hunters.


19 kya. Lascaux cave paintings, Southwest France, discovered in 1940.

lascaux copy

below, from Consciousness and the Brain: Deciphering how the Brain Codes Our Thoughts (an excellent book, not mainly about cave paintings)

Deep inside the Lascaux cave, past the world-renowned Great Hall of the Bulls, where Paleolithic artists painted a colorful menagerie of horses, deer, and bulls, starts a lesser-known corridor known as the Apse. There, at the bottom of a sixteen-foot pit, next to fine drawings of a wounded bison and a rhinoceros, lies one of the rare depictions of a human being in prehistoric art. The man is lying flat on his back, palms up and arms extended. Next to him stands a bird perched on a stick. Nearby lies a broken spear that was probably used to disembowel the bison, whose intestines are hanging out.

The person is clearly a man, for his penis is fully erect. And this, according to the sleep researcher Michel Jouvet, illuminates the drawing’s meaning: it depicts a dreamer and his dream. As Jouvet and his team discovered, dreaming occurs primarily during a specific phase of sleep, which they dubbed “paradoxical” because it does not look like sleep; during this period, the brain is almost as active as it is in wakefulness, and the eyes ceaselessly move around. In males, this phase is invariably accompanied by a strong erection (even when the dream is devoid of sexual content). Although this weird physiological fact became known to science only in the twentieth century, Jouvet wittily remarks that our ancestors could easily have noticed it. And the bird seems the most natural metaphor for the dreamer’s soul: during dreams, the mind flies to distant places and ancient times, free as a sparrow.

This idea might seem fanciful were it not for the remarkable recurrence of imagery of sleep birds, souls, and erections in the art and symbolism of all sorts of cultures. …