Tag Archives: demic expansions

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.

toba

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.

Small tools, and dingoes, and Oz

Australia can seem like the Land That Time Forgot. Australian marsupials were largely isolated from competition with placental mammals from other continents. (Although humans seem to have done in a lot of the megafauna when they arrived.) And many discussions of human prehistory assume that outside contact had little effect on Australia from the time of first settlement, before forty thousand years ago, and the English settlement of 1788. Populations rose and fell with changes in climate and home-grown innovations in technology.

But there have always been hints that the story was more complicated. Some of the evidence comes from language distributions. The distribution of Australian languages is extremely lopsided. There are a lot of language families in and around the northern peninsula of Arnhemland. Then just one family, Pama-Nyungan, (distantly related to some or all of the others) covers about 80% of the continent.

This sort of distribution is seen with other language families, where it looks like the signature of population spreads. For example, three of four major branches of the Austronesian language family are found only on Taiwan. The fourth branch, Malayo-Polynesian, spans the world from Madagascar, to island Southeast Asia, to Easter Island. The explanation, supported by many lines of evidence, and almost universally accepted, is that Proto-Austronesian first diverged into separate languages on Taiwan. One bunch of Austronesian speakers then sailed to the Philippines, and thence to farther isles, their languages diverging along the way … and the rest is history (actually prehistory). A similar argument roots the Bantu expansion in the Nigeria-Cameroon border area.

So it looks like there was a similar expansion in Australia. Language change is hard to date, but it’s very hard to believe the expansion happened forty thousand years ago. There is other evidence suggesting a recent date. Some time before 2000 BCE, a new archeological culture, the Small Tool Tradition, swept over Australia. At the same time, natives began exploiting a far wider range of habitats and food sources than previously. And – strikingly – a new animal makes its appearance – the dingo, which must have been introduced from overseas. All of this makes it look like some outside contact, jump-started a continent-scale cultural expansion with new technology, and maybe new forms of social organization.

Just published work on Australian genetics suggests a complicated picture. Northeastern and southwestern Australia, separated by the interior desert, are also strongly genetically differentiated, reflecting a split probably going back sometime between 32 and 10 thousand years ago, i.e. well after initial colonization, but well before the spread of the Small Tool Tradition. But there is also evidence that the last 10 thousand years saw a population expansion in the northeast, a population bottleneck in the southwest, and some gene flow between the two. A scenario that fits the data would be Pama-Nyungan speakers expanding in northeast, and then some northeasterners migrating to the southwest, spreading their language and culture, but not arriving in such large numbers as to replace local populations.

On the other hand a story about gene flow from India (or an India-like population) that I blogged about last year may not hold up.

We can expect that the picture will grow clearer as more data accumulate.

First farmers

8.5 kya. Farming is now spreading out of the Fertile Crescent. Farmers have crossed the Aegean, and appear in the Balkans and Greece. (They got to Cyprus more than a thousand years earlier.) Farmers have also begun spreading out of the Yellow River and Yangzi River valleys in China.

There’s an argument among philosophers of a utilitarian bent, started by Derek Parfit, over which is better: a world with just a few very happy people (more happiness per capita), or a world crowded with a multitude of people for whom life is just barely worth living (more total happiness)? (The choice of the latter has been dubbed the “Repugnant Conclusion.”) Whatever the philosophical merits of one possible world or another, there’s little doubt about which direction social evolution usually takes. “God favors the side with the largest battalions” (a saying often attributed to Napoleon, but actually predating him), and agricultural populations have mostly managed to replace hunter-gatherers, even though they are probably worse fed and sicker on average. The DNA evidence shows that in Mediterranean Europe it’s replacement we’re talking about, not just the spread of new technologies. Migrants originally from Anatolia pushed aside indigenous hunter-gatherers without much interbreeding.

For a while, a decade ago, it looked as if the spread of agriculture might also explain much of the distribution the world’s major language families. Peter Bellwood’s book First Farmers made this case. According to this theory, the first farmers in Europe were speakers of an early Indo-European language that eventually gave rise to most of the languages of Europe, as well as Iran and northern India. We’ll see in days to come on Logarithmic History that the story turns out to be more complicated.

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.

toba

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.

Toba

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.
 
toba

The Toba eruption coincides with a shift back to glacial conditions, and it has been argued 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 goes 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 10,000. It’s just that the others 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 70,000 years ago didn’t leave descendants. We will see later on in Logarithmic History that population replacements have been a common occurrence in human history.

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.