David Reich: uncertainty

Transcript context

…How many humans are around at this time? To the extent that all modern humans are descendants of this group, how many different groups of humans are there—no genetically distinct necessarily, but just separate locations or so forth—such that there's enough gene flow between all of them and there's a shared common descent. I don't know. Here’s one thing that is really interesting. A couple of years ago, we published a paper on relatively recent hunter-gatherer populations from mostly eastern and central Africa in order to be able to discern these deep population exchanges that we would really like to know in order to understand human evolution. This included individuals going back up to about 15,000 years ago, which is the oldest DNA from sub-Saharan Africa, which is not very old. Really we would like to be able to probe 2 million years ago, but we can't. But with 15,000-year-old individuals, what you see is many groups at many places all with very reduced diversity. In other words, they look like they're living in tiny populations of hundreds of people and not exchanging DNA with each other very often at all over time. We see this again and again. You take such a population and put it into a model. If it's this small, what will happen over time? It will lose its diversity over time, and it will become very non-diverse. Over time, Africa will have very little diversity. But of course, Africa today has great human diversity in it. What seems to be happening is that the whole continent of sub-Saharan Africa, and probably Eurasia at this time, is full of hundreds, thousands, tens of thousands of little groups that are communicating hardly at all with each other. They are in very small sizes and losing diversity. When we sample them, this is a group that leaves hardly any descendants at all, maybe none, amongst modern people. What's actually happening is that occasionally these groups merge together and recharge their diversity. Diversity is maintained in the ensemble of rarely mixing groups. You can't really appreciate the diversity by studying any one group. You actually have to think about the whole ensemble of hundreds or thousands of tens of thousands of them as preserving the diversity. There's some question about the migration rate amongst these groups, an archipelago of little groups losing diversity and going extinct at some level. But together there is enough recontact to recharge the diversity and create the incredibly diverse populations you see today, for example, in southern Africa or western Africa or central Africa. I want to go back to what you were saying. For hundreds of thousands of years—not just with modern humans, but with even the so-called archaic humans, Neanderthals, and other species—there's been selective pressure for larger brains. This is despite the fact that they're in different parts of the world. If you're in Eurasia or if you're in sub-Saharan Africa, either way we finally got to a state where the niche we're in rewards marginal increases in intelligence and is willing to bear the cost of that and keep chugging on that variable. Do we know why that was the case? What was happening in the world? What was happening with maybe primate brains such that the selective pressure was turning towards greater intelligence?…

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