David Reich

But in a period between about 2 million to 500,000 years ago, I think it's not at all clear where the main ancestors leading to modern humans were.

There were humans throughout many parts of Eurasia and throughout many parts of Africa with...

a parallel increase in brain size and not obviously closer ancestrality to modern humans in one place than in the other.

It's not clear where the main lineages were.

Maybe they were in both places and mixed to form the lineages that gave rise to people today.

So I think there's been an assumption where Africa's been at the center of everything.

Yeah.

for many, many millions of years.

And certainly it's been absolutely central at many periods in human history.

But in this key period when modern humans develop from Homo habilis and Homo erectus all the way to Homo heidelbergensis and the shared ancestor of Neanderthals, modern humans, and Denisovans, that time period, which is when a lot of important change happens, it's not clear, as I understand it, based on the archaeology and also certainly based on the genetics where that occurred.

Well, the simplest version of this is that the main lineage leading to modern humans is in Africa at this point.

And Africa, as I understand it from talking with the archaeologists and the climatologists, is that Africa and the Near East are continuous ecological spaces at certain periods of time.

And so there's no difference between what's now the Near East and Africa.

The

fauna and the flora are pumped from Africa into the Near East or pumped from the Near East into Africa.

And so the African range goes into that region.

And so it's a place of overlap between Eurasian fauna and flora and African flora and fauna.

And so that's a very natural place for interactions to occur, especially in periods of climate change.

Animals, for example, from one region get pumped into the Near East, and then in another period of climate change, they get pumped into Eurasia or the rivers.

I think there's always a land bridge.