When geneticists first started measuring genetic differences between people, they realized that the population must have once been a lot smaller.They came up with the idea of a genetic “bottleneck”, a period in which the human population might have been very small.The story of the rest of the world during the last 100,000 years is not the story of Africa.

A fossil that shares some human traits might be part of that delta network leading to some living populations.

But just as easily, it might instead be off on its own branch, flowing until it disappears into the desert sands.

Those people further mixed a small fraction with archaic humans, the Neanderthals and, for a few, the Denisovans.

That founder effect unfolded within the last 100,000 years.

The last month has seen more shake-ups to the modern human origins story than any time I can remember.

Here’s what we have learned in the last few weeks about this key time period in Africa. We’re 99.9% genetically the same, and that impressive genetic similarity comes from inbreeding.But the last few years have added a lot of complications to this simple picture.A result from PSMC studies of human genomes from different populations (indicated).It’s a good reminder that within humans, a genetic “split” is not really a split. Genetics tells us that the ancestry of African groups is like a river delta, spreading from some 260,000 years ago up to the present.But anthropologists really disagree about how to identify “modern humans” in the fossil record.Very ancient peoples mixed too, but migration and mixing were less in the distant past than in recent history.