The first thing you see is obvious: our ancestors went through two different phases of population “bottlenecking” (constriction): one occurred about three million years ago, when a large population declined to around 10,000 individuals. The authors note that while this may reflect population size decline associated with the origin of hominins after our split with the lineage that produce modern chimps, they also say that this could be an artifact of ancient genetic polymorphisms maintained by natural selection.
The second bottleneck is the one of interest, for it’s the one associated with a reduced population size as humans left Africa. For the Chinese, Korean, and European genomes, effective population size fell from about 13,500 (at 150,000 years ago) to about 1200 between 20,000 and 40,000 years ago.
Finally, we also see the population recover in size, with a huge increase in all populations beginning roughly 20,000 years ago. This clearly reflects population growth in both Africa and in areas colonized from Africa as humans expanded around the globe.
There are two other interesting points:
- All the data clearly show that all modern humans, African and non-African alike, descent from one “homogeneous ancestral population in the last 100,000 years, with subsequent minor admixture out of Africa from Neanderthals.” This goes against earlier theories that there is a much older divide separating West African from non-African populations.
- Also contrary to earlier assumptions that after Homo sapiens left Africa (ca. 60,000 years ago) there was little interbreeding between African and non-African populations, the new data show that genetic interchange between these populations continued up until 20,0000-40,000 years ago. This conclusion, though, is provisional because it depends on estimates of mutation rates which are necessarily indirect.