Out of Africa

A Better Picture of Human Expansion

A new, comprehensive review of humans' anthropological and genetic records gives the most up-to-date story of the "Out of Africa" expansion that occurred about 45,000 to 60,000 years ago.

This expansion, detailed by three Stanford geneticists, had a dramatic effect on human genetic diversity, which persists in present-day populations. As a small group of modern humans migrated out of Africa into Eurasia and the Americas, their genetic diversity was substantially reduced. In studying these migrations, genomic projects haven't fully taken into account the rich archaeological and anthropological data available, and vice versa. This review integrates both sides of the story and provides a foundation that could lead to better understanding of ancient humans and, possibly, genomic and medical advances. "People are doing amazing genome sequencing, but they don't always understand human demographic history" that can help inform an investigation, said review co-author Brenna Henn, a postdoctoral fellow in genetics at the Stanford School of Medicine. "We wanted to write this as a primer on pre-human history for people who are not anthropologists." This model of the Out of Africa expansion provides the framework for testing other anthropological and genetic models, Henn said.

It will allow researchers to constrain various parameters on computer simulations, which will ultimately improve their accuracy. The anthropological information can inform geneticists when they investigate certain genetic changes that emerge over time. For example, geneticists have found that genes for lactose intolerance and gluten sensitivity began to emerge in populations expanding into Europe around 10,000 years ago. The anthropological record helps explain this: It was around this time that humans embraced agriculture, including milk and wheat production. The populations that prospered -- and thus those who survived to pass on these mutations -- were those who embraced these unnatural food sources. This is an example of how human movements drove a new form of natural selection. "If you know something about the demographic history of populations, you may be able to learn something about the reasons why a group today has a certain genetic abnormality -- either good or bad," said Marcus Feldman, a professor of biology at Stanford.


"That's one of the reasons why in our work we focus on the importance of migration and history of mixing in human populations."


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