How ancient DNA is rewriting human history

Michael White writes: There are no written records of the most important developments in our history: the transition from hunting and gathering to farming, the initial colonization of regions outside Africa, and, most crucially, the appearance of modern humans and the vanishing of archaic ones. Our primary information sources about these “pre-historic” events are ancient tools, weapons, bones, and, more recently, DNA. Like an ancient text that has picked up interpolations over the millennia, our genetic history can be difficult to recover from the DNA of people alive today. But with the invention of methods to read DNA taken from ancient bones, we now have access to much older copies of our genetic history, and it’s radically changing how we understand our deep past. What seemed like an episode of Lost turns out to be much more like Game of Thrones: instead of a story of small, isolated groups that colonized distant new territory, human history is a story of ancient populations that migrated and mixed all over the world.

There is no question that most human evolutionary history took place in Africa. But by one million years ago—long before modern humans evolved — archaic human species were already living throughout Asia and Europe. By 30,000 years ago, the archaic humans had vanished, and modern humans had taken their place. How did that happen?

From the results of early DNA studies in the late 1980s and early ’90s, scientists argued that anatomically modern humans evolved in Africa, and then expanded into Asia, Oceania, and Europe, beginning about 60,000 years ago. The idea was that modern humans colonized the rest of the world in a succession of small founding groups — each one a tiny sampling of the total modern human gene pool. These small, isolated groups settled new territory and replaced the archaic humans that lived there. As a result, humans in different parts of the world today have their own distinctive DNA signature, consisting of the genetic quirks of their ancestors who first settled the area, as well as the genetic adaptations to the local environment that evolved later.
There are very few isolated branches of the human family tree. People in nearly every part of the world are a product of many different ancient populations, and sometimes surprisingly close relationships span a wide geographical distance.

This view of human history, called the “serial founder effect model,” has big implications for our understanding of how we came to be who we are. Most importantly, under this model, genetic differences between geographically separated human populations reflect deep branchings in the human family tree, branches that go back tens of thousands of years. It also declares that people have evolutionary adaptations that are matched to their geographical area, such as lighter skin in Asians and Europeans or high altitude tolerance among Andeans and Tibetans. With a few exceptions, such as the genetic mixing after Europeans colonized the Americas, our geography reflects our deep ancestry.

Well, it’s time to scrap this picture of human history. [Continue reading…]

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