Top Story: March 21, 2011

Out of (Southern) Africa?

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By Anne Holden

The origin of our species was once firmly rooted in eastern Africa, but a new discovery may have shifted those roots much further to the south.

Exactly when and from where our species, Homo sapiens, first evolved and left Africa has been the subject of fierce debate. Most fossil and genetic evidence placed these origins in eastern Africa between 150,000 and 200,000 years ago. But a new study published earlier this month online in the Proceedings of the National Academy of Sciences reveals that our species may have evolved in southern, not eastern Africa.

Most Africans south of the Sahara are descended from early farmers spreading from western Africa about 8,000 years ago. But nestled within this vast dispersal of farmers are pockets of hunter-gatherers: The Khomani Bushmen of the Namibian Desert, the Pygmies of the central African rainforests, and the click-speaking Hadza and Sandawe of Tanzania. These peoples’ anatomy, culture, and language are distinct from their neighbors, and many believe that they offer a window into our species’ earliest days. But genetic data of these groups has been limited, and many questions on their origins remain.

The study’s authors, led by Brenna Henn of Stanford University, sought to fill in the gaps. “We started [this] project because southern Africa has been poorly sampled. Very few other studies have ever published on them in the last decade, certainly never with more than a dozen individuals,” says Henn.

Henn and colleagues analyzed over 55,000 individual points on the genomes of people from six hunter-gatherer populations, comparing them alongside other African populations. The team used this data to construct a genetic map of prehistoric Africa.

Not only had the hunter-gatherers been genetically isolated from the farming groups for thousands of years, they were also genetically distinct from each other.

In addition, long before the farmers swept across the continent, these hunter-gatherer groups were already well-established in their respective locales.

How does this relate to modern human origins? By using genomic data and a computer model, the team found the most likely starting point to be closest to the most ancient populations: southern Africa. Other experts have hypothesized this to be the case, and recent archaeological discoveries and climatic evidence have lent additional support to the fact that this region of Africa hosted our most ancient ancestors.

But the team has many questions left to answer. Henn and her colleagues also detected rapid evolutionary change among the groups, which may trace back thousands of years. They found that the Hadza of Tanzania have been going through a rapid decline, called a bottleneck, but have yet to understand why. “We would like to know when this bottleneck started - did it happen when the agriculturalists moved in?  Why don't all hunter-gatherer populations show this signature?” says Henn.

Anne Holden, a docent at the California Academy of Sciences, is a PhD trained genetic anthropologist and science writer living in San Francisco.

Image of the Khomani courtesy of Brenna Henn

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