A view of the exact region in Africa where humans emerged

There's still a lot we don't know about our evolutionary history, but one generally agreed upon point is that humans originated in eastern Africa, around what's now Ethiopia. But a new genetic study suggests we came from somewhere else entirely.

Stanford researchers examined the genetic diversity of modern hunter-gatherer populations. As co-author Brenna Henn explains, they made two crucial discoveries:

"One is that there is an enormous amount of diversity in African hunter-gatherer populations, even more diversity than there is in agriculturalist populations. These hunter/gatherer groups are highly structured and are fairly isolated from one another and probably retain a great deal of different genetic variations - we found this very exciting.

"The other main conclusion was that we looked at patterns of genetic diversity among 27 (present-day) African populations, and we saw a decline of diversity that really starts in southern Africa and progresses as you move to northern Africa. Populations in southern Africa have the highest genetic diversity of any population, as far as we can tell. So this suggests that this might be the best location for (the origins) of modern humans."

Why does greater genetic diversity point to southern Africa as humanity's birthplace? The logic is actually quite simple. Whenever part of a population leaves to find a new home elsewhere, they necessarily will have less genetic diversity than the group they left behind, and their descendants will maintain roughly the same genetic diversity as their founding ancestors. That effect is particularly true when it's a very small group of individuals relative to the original population.

It's a fascinating finding, but how can it fit with what we already know about hominid origins, which places our ancestor species squarely in eastern Africa for the past millions of years? Chris Stringer, a paleontologist at London's Natural History Museum, offers one persuasive explanation:

"The new paper... suggests that the genes of the Namibian and Khomani bushmen (southern Africa), Biaka pygmies (Central Africa) and the Sandawe (East Africa) appear to be the most diverse, and by implication these are the most ancient populations of Homo sapiens. This is a landmark study, with far more extensive data on... hunter gatherer groups than we have ever had before, but I am cautious about localising origins from it. It seems more likely that the surviving hunter-gatherer groups are now localised remnants of populations that formerly ranged across much of sub-Saharan Africa 60,000 years ago."

So then, humanity once roamed across all of sub-Saharan Africa, and this may have somewhat scrambled the link between genetic diversity and the ancientness of that particular population. Indeed, if we can greatly simplify the story of human migration out of Africa, we might imagine a single "original" population slowly moving southwards to these present locations while breakaway groups kept migrating north, with some of these new populations eventually taking us out of our birth continent.

PNAS via BBC News. Image via PSU.