Anthropologists are challenging the long-held view that humans evolved from a single ancestral population in one region of Africa. Instead, a scientific consortium has found that human ancestors were diverse in form and culture and scattered across the continent. These populations were subdivided by different habitats and shifting environmental boundaries, such as forests and deserts.
The journal Trends in Ecology and Evolution published the findings, which drew from studies of bones (anthropology), stones (archaeology) and genes (population genomics), along with new and more detailed reconstructions of Africa’s climates and habitats over the last 300,000 years.
Emory University anthropologist Jessica Thompson was one of 23 authors on the paper. The research was led by the Max Planck Institute for the Science of Human History in Germany and the University of Oxford in England. In the following Q&A, Thompson explains the paper and its significance.
Can you provide some background on our understanding of human evolution?
Jessica Thompson: Even as early as 20 years ago, fossils were the main material we had to try to answer the question of where humans originated. A multi-regionalist theory hypothesized that Homo sapiens emerged in different places at the same time, evolving at the same rate across the Old World. This would mean that there was extensive gene exchange across ancient Asia, Europe and Africa, and that groups such as Neanderthals would not be a separate species but just a localized form of Homo sapiens. But it is difficult to get that level of resolution from bones alone.
By the 1990s, mitochondrial DNA analyses provided growing genetic evidence for the competing theory — that all modern humans originated in Africa and then dispersed from there around the globe. The implication of this is that groups such as the Neanderthals would actually have been different species, and that they were replaced by modern human groups dispersing from Africa.
Intense debate continued over the two theories but, by the early 2000s, it was clear that the out-of-Africa group had won. Only a small percentage of modern humans from the total population living in Africa actually left the continent, creating a genetic bottleneck in populations outside of Africa. So there is more diversity within the genomes of some living peoples in Africa today than there is, say, between an Australian aboriginal person and a Norwegian person.
As a final twist, whole-genome DNA now shows that there was some gene flow with Neanderthals as those first modern populations emerged from Africa. This could have happened several times over many thousands of years, and so a “leaky out-of-Africa” model seems to be the best fit for the data.
How does the current paper fit into this model?
JT: While it was well established that modern humans originated in Africa, there was still the question of where in Africa. East Africa and South Africa have been strong candidates, but that could be due to the long historical bias of where fossils were being found.
Our paper takes the global idea of multi-regionalism and shrinks it down to the boundaries of Africa. The answer to where humans originated appears to be lots of places within the continent, often separated for long periods, but again with leaky boundaries. Essentially, there is not a single ancestral human population. Who we are today probably evolved as a mosaic of populations of very near modern humans who were separated by geographic and cultural boundaries but were also all interacting with one another at different points in time. Our origin story is one of lots and lots of different humans that came together and then separated and later came together again in this really confusing manner. There’s a lot of moving parts. Humans, for a very long time, have been a culturally and phenotypically diverse bunch.
What new questions does this paradigm shift bring up?
JT: Instead of seeking the origin of humans in one spot, we need to look for pieces of the puzzle in many different places. Then we can ask, what adaptations did different populations have that contributed to who we are today? How did they come to be present in the single species we are now? And, perhaps more philosophically, what are the unifying characteristics that bind us together as that species, in spite of our differences?
While we need more data from places like East Africa and South Africa, it’s apparent now that West Africa and Central Africa are also key players in the story. They’re at the crossroads for much of the continent and yet we know very little about ancient populations from those regions. I’m hoping I can contribute to that effort with my current work in Malawi, which is positioned between southern and eastern Africa. There, we find a long, but relatively unexplored cultural record of human behavior that goes back into the last Ice Age.
We also recently recovered some of the oldest DNA in Africa from a site in Malawi, which we published last year. This helped to actually show some of those ancient interactions between populations at least over the last 10,000 years or so — as well as some of the differences between them. The implications are that this kind of structure went back even farther in time, to our origins as a species.
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