Humans did not emerge from a single region of Africa, suggests a powerful modelling study. Rather, our ancestors moved and intermingled for millennia
The widely held idea that modern-day humans originated from a single region of Africa is being challenged. Models using a vast amount of genomic data suggest that humans arose from multiple ancestral populations around the continent. These ancient populations — which lived more than one million years ago — would have all been the same hominin species but genetically slightly different.
The models supporting this theory rely on new software and genomic-sequencing data from current African and Eurasian populations, as well as Neanderthal remains. Researchers published the results on 17 May in Nature.
The study contributes more evidence to the idea that there is “no single birthplace in Africa, and that human evolution is a process with very deep African roots”, says Eleanor Scerri, an evolutionary archaeologist at the Max Planck Institute of Geoanthropology in Jena, Germany.
The single-origin theory has been popular for decades, partly on the basis of fossil records. But the theory doesn’t fit the data well, says Scerri. All of the tools and physical traits attributed to Homo sapiens cropped up throughout Africa around a similar time, 300,000 to 100,000 years ago. If humans had radiated from a single location, archaeologists would expect to see more recent fossils farther away from a central point, and older ones closer to it.
Ancestral stem
The ancient hominin species, or ‘ancestral stem’, had localized populations which are thought to have interbred with each other over millennia, sharing any genetic differences that they had evolved. They also moved around Africa over time. “Our roots lie in a very diverse overall population made up of fragmented local populations,” says Scerri. The intertwining of these stems, separated only weakly by their genetic differences, gave rise to a concept of human evolution that the researchers described as a “weakly structured stem” — more like a tangled vine than a ‘tree of life’.
Although the idea of multiple origin points isn’t new, the weakly structured stem explanation is. Other models exploring multiple origins have not used as many parameters as this study used, says co-author Brenna Henn, a human geneticist at the University of California, Davis.
The team used software developed by co-author Simon Gravel at McGill University in Montreal, Canada, that could co-ordinated the extensive computing power needed for expanded modelling. Previous work fell short on genetic data, because it focused mainly on West Africa, meaning that not all of the continent’s vast genetic diversity was incorporated. That created an incomplete picture of how modern humans’ ancestors might have mingled and moved across the landscape, and required scientists to make assumptions to fill knowledge gaps.
Genomic data
This study incorporated genome-sequencing data from existing eastern and western African populations and the Nama people of southern Africa. This spread of genomic data helped the researchers to understand and track the historical movement of genes across generations.
“We really wanted to sit down and very systematically evaluate the models in a more creative way,” says Henn. “It’s a new model for human evolution that is concrete.”
The models used variables such as migration and population merging to predict gene flow over the course of thousands of years. Those predictions were then compared with the genetic variation seen today to determine which models matched the data best.
One previously proposed explanation2 for today’s human diversity is that H. sapiens mixed with other archaic human species that had branched off and become isolated. But Henn and her colleagues found that the weakly structured stem model was the better fit, giving a clearer explanation for the variation seen in humans today.
Ultimately, questions still abound about humans’ origins. Henn wants to add more DNA from other African regions to the models to see if that changes their results. She also hopes to use the data to make predictions about the fossil record, such as what features would be found in human fossils from a particular area. (Nature)