The prevailing notion that modern humans originated from a single region in Africa is being challenged. Models using vast amounts of genomic data suggest that humans originated from multiple ancestral groups across the continent. These ancient populations – which lived more than a million years ago – were all the same species of hominin but slightly different genetically.
The models that support this theory are based on new software and genetic sequence data from current populations in Africa and Eurasia, as well as Neanderthal remains. The researchers published the results May 17 in the journal nature1.
The study contributes further evidence to the idea that “there is no single place of birth in Africa, and that human evolution is a process with very deep African roots,” says Eleanor Skerry, an evolutionary archaeologist at the Max Planck Institute for Geosciences in Jena, Germany.
The single origin theory has been popular for decades, in part based on fossil records. The theory doesn’t fit well with the data, Skerry says. All tools and physical attributes are attributed to Homo sapiens They appeared throughout Africa at about the same time, between 300,000 and 100,000 years ago. If humans rose from one place, archaeologists would expect to see more recent fossils further away from a central point, and older ones closer to it.
Ancient hominin species, or ‘ancestral stems’, had local populations that are believed to have interbred with each other over thousands of years, sharing any genetic differences that might have evolved. They also moved across Africa over time. “Our roots lie in a very diverse population made up of fragmented local populations,” says Skerry. The interweaving of these stems, separated only weakly by genetic differences, has given rise to a concept of human evolution that researchers describe as a “poorly built trunk” — more like a tangled vine than a “tree of life.”
Although the idea of multiple points of origin is not new, the poorly structured stem interpretation is. Other models that explore multiple origins did not use many of the parameters that this study did, 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 can orchestrate the comprehensive computing power needed for extended modeling. Previous work was limited to genetic data, because it focused mainly on West Africa, meaning that not all of the continent’s vast genetic diversity was incorporated. This created an incomplete picture of how the ancestors of modern humans mixed and moved across the landscape, requiring scientists to make assumptions to fill in knowledge gaps.
This study included genome sequence data from present-day East and West African populations and the Nama people of southern Africa. The spread of genomic data has helped researchers understand and track the historical movement of genes across generations.
“We really wanted to sit down and evaluate the models very systematically in a more creative way,” Henn says. “It is a tangible new paradigm of human evolution.”
The models used variables such as migration and population integration to predict gene flow over thousands of years. These predictions were then compared with the genetic variation seen today to determine which models best match the data.
One explanation has been suggested previously2 Because today’s human diversity is just that H. sapiens mixed with other ancient human species that branched out and became solitary. But Henn and her colleagues found that the stubby trunk model was the most appropriate, giving a clearer explanation for the diversity we see in humans today.
In the end, questions still abound about the origins of humans. Henn wants to add more DNA from other African regions to the models to see if that changes their results. It also hopes to use the data to make predictions about the fossil record, such as what features might be found in human fossils from a particular area.
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