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 Nature1.

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...

...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...
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...

a,b, In the two best-fitting parameterizations of early population structure, continuous migration (a) and multiple mergers (b), models that include ongoing migration between stem populations outperform those in which stem populations are isolated. Most of the recent populations are also connected by continuous, reciprocal migration that is indicated by double-headed arrows (labels matched to migration rates and divergence times in Table 1). These migrations last for the duration of the coexistence of contemporaneous populations with constant migration rates over those intervals. The merger-with-stem-migration model (b, with LL = −101,600) outperformed the continuous-migration model (a, with LL = −115,300). Colours are used to distinguish overlapping branches. The letters a–i represent continuous migration between pairs of populations, as described in Table 1.