Wednesday, July 11, 2018
Humans evolved in partially isolated populations scattered across Africa
The textbook narrative of human evolution casts Homo sapiens as evolving from a single ancestral population in one region of Africa around 300,000 years ago. However, in a commentary published July 11 in the journal Trends in Ecology & Evolution, an interdisciplinary group of researchers concludes that early humans comprised a subdivided, shifting, pan-African meta-population with physical and cultural diversity. This framework better explains existing genetic, fossil, and cultural patterns and clarifies our shared ancestry.
"In the fossil record, we see a mosaic-like, continental-wide trend toward the modern human form, and the fact that these features appear at different places at different times tells us that these populations were not well connected," says Eleanor Scerri, a British Academy postdoctoral fellow in archaeology at the University of Oxford and the Max Planck Institute for the Science of Human History. "This fits with a subdivided population model in which genetic exchanges are neither random nor frequent. This allows us to start detailing the processes that shaped our evolutionary history."
Explaining this poor connectivity was a series of shifting rivers, deserts, forests, and other physical barriers separating these subpopulations, as highlighted in the ecological record. "These barriers created migration and contact opportunities for groups that may previously have been separated, and later fluctuation might have meant populations that mixed for a short while became isolated again," says Scerri.
The theory that there was mingling and isolation of subpopulations from the southern tip to the northern coasts of Africa is a much better fit with the fossil and genetic data than is a single population model. Examination of H. sapiens fossils paired with inferences made from contemporary DNA samples suggested levels of early human diversity that supported the researchers' shifting subdivided population model.
"For the first time, we've examined all the relevant archaeological, fossil, genetic, and environmental data together to eliminate field-specific biases and assumptions and confirm that a mosaic, pan-African origin view is a much better fit with the data that we have," says Scerri. "To understand our genetic and cultural diversity or where being human comes from - our behavioral flexibility and biological plasticity - we have to look at an ancient history of population subdivision and diverse ecologies across Africa."
Moving forward, this research will allow our models of human evolutionary history to reject the simple linear progression from what might be termed "archaic morphology" toward a recognizably human form in favor of a more accurate account of the complexity and irregularity involved in our evolution and an acknowledgment of a pan-African origin of our species.
"In bringing together people from such diverse fields, we've arrived at a place where we can begin to address some key questions about our shared ancestry and even emerge with new questions we haven't known to ask before," Scerri says. "We are an evolving lineage with deep African roots, so to understand this history, we must re-examine evidence from diverse sources without a priori conceptions."