Our species arose in Africa more than 300,000 years ago, with the oldest-known Homo sapiens fossils discovered at a site in Morocco called Jebel Irhoud, located between Marrakech and the Atlantic coast.
But the scarcity of Homo sapiens fossils from early in our evolutionary history and the geographical spread of those remains in Africa in places like Ethiopia and South Africa have made it difficult to piece together how our species emerged and dispersed across the continent before trekking worldwide. A new study tapping into genome data from modern-day African populations is offering insight into how this may have unfolded.
The research indicated that multiple ancestral groups from across Africa contributed to the emergence of Homo sapiens in a patchwork manner, migrating from one region to another and mixing with one another over hundreds of thousands of years. It also found that everyone alive today can trace their ancestry to at least two distinct populations that were present in Africa dating back about a million years.
The findings did not support a longstanding hypothesis that a single region in Africa gave rise to Homo sapiens or a scenario involving mixture with an unidentified closely related species in the human evolutionary lineage within Africa.
"All humans share relatively recent common ancestry, but the story in the deeper past is more complicated than our species evolving in just a single location or in isolation," said University of Wisconsin-Madison population geneticist Aaron Ragsdale, lead author of the study published this week in the journal Nature.
The ancestral groups were likely spread across a geographic landscape in a population structure that, Ragsdale said, "was 'weak,' meaning that there was ongoing or at least recurrent migration between groups, and this maintained genetic similarity across ancestral populations."
Because of the paucity of fossil remains and archaeological evidence, the researchers turned to genome data from living people to find clues about the past. They examined genome data from 290 people, mostly from four geographically and genetically diverse African peoples, to trace the similarities and differences between the populations and identify genetic interconnections over hundreds of thousands of years.
These included: 85 individuals from a West African group called the Mende from Sierra Leone; 44 individuals from the Nama Khoe-San group from southern Africa; 46 individuals from the Amhara and Oromo groups in Ethiopia; and 23 individuals from the Gumuz group, also from Ethiopia. Genome data also was examined from 91 Europeans to account for post-colonial era influence and from a Neanderthal, the extinct human species that was concentrated in Europe until about 40,000 years ago.
The fossil record is scanty in the time period that would be most informative about the emergence and spread of Homo sapiens, and there is no ancient DNA from skeletal or dental remains from these time periods, the researchers said.
"While we find evidence of anatomically modern human remains and artifacts in different parts of Africa, they are so sparse in space and time that it is difficult to understand their relationships with each other, and with us," said study geneticist and study co-author Simon Gravel of McGill University in Montreal. "Were they related to each other? Are they related to our ancestors, or were they local populations who went extinct?"
"Genetic data was inherited from a continuous chain of transmissions dating back to well before the origins of modern humans. The relatedness among contemporary humans contains a lot of information about this chain of events," Gravel added. "By building models of how these transmissions occurred, we can test detailed models that relate past populations to present-day populations."