Genetic analysis of ancient African skeletons has resulted in an 18,000-year-old human genome being sequenced, providing clues as to how the continent’s former occupants lived, traveled, and reproduced.

In the journal Nature, researchers explain that early African populations began to form extensive social networks about 50,000 years ago, yet became more fragmented around the time of the Last Glacial Maximum.

Archaeological evidence suggests that ancient Africans first started trading obsidian and other symbolic items as early as 300,000 years ago, yet it wasn’t until the Later Stone Age that long-distance exchange really intensified. Researchers have often speculated that this transfer of goods may have gone hand-in-hand with the movement of people, yet a scarcity of genetic samples has made this theory impossible to confirm.

Generally, DNA can’t survive for long periods in Africa’s hot and humid climate, and researchers had never previously managed to sequence a sub-Saharan African human genome older than 9,000 years. However, the authors of this latest study recovered genetic material from six individuals buried in Malawi, Tanzania, and Zambia who lived between 5,000 and 18,000 years ago.

They also examined records of 28 previously reported individuals recovered from burial sites across the continent, generating improved genetic data for 15 of them. After analyzing the DNA, the researchers discovered that these 34 individuals were descended from three distinct source populations originating in northeastern, central and southern Africa.

This implies that the continent was once inhabited by three separate groups, who must have remained disconnected from one another for vast amounts of time. However, the intermingling of these three lineages at every burial site indicates an exchange of genetic information between the three populations began sometime before 20,000 years ago.

According to the study authors, this process probably started roughly 50,000 years ago, as it is around this time that the archaeological records begin to show an increase in the movement of goods across long distances. As trading networks spread across the continent, it seems likely that people also began having children with partners from far-flung regions.

However, the genetic data also reveals that this long-range DNA exchange decreased considerably from about 20,000 years ago, indicating that people began reproducing with their close neighbors from that point onwards. According to the study authors, this timeframe coincides with the Last Glacial Maximum, when the changing climate may have restricted the movement of people and forced populations to become more sedentary.

“At first people found reproductive partners from wide geographic and cultural pools,” explained study author Jessica Thompson in a statement. “Later, they prioritized partners who lived closer, and who were potentially more culturally similar.” 

"Maybe it was because by that point, previously established social networks allowed for the flow of information and technologies without people having to move," adds co-author Elizabeth Sawchuk.

Importantly, the discovery of such ancient DNA in sub-Saharan Africa allows researchers to confirm some widely-disputed hypotheses regarding how and when the continent’s ancient inhabitants first began traveling long distances to find partners.

Previously, archaeologists have had to rely on material artifacts to theorize about the social networks that may have existed across the continent, yet the addition of genetic data allows scientists to paint a more robust picture.

“It has been difficult to reconstruct events in our deeper past using the DNA of people living today, and artifacts like stone tools and beads can’t tell us the whole story,” said Sawchuck. “Ancient DNA provides direct insight into the people themselves, which was the missing part of the puzzle.”