A new analysis of ancient human DNA shows that people moved and chose their reproductive partners along complex social networks that spanned large swaths of Africa between 80,000 and 20,000 years ago, according to a study. study co-directed by Yale anthropologist Jessica Thompson.
These movements continued for tens of thousands of years before people started living more locally and long before they started farming and raising livestock.
The study, published Feb. 23 in the journal Nature, provides the first genetic evidence of major demographic changes among hunter-gatherer populations in East and South-Central Africa during the last ice age. The analysis includes the oldest DNA extracted from ancient human remains in Africa and the oldest from anywhere in the tropics.
The results reveal that these foragers shared genetic connections across an area that now stretches thousands of miles from Ethiopia to South Africa, and into the central African rainforest. They also support hypotheses developed by archaeologists that demographic changes accompanied well-documented changes in material culture that occurred in the later Stone Age, starting around 50,000 years ago, said Thompson, assistant professor of anthropology at Yale School of Arts and Sciences.
“The first innovation of things like personal ornaments and clothing took place in Africa as early as 130,000 years ago, but around 50,000 years ago we begin to see an overall increase in the complexity and diversity of objects made by Africa. ‘man,” Thompson said. “Seeing this widespread genetic mixing shows that people were really moving around a lot, exchanging their genes and their ideas.”
The study’s 41 co-authors include researchers from institutions in Malawi, Kenya, Zambia, South Africa and Tanzania. Thompson helped lead the research, along with researchers from Harvard University, Rice University and the University of Alberta. (Watch a research video.) Yale graduate student Alex Bertacchi is a co-author of the paper and assisted Thompson in the 2019 digs in Malawi that recovered two of the first skeletons examined in the study. He is studying animal remains from the same site for his thesis.
The researchers analyzed the DNA of 34 ancient individuals to discern the ancestral relationships of people who lived throughout sub-Saharan Africa. Their analysis included new DNA from six individuals who lived between 5,000 and 18,000 years ago at five sites in southern eastern and central Africa. The three oldest skeletons – two excavated in Malawi and one in Tanzania – date from between 14,000 and 18,000 years ago, representing the earliest human DNA recovered in Africa. Additionally, the researchers analyzed the DNA of 28 previously studied ancient Africans, including newly improved genetic data from 15 of these individuals.
Recent DNA has limited utility in reconstructing the composition of ancient populations due to the enormous amount of demographic change that has occurred over the past 5,000 years as a result of the spread of organized food production – agriculture and animal husbandry. Understanding past population dynamics requires analyzing the DNA of hunter-gatherers who predated the rise of food production, Thompson explained. It’s a difficult task because DNA degrades rapidly in the heat and humidity of tropical central Africa, she said.
“At the start of this project I was told by colleagues that the sites would not be very old, or that DNA could not be recovered from them, or that modern DNA was sufficient to understand these ancient dynamics,” said Thompson. “None of that turned out to be true. There’s still plenty on this record to surprise us, if we just invest in research.
The three individuals Thompson’s team found in Malawi were infants. Thompson excavated them from sites that more than 70 years ago yielded other ancient human skeletons. Thompson was looking for artifacts to put the lives of these people into context when his team unearthed two small skeletons from a site. Their depth below the surface suggested they were significantly older than previous finds. Both skeletons had well-preserved ear bones, which are one of the best skeletal sources for recovering ancient human DNA.
“I had never found anything like it,” Thompson said. “When we saw that these tiny bones were preserved, we just couldn’t believe it. Their age alone was important, but when we saw these bones, we knew ancient DNA could also be a real possibility.
The ear bones visited Yale where they were analyzed in a micro-CT scanner, which captured detailed data on their composition that may support further research. The bones then traveled to the lab of study co-author David Reich, a professor of genetics at Harvard University, where their DNA was extracted and analyzed. All other remains have been entrusted to the Department of Museums and Monuments of Malawi.
Finding large amounts of Central African ancestry in individuals from eastern and southern Africa was a key finding, said Mary Prendergast, associate professor of anthropology at Rice University and lead author of the study.
“Clearly a lot of people were moving from what are now the central African rainforests to places to the south and east, which surprised us,” Prendergast said. “This suggests to archaeologists that we should pay more attention to Central Africa, which has been understudied due to limited funding, geographic challenges and geopolitical instability.”
Researchers estimate that the period of demographic change began 80,000 years ago and lasted until around 20,000 years ago, when the amount of movement and mixing between populations decreased and people began to live locally.
“It’s possible that by this point people have established networks and can move technology and information around without finding reproductive partners elsewhere,” said Elizabeth Sawchuk, postdoctoral fellow at the University of Alberta, Canada. , who helped lead the study. “Their social networks persisted and objects continued to move along them until about 5,000 years ago when this ancient structure was crushed by more recent demographic changes.”
The research team thanked their colleagues and curators from African museums for protecting and preserving the remains that formed the basis of the study.
“This work shows why it is so important to invest in the management of human remains and archaeological artefacts in African museums,” said Potiphar Kaliba, research director at the Department of Museums and Monuments of Malawi and co-author of the study, adding that some of the skeletons provided usable genetic information despite being excavated half a century ago.