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(THE CONVERSATION) Every person living on the planet today is descended from people who lived as hunter-gatherers in Africa.
The continent is the cradle of human origins and ingenuityand with each new fossil and archaeological discovery, we learn more about our common African past. This research tends to focus on when our species, Homo sapiens, spread to other landmasses 80,000–60,000 years ago. But what happened in Africa after that, and why isn’t more known about the people who stayed?
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Our 2022 study, led by an interdisciplinary team of 44 researchers based in 12 countries, help answer these questions. By sequencing and analyzing ancient DNA (aDNA) from people who lived 18,000 years ago, we have roughly doubled the age of sequenced aDNA from sub-Saharan Africa. And this genetic information helps anthropologistsAswe better understand how modern humans moved and intermingled in Africa long ago.
Tracing our human past in Africa
Beginning around 300,000 years ago, Africans who looked like us – the first anatomically modern humans – also began to behave in ways that seem very human. They made new types of stone tools and began to transport raw materials up to 250 miles (400 kilometers), probably through commercial networks. 140,000 to 120,000 years ago, people made animal skin clothing and started to adorn yourself with openwork marine shell beads.
While the first innovations appeared patchily, a more widespread change took place around 50,000 years ago – around the same time people started moving to places as far away as Australia. New types of stone and bone tools became common and people began shape and swap ostrich eggshell beads. And while most rock art in africa is undated and badly altered, an increase of ocher pigment at archaeological sites alludes to an explosion of art.
What caused this change, known as Later Stone Age transition, is a long-standing archaeological mystery. Why would certain tools and behaviors, which until then had appeared in a fragmentary way across Africa, suddenly become generalized? Did it have something to do with changes in the number of people or how they interacted?
The challenge of accessing the deep past
Archaeologists piece together human behavior in the past primarily through the things people left behind – the remains of their meals, tools, ornaments and sometimes even their body. These records can accumulate over thousands of years, creating views of daily livelihoods that are actually averages over long periods. However, it is difficult to study ancient demography, or how populations changed, from archaeological records alone.
This is where DNA can help. When combined with evidence from archaeology, linguistics, and oral and written history, scientists can piece together how people moved and interacted based on groups that share genetic similarities.
But the DNA of living people can’t tell the whole story. African populations have been transformed over the past 5,000 years by the spread of animal husbandry and agriculturethe city development, ancient pandemics and the ravages of colonialism and slavery. These processes caused some lines to disappear and bring the others togetherforming new populations.
Using today’s DNA to reconstruct ancient genetic landscapes is like reading a letter left in the rain: some words are there but blurry, and others have completely disappeared. Researchers need ancient DNA from archaeological human remains to explore human diversity in different places and times and to understand what factors shaped it.
Unfortunately, aDNA from Africa is particularly difficult to recover because the continent straddles the equator and heat and humidity degrade DNA. While the Eurasia’s oldest DNA is about 400,000 years oldall sequences from sub-Saharan Africa to date have been less than about 9,000 years old.
Breaking the “tropical ceiling”
Because each person carries a genetic heritage inherited from generations of their ancestors, our team was able to use the DNA of individuals who lived between 18,000 and 400 years ago to explore how people interacted as far back as 80,000 to last 50,000 years. This allowed us, for the first time, to test whether demographic change played a role in the later Stone Age transition.
Our team sequenced the aDNA of six people buried in what is now Tanzania, Malawi and Zambia. We compared these sequences to previously studied aDNA from 28 individuals buried at sites stretching from Cameroon to Ethiopia and into South Africa. We also generated new and improved DNA data for 15 of these people, trying to extract as much information as possible from the small handful of ancient African individuals whose DNA is well enough preserved to be studied.
This created the largest genetic data set to date for studying the population history of ancient African foragers – people who hunted, gathered or fished. We used it to explore the population structures that existed before the drastic changes of the last thousand years.
DNA weighs in on a long-running debate
We found that people actually changed the way they moved and interacted around the later Stone Age transition.
Although separated by thousands of miles and years, all of the ancient individuals in this study were descended from the same three populations related to ancient and present-day East, South, and Central Africans. The presence of East African ancestry as far south as Zambia and South African ancestry as far north as Kenya indicates that people moved long distances and had children with people located far from their Place of birth. The only way this population structure could have emerged is if people moved long distances over several millennia.
Moreover, our research has shown that almost all ancient East Africans shared a surprisingly large number of genetic variations with the hunter-gatherers who live in the rainforests of central Africa today, making ancient Africa of the East a veritable genetic melting pot. We could say that this mixing and shifting happened about 50,000 years ago, when there was a major split in the forage populations of Central Africa.
We also noted that the individuals in our study were genetically most similar to their closest geographic neighbors. This tells us that around 20,000 years ago, foragers in certain African regions found their mates almost exclusively locally. This practice must have been extremely strong and persisted for a very long time, as our results show that some groups remained genetically independent of their neighbors for several thousand years. This was particularly clear in Malawi and Zambia, where the only close relationships we detected were between people buried around the same time at the same sites.
We don’t know why people started “living locally” again. Changing environments as the last ice age peaked and declined around 26,000 to 11,500 years ago may have made it more economical to forage for food closer to home, or perhaps networks Elaborate exchanges have reduced the need for people to travel with items.
Alternatively, new group identities may have emerged, restructuring the rules of marriage. If so, we would expect to see artifacts and other traditions like rock art diversify, with specific types clustered in different regions. In effect, this is exactly what archaeologists find – a trend known as regionalization. We now know that this phenomenon has not only affected cultural traditions, but also gene flow.
Like always, aDNA research raises as many questions as answers. Finding Central African ancestry throughout eastern and southern Africa prompts anthropologists to reconsider how interconnected these regions were in the distant past. This is important because Central Africa has remained archaeologically understudied, in part because of political, economic, and logistical challenges that make research difficult.
Moreover, while genetic evidence supports a major demographic transition in Africa 50,000 years ago, we still don’t know the main drivers. Determining what triggered the Later Stone Age transition will require further examination of regional environmental, archaeological, and genetic records to understand how this process unfolded across sub-Saharan Africa.
Finally, this study is a stark reminder that researchers have still much to learn from ancient individuals and artifacts preserved in African museums, and highlights critical role of preservatives who manage these collections. While some human remains in this study have been found over the past decade, others have been in museums for half a century.
Even as technological advances push the temporal boundaries of aDNA, it is important to remember that scientists are only just beginning to understand human diversity in Africa, past and present.
This article is republished from The Conversation under a Creative Commons license. Read the original article here: https://theconversation.com/ancient-dna-helps-reveal-social-changes-in-africa-50-000-years-ago-that-shaped-the-human-story-175436.
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