Africa is the cradle of mankind. We have acquired a wealth of important information about our ancestors on this continent, and with each new fossil and archaeological discovery, we have a better understanding of human history.
In the past, much of the research on Africa focused on 60,000 to 80,000 years ago, and scientists are more concerned with the story of Homo sapiens migrating to other continents during this period. But what happened in Africa after that time? Why don't we learn more about the people who remain in Africa?
Recently, a study conducted by an international interdisciplinary team helped answer these questions. The study gained insight into the demographic structure of ancient foragers in sub-Saharan Africa by sequencing and analyzing the ancient DNA (aDNA) of humans who lived between 5,000 and 18,000 years ago. This genetic information can help shed light on the social story of humanity in Africa, telling how they migrated and blended into the land long ago. The study was recently published in Nature.
Trace the human history of Africa
From about 300,000 years ago, the paleo-humans of Africa did not look much different from us, they were the earliest anatomically modern humans, and they began to act in a very "human" way.
They built new types of stone tools, began transporting raw materials hundreds of kilometers away, and most likely established a trading network. By 120,000 to 140,000 years ago, people would have made clothes from animal skins and decorated themselves with perforated sea shell beads.
While early innovations emerged in a piecemeal fashion, the broader shift occurred about 50,000 years ago. New types of stone tools and bone tools became more common, and people began to make and exchange beads made from ostrich eggshells. While most petroglyphs in Africa are not exactly dated and are heavily weathered, the increase in ochre pigments in archaeological sites hints at an explosion of art. What led to this shift, the so-called Late Stone Age transition, has always been an archaeological mystery.
Beads made from ostrich eggshells were widely used in ancient social networks. | Image credit: Jennifer Miller, CC BY-ND
Archaeologists reconstruct past human behavior primarily through what people left behind, including their diets, tools, ornaments, and sometimes even their bodies. However, it is difficult to study ancient demography, that is, how ancient populations changed, from the archaeological record alone. That's where DNA can help. When genetics is combined with evidence from archaeology, linguistics, and oral and written history, scientists can piece together how people migrate and interact based on which groups have genetic similarities.
However, DNA from modern times does not tell all the stories in its entirety. Over the past few thousand years, Africa's population has changed dramatically. These processes cause some lineages to disappear, allowing some lineages to fuse together to form new populations.
Using modern DNA to reconstruct ancient genetic landscapes is like reading a letter abandoned in the rain, some of which is obscure and some of which is completely invisible. Researchers need a DNA from human remains from archaeological excavations to explore human diversity in different regions and eras and to understand what factors have shaped this diversity.
Unfortunately, aDNA in Africa is particularly difficult to restore because it spans the equator and heat and humidity allow DNA to degrade naturally. While the oldest aDNA from Eurasia is about 400,000 years old, most sequences from sub-Saharan Africa to date are no more than 9,000 years old.
Travel back in time to the older past
Because each person carries the genetic "legacy" of generations of ancestors, scientists were able to use the DNA of individuals who lived 400 to 18,000 years ago to explore how people from as far back as the past 50,000 to 80,000 interacted. This allowed scientists to test for the first time whether demographic changes played a role in the late Stone Age transition.
All published paleogenous maps, the size of the black dots representing the number of individual genomes. Blue dots represent late Stone Age foragers compared to the individuals in our study. Red Star represents the first individual reported in our study. The minimap in the lower left highlights the gap between Africa and the rest of the world in the published ancient genome. Very little ancient DNA has been preserved between the Tropic of Cancer. | Image credit: Mary Prendergast; basemaps by Natural Earth, CC BY-ND
The team sequenced aDNA from six individuals buried in present-day Tanzania, Malawi and Zambia, comparing these sequences to the aDNA of 28 individuals previously studied. They also made improved new DNA data for 15 of them.
This creates the largest genetic dataset to date used to study the population history of ancient African foragers( those who made a living hunting, gathering, or fishing). Researchers use it to explore the demographics that existed in the past.
The study found that people did change the way they moved and interacted before and after the late Stone Age transition. Although far apart and years apart, all of the ancient individuals in the study were descendants of three groups of people associated with ancient and present-day Eastern, Southern and Central Africans. The ancestors of East Africans reached as far south as Zambia, while the ancestors of southern Africans extended north of Kenya, suggesting that people were migrating long distances and producing offspring with people far from where they were born. The only way this demographic structure emerges is when people migrate long distances over thousands of years.
In addition, studies have shown that almost all ancient East Africans shared a large amount of genetic variation with hunter-gatherers living in the rainforests of Central Africa today, making ancient East Africa truly a genetic melting pot. As can be seen from the findings, this mixing and movement occurred about 50,000 years ago, when the forager population in Central Africa was significantly divided.
The researchers also noticed that about 20,000 years ago, foragers in some african regions would almost only look for a local partner. This practice must have been very obvious and went on for a long time. This phenomenon is particularly pronounced in Malawi and Zambia, where the closest relationship exists almost exclusively between people buried in the same place at the same time.
This means that people have started a "local life" compared to before, but the reasons behind it are not clear. Perhaps during the last glacial period, around 11,500 to 26,000 years ago, environmental changes tended to forage closer to home, or perhaps well-designed exchange networks reduced the need for people to travel with their belongings.
Researchers unearthed two ancient individuals in Malawi and conducted detailed aDNA studies. | Image credit: Jacob Davis, CC BY-ND
Or maybe a new group identity may have emerged, reorganizing the rules of marriage. If so, we will also see a trend toward the diversification of artisanals and other traditions, with specific types likely to congregate in different regions. In fact, this is exactly what archaeologists have discovered, a trend known as regionalization. This phenomenon may affect not only cultural traditions, but also the flow of genes.
New data creates new problems
As with many previous studies, aDNA research raises no less questions than answers. These results have prompted anthropologists to rethink how these regions of Africa were interconnected in the distant past.
Moreover, while genetic evidence supports a major demographic shift in Africa after 50,000 years ago, the key driver remains a mystery. Determining what triggered the Late Stone Age transition also requires a more detailed examination of the local environmental, archaeological, and genetic records to understand exactly how the process developed in sub-Saharan Africa.
The study also reminds scientists that researchers can still learn a lot from the museum's collection of ancient remains and artifacts. While some of the human remains in this study were excavated over the past decade, others have been preserved in museums for half a century.
#创作团队:
Elizabeth Sawchuk (Banting Postdoctoral Fellow and Adjunct Professor of Anthropology, University of Alberta)
Jessica Thompson (Assistant Professor of Anthropology, Yale University)
Mary Prendergast (Associate Professor of Anthropology, Rice University)
Compiled: Gaviota
Typography: Wenwen
#参考来源:
https://theconversation.com/ancient-dna-helps-reveal-social-changes-in-africa-50-000-years-ago-that-shaped-the-human-story-175436
https://news.rice.edu/news/2022/ancient-dna-reveals-surprises-about-how-early-africans-lived-traveled-and-interacted
#图片来源:
Cover image: Nina R/Wikimedia Commons, CC BY
首图：Nina R/Wikimedia Commons, CC BY