Are Africans really the only modern people?

Study on the origin of mankind : Are we all from northern Botswana?

White crust covers the ground. Individual bushes grow here, now and then a tree. There is hardly any protection from the sun, the horizon seems endless. The Makgadikgadi salt pans in northern Botswana are desolate and barren today. But this area in southern Africa may have played a crucial role in human history. Here, south of the Zambezi River, the early humans started their migrations all over the world. This is what an international team of researchers reports in the current issue of the journal "Nature".

It has long been accepted that Africa must have been the cradle of modern humans: This is where the ancestors of today's humans lived, who now inhabit the entire planet. That was around 200,000 years ago - the physique of Homo sapiens has not changed significantly since then.

But the early humans lived in completely different parts of the continent. Fossil finds so far indicated that the origin of modern humans lay in the east of the continent. In contrast, genetic analyzes indicated southern Africa. As recently as March, researchers in another study reported that human populations first increased sharply in East Africa around 70,000 years ago and from there reached other continents via the Arabian Peninsula. But where did the people come from before they came to East Africa?

Where there is salt today, there used to be a huge lake

This is what a team around the Australian human geneticist Vanessa Hayes from the Garvan Institute of Medical Research in Sydney wants to have reconstructed. On the one hand, the researchers analyzed genetic data from more than 1200 people living in this region today. On the other hand, they compared the distribution of today's cultures and languages ​​in the region and also simulated the climatic developments 200,000 years ago.

Because the area around today's Makgadikgadi salt pans was not always inhospitable. Its condition as you can see it today is only around 10,000 years old. Until about 200,000 years ago, on the other hand, a huge lake stretched across the lowlands. It must have been about twice the size of today's Lake Victoria. Its banks were covered with lush greenery, giraffes, lions and zebras lived here.

Around 200,000 years ago, the landscape changed with the beginning of a warm period. "When the early humans first got here, the lake was splitting into smaller lakes and extensive wetlands," says Hayes. It became a fertile region that offered hunters and gatherers a good livelihood. People settled in this area and did not leave it for a long time - also because the surrounding area was even drier and more inhospitable than it is today.

These people could not have undertaken great migrations outside of the wetland: Even today, individual individuals from hunter-gatherer communities do not move far away from their group, says Hayes. And the animals that lived in this area apparently did not migrate at that time. The genome sequences of these people have barely changed for over 70,000 years, the researchers' data show. "That means these early people must have stayed in the region and not left," says Hayes.

Green corridors made the hike possible

But about 130,000 years ago the climate changed again - and with it the landscape. To the north-east of the area, which had previously been desert, there was now increased rainfall. Plants and animals spread in this direction - and eventually humans too. On their wanderings they finally reached East Africa. Another 20,000 years later, such green corridors also emerged in the landscape towards the southwest, so that people from the Makgadikgadi wetland also reached the south coast of Africa.

The researchers base their analyzes primarily on the genome data that they used in the study. They examined a certain part of the genetic make-up of 1217 living representatives of various African tribes: the mitochondrial DNA. This is the name given to the genetic material found in the mitochondria of every cell in the body. The researchers could not use the genome of fossils for this study because it is often too poorly preserved.

The mitochondrial DNA does not provide the main information for the body plan, as is the case with the cell nucleus DNA. But it has a great advantage when it comes to evolutionary studies: it is only inherited from the mother's side. By comparing the mitochondrial DNA of different people, their maternal lineage can be reconstructed well. "And women must have had men," says Hayes - that's why mitochondrial DNA serves as a guide to general ancestry.

One researcher questions the conclusions

Nevertheless, the researchers' conclusions should be viewed with caution, says Carina Schlebusch. The evolutionary biologist at the University of Uppsala in Sweden also researches the origins and origins of modern people and was not involved in the study.

The number of analyzed data sets is relatively small, she says - and the result is therefore high. In addition, the methodology is a standard procedure. "But the mitochondrial DNA represents only a very small part of the human genome and basically only tells us the story of a single ancestor," says the evolutionary biologist.

But the human genome contains various parts that may have been added in other ways. Therefore, according to Schlebusch, it is impossible to draw general conclusions about human origins by looking only at mitochondrial DNA. "This study therefore only shows the origin of the mitochondrial DNA - and nothing more," says Schlebusch.

Other studies, on the other hand, which refer to the entire genome, rather point to different places of origin of humans in Africa. "But these studies are completely omitted by the authors." The general view of geneticists and paleontologists, however, is that the human origins in Africa include different regions - and are not traced back to a single location.

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