In Neanderthal DNA, signs of human migration

With fossils and DNA, scientists are piecing together a picture of humanity’s beginnings, an origin story with more twists than anything you would find at the movie theatre. The expert consensus now is that Homo sapiens evolved at least 3,00,000 years ago in Africa. Only much later — roughly 70,000 years ago — did a small group of Africans establish themselves on other continents, giving rise to other populations of people today. To Johannes Krause, the director of the Max Planck Institute for Human History in Germany, that gap seems peculiar. “Why did people not leave Africa before?” he asked. After all, he observed, the continent is physically linked to the Near East. “You could have just walked out.”

A comprehensive picture

In a recent study published in Nature Communications, Johannes and his colleagues report that Africans did indeed walk out — over 2,70,000 years ago. Based on newly discovered DNA in fossils, the researchers conclude that a wave of early Homo sapiens made their way from Africa to Europe. There, they interbred with Neanderthals. Then the ancient African migrants disappeared. But some of their DNA endured in later generations of Neanderthals. “This is now a comprehensive picture,” Johannes said. “It brings everything together.”

As a graduate student in the mid-2000s, Johannes travelled to museums to drill bits of bone from Neanderthal fossils. In some of them, he and his colleagues managed to find fragments of DNA that they could study. The vast majority of our genes are in a pouch in each cell called the nucleus. We inherit so-called nuclear DNA from both parents. But we also carry a small amount of DNA in the fuel-generating factories of our cells, called mitochondria. Years ago, Johannes and his colleagues started their search for ancient Neanderthal genes in a fossil by looking for mitochondrial DNA. After discovering mitochondrial DNA in some fossils, they later managed to find nuclear DNA.

The genes held some surprises. For example, bits of DNA in living people of non-African ancestry come from Neanderthals. When modern humans expanded out of Africa, they seem to have interbred several times with Neanderthals. Those children became part of human society, passing on their genes. But a finger bone and a tooth from a Siberian cave called Denisova left Johannes and his colleagues with a baffling puzzle. Inside those fossils, the scientists found sequences of mitochondrial DNA that were not human or Neanderthal, but something else — a distant branch of the family tree.

Later, the researchers recovered the nuclear DNA from the Denisovan finger bone, which showed Denisovans and Neanderthals were more closely related to each other. As scientists found ancient DNA in more fossils, our history has come into sharper focus. Scientists now estimate that the common ancestor of modern humans, Neanderthals and Denisovans, lived between 5,50,000 and 7,65,000 years ago. About 4,45,000 to 4,73,000 years ago, that common ancestor’s descendants split into two lineages. One eventually led to modern humans, while the other led to Neanderthals and Denisovans.

After years of investigation, however, Johannes still did not understand why the nuclear DNA and mitochondrial DNA of Neanderthals seemed to have different histories. The former pointed to a link with Denisovans, the latter to humans. In 2013, one of Johannes’ graduate students, Cosimo Posth, examined a Neanderthal fossil from a German cave called Hohlenstein-Stadel. He was able to reconstruct all of its mitochondrial DNA.

Parting ways, genetically

Cosimo estimated that the Neanderthal fossil was 1,20,000 years old and that it belonged to a branch of the Neanderthal family tree with a long history. He and his colleagues determined that all known Neanderthals inherited their mitochondrial DNA from an ancestor who lived 270,000 years ago. The common ancestors of Neanderthals and Denisovans spread across Europe and Asia over half a million years ago. Gradually the eastern and western populations parted ways, genetically speaking. In the east, they became Denisovans. In the west, they became Neanderthals. The 4,30,000-year-old fossils found at the Spanish cave, Sima de los Huesos, captures the early stage of that split. At some point before 2,70,000 years ago, African humans moved into Europe and interbred with Neanderthals. Their DNA entered the Neanderthal gene pool.

Over many generations, most of that new DNA disappeared. But the mitochondrial DNA survived, passed down from mothers to their children. In fact, eventually all the Neanderthals inherited it, for some reason discarding the mitochondrial DNA that the species once had. Cosimo said it was possible that early members of our own species moved from North Africa into Europe. Supporting this idea was the discovery reported recently of fossils of Homo sapiens in Morocco dating back 3,00,000 years. But Cosimo said it was too soon to rule out another possibility: that these migrants belonged to another species in Africa closely related to us that scientists have yet to document.

The New York Times

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