Shining new light on how humans are related to Neanderthals

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 theater.

The expert consensus now is that Homo sapiens evolved at least 300,000 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 in an interview. After all, he observed, the continent is physically linked to the Near East. “You could have just walked out.”

In a study published Tuesday in Nature Communications, Krause and his colleagues report that Africans did indeed walk out — over 270,000 years ago.

Based on newly discovered DNA in fossils, the researchers conclude that a wave of early Homo sapiens, or close relatives of our species, 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,” Krause said. “It brings everything together.”

Since the 1800s, paleontologists have struggled to understand how Neanderthals are related to us. Fossils show that they were anatomically distinct, with a heavy brow, a stout body and a number of subtler features that we lack.

The oldest bones of Neanderthal-like individuals, found in a Spanish cave called Sima de los Huesos, date back 430,000 years. More recent Neanderthal remains, dating to about 100,000 years ago, can be found across Europe and all the way to southern Siberia.

Then, 40,000 years ago, Neanderthals vanish from the fossil record.

As a graduate student in the mid2000s, Krause traveled 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.

Scientists who study ancient genes search for two kinds of genetic material. 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. We inherit mitochondrial DNA only from our mothers, because a father’s sperm destroys its own mitochondrial DNA during fertilization.

Years ago, Krause 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 Krause 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. The Neanderthal mitochondrial DNA was much closer to our own. Later, the researchers recovered the nuclear DNA from the Denisovan finger bone, which showed Denisovans and Neanderthals were more closely related to each other.