Ancestors of modern humans interbred with Neanderthals and another extinct line of humans, known as the Denisovans, at least, four times. The multiple encounters may have given modern humans genes that made them immune to pathogens, researchers said.

The team at Binghamton University in New York identified fragments of Neanderthal and Denisovan DNA among the genomes of modern Melanesian people, which suggested that the three groups, modern humans, Neanderthals and Denisovans, shared a common ancestor who lived around 600,000 years ago, according to the New York Times.

A group of Neanderthals in a cave. Image credit: Tyler B. Tretsven.

“I am surprised that these Neandertal and Denisovan genomes made it out to this remote place,” said Andrew Merriwether, a molecular anthropologist at Binghamton. “We know people have been there for at least 48,000 years because we find human remains that go back that far, but no one has ever been able to connect them to any other place,” he added.

The first clues to ancient interbreeding have been surfacing since 2010 when scientists discovered that some modern humans, mostly Europeans, carry DNA that matched material recovered from Neanderthal fossils.

Later, the investigation went further and researchers found that the forebears of modern humans first encountered Neanderthals after expanding out of Africa more than 50,000 years ago.

Recently, it was found that the previous extinct group was not the only one who interbred with modern human ancestors. A finger bone discovered in a Siberian cave provided DNA from another group of ancient humans, the Denisovans. This finding later proved the existence of the common ancestor between the three species.

How often did they encounter with each other?

The findings are been qualified as another genetic nail in the coffin of the over-simplistic models of human evolution, said Carles Lalueza-Fox, a research scientist at the Institute of Evolutionary Biology in Barcelona who was not involved in the study.

Scientists have developed new ways to study the DNA of living people to tackle the mystery evolution holds. A database of 1,488 genomes from people around the world were analyzed by Joshua M. Akey, a geneticist at the University of Washington and his colleges.

They added the 35 genomes from people in New Britain and other Melanesian islands in an effort to learn more about Denisovans in particular. They found that all non-Africans in their study had Neanderthal DNA while the Africans had very little or none. The finding supported previous studies.

As the best explanation for the pattern, researchers concluded that the ancestor of human acquired Neanderthal DNA on three different occasions. Until now the encounters were suspected but lacked hard data.

When it came for the Melanesians, after a single interbreeding with Neanderthals, Dr. Akey found, their ancestors went on to interbreed just once with Denisovans, as well. The location is still unknown.

Source: The New York Times