Humans and Neanderthals only had sex for a brief period, but it still fundamentally changed our DNA

• Modern humans have a small amount of Neanderthal DNA, and those genes still impact our health today.
• Scientists think they've figured out when the two groups started interbreeding and swapping DNA.
• The results can help scientists understand how Neanderthal genes evolved in humans over millennia.

We might consider Neanderthals to be ancient and unknowable relatives, but humans were once on very intimate terms with them.

Our ancestors, for example, had babies with the shorter, stockier species and swapped DNA for thousands of years.

Now researchers think they've figured out when much of this interspecies intermingling took place, which could provide clues about why humans outlasted their ancient cousins.

Studying genomic data from both ancient and modern humans, scientists learned that Neanderthals and humans were having sex around 47,000 years ago then stopped less than 7,000 years later — relatively brief in terms of evolution.

"This study gives us the most accurate picture showing how some Neanderthals joined into the modern human gene pool, and then what happened to their genes afterward," John Hawks, an anthropologist at the University of Wisconsin–Madison who was not involved in the research, told Business Insider.

All humans have at least a little Neanderthal DNA, a 2020 study found. These genes may impact everything from metabolism to our risk for diabetes.

Finding out when humans acquired them can help scientists understand how these genes evolved and why they might have been beneficial to our ancestors and stuck around in our DNA for millennia.

When did humans and Neanderthals start interbreeding?

The basic story of human-Neanderthal relationships goes like this: We shared a common ancestor over 500,000 years ago then went our separate ways.

Then, around 75,000 years ago, Neanderthals were living in Asia and Europe when some humans started moving out of Africa. The two groups began having children together shortly after that, sometime between 60,000 and 50,000 years ago.

But by 40,000 years ago, the Neanderthals were extinct.

For this latest study, researchers at the Max Planck Institute for Evolutionary Anthropology and the University of California, Berkeley looked at DNA from 59 humans who lived between 45,000 and 2,200 years ago.

To figure out when our ancient human ancestors hooked up with Neanderthals, the researchers compared stretches of Neanderthal DNA in their genomes. Individuals from about 40,000 years ago had long stretches of gene-containing chromosomes that matched the other species.

The researchers saw that after just a few thousand years, these segments were shorter. This means those humans were more distantly related to Neanderthals, Hawks said.

Using computer software, the scientists created models to estimate how many generations had passed since a Neanderthal entered their family tree. From that, they set the starting date of human-Neanderthal hookups at around 47,124 years ago and estimated that interbreeding lasted for about 6,832 years.

"The conclusion is fairly strong," Hawks said, because the estimates fit both the earlier and later human genomes.

The Neanderthals that live on inside us

In addition to ancient human DNA, the researchers also studied the genomes of 275 modern people from various backgrounds to learn more about how humans and Neanderthal genes have evolved over the millennia.

"It is possible to track the fate of single parts of chromosomes that were once in a Neanderthal individual, came into that individual's modern descendants, and were passed on until the present day," Hawks said.

For example, the researchers found both current and ancient humans have Neanderthal genes that affect metabolism, immunity, and skin pigmentation.

They speculate that these characteristics may have benefited ancient humans who inherited them and then kept passing them on. One hypothesis is that paler skin would have helped increase Vitamin D levels in gloomier climates.

Other studies have linked Neanderthal genes to autoimmune diseases and diabetes. On the other hand, some DNA from the species seems to protect people from West Nile, hepatitis C, and SARS.

The study's results don't necessarily apply to everyone. For instance, no present-day people of African ancestry were included because only about 0.5 % of their DNA comes from Neanderthals.

However, almost everyone else in the world is about 2% Neanderthal, so learning more about the species' genes could shed light on many peoples' susceptibility to certain diseases.

The researchers published their work in a pre-print study, which hasn't been peer-reviewed yet.