Interbreeding Drove Evolution

Tens of thousands of years ago, anatomically modern humans shared the planet with Neanderthals, and things got frisky.

Archaeological and genetic evidence shows that modern humans of European and Asian descent received one to four percent of their DNA from Neanderthals.

Anatomically modern humans left Sub-Saharan Africa and migrated into Europe between 60,000 and 70,000 years ago. Until recently, the scientific community generally agreed that the arrival of early humans precipitated the extinction of pre-existing Neanderthal populations, due to violent conflict and the spread of disease.

A new comprehensive study of over 400 samples of Neanderthal remains reveals, however, that the two species may have coexisted in Europe for as long as 5,400 years — ten times longer than previously thought.

“We can set aside the idea of a rapid extinction of Neanderthals caused solely by the arrival of modern humans,” said Thomas Higham, lead researcher and a professor at the University of Oxford. “Instead we see a more complex process in which there is a much longer overlap between the two populations where there could have been exchanges of ideas and culture.”

This prolonged cohabitation also resulted in another, more devious consequence: humans and Neanderthals mated with one another on a regular enough basis that genetic remnants of these encounters remain buried in our DNA today.

In 2002, researchers uncovered a human mandible in Pestera cu Oase, the famed “cave of bones” in southwestern Romania. With carbon dating placing the bone’s age between 37,000 and 42,000 years old, it is the oldest modern human specimen found in Europe.

The Oase individual provides an unparalleled opportunity for researchers to study the relationship between early humans and Neanderthals. Although the prehistoric jawbone only contained trace amounts of DNA, a team of scientists at Harvard Medical School’s Reich Laboratory were able to determine that as much as six to nine percent of the Oase individual’s genome is derived from Neanderthals, far more than any modern human ever before sequenced.

These incredible findings indicate that whoever the jawbone belonged to had a Neanderthal ancestor only four to six generations back.

“In the last few years, we’ve documented interbreeding between Neanderthals and modern humans,” said David Reich, a professor of genetics and study co-author, “but we never thought we’d be so lucky to find someone so close to that event.”

From where we stand in the present day, it may seem difficult to look back across a yawning chasm of 40,000 years and imagine that people just like us interbred with a different species. But hybridization is not entirely uncommon in the animal kingdom, and humans may have benefitted evolutionarily from such interactions.

As ancient humans left Africa and moved into new environments, mating with other hominid species may have provided them with the opportunity to adapt more quickly to the drastic changes in their surroundings. In fact, we owe the robust immune systems that we enjoy today in part to genes passed on to us from other hominid species.

In 2011, a group of researchers focused on a class of essential disease-fighting genes called HLA in hopes of better understanding how we inherited certain immune defenses. They found that an archaic variation of HLA genes actually originated in Neanderthals and Denisovans (another ancient hominid species) before appearing in humans, proving that intermingling with extinct lineages did in fact shape our evolution.

According to researcher and immunogeneticist Peter Parham, “The same is likely to be true for the reproductive system and the nervous system. The power of future studies will be enhanced by the study of larger numbers of Neanderthal and Denisovan individuals.”

As DNA cascades through generations, its segments break up and recombine so that ancestral genetic information blends with that of more recent individuals. So, why do we only find evidence of Neanderthal ancestors in the DNA of modern humans of European and Asian descent? What about hybridizations that may have occurred before the exodus from Sub-Saharan Africa?

Quite simply, scientists do not yet have the ability to sequence DNA that ancient. Until innovations in genomics allow for the analysis of much older human specimens, interbreeding between the earliest anatomically modern humans in Africa and their extinct hominid counterparts remains speculative.

For the rest of us, we may now have an excuse for moving a bit more slowly in the morning. We are part Neanderthal, after all.

Photos courtesy of Wikimedia Commons.