According to two separate research teams, even though the Neanderthal species of humans vanished from existence around 30,000 years ago, their interbreeding with Homo sapiens in prehistoric times may still be having an effect on modern human health.
Researchers say that this interbreeding with Neanderthals occurred after humans left Africa about 100,000 years ago. However, according to David Reich, senior author of the Nature study, not all sexual intercourse between these two groups produced offspring. The two species were “at the edge of biological incompatibility,” says Reich, meaning that offspring between the two might have been infertile or having less than optimal fertility.
Certain human genes, which were derived from our Neanderthal ancestors, also appear to increase our risk for medical conditions such as diabetes, lupus, Crohn’s disease, cirrhosis of the liver and chronic depression, scientists say. These findings are based upon genetic studies of people in modern-day East Asia and Europe.
The results of interbreeding were not all bad news, however. It is likely that the DNA that we inherited also helped to protect us from the ravages of Ice Age temperatures and the weaker sunlight that was faced upon migration out of Africa. Most likely these protective effects occurred due to changes in hair and skin which the Neaderthals had already developed and were able to pass on to human-Neanderthal children.
Both research teams found that Neaderthal DNA has a much broader influence on modern genomes than was previously thought. Previous studies seemed to indicate that non-African humans have about 1 to 3 percent Neanderthal DNA in their genome. However, when hundreds of human genomes were examined from various places around the world, it was found that Neanderthals had a much greater impact than was previously thought. In the study found in the journal Science, which examined genomes from 665 modern-day humans, it was found that there was a 20 percent overlap between the human and Neanderthal genomes. In addition, the study found in Nature, which analyzed 1,004 modern-day human genomes, found an even higher figure of 30 percent.
Geneticist Joshua M. Akey, who led the Science study, said these higher figures came about because the 1 percent of Neaderthal sequences that one individual has might be in different places than the 1 percent that another person has. Akey adds that depending upon which computer model is proven to work better, the overlap could go as high as 60 percent if every modern human genome were entered into the analysis.
Akey further adds that it’s very difficult to label Neanderthal interbreeding as being particularly good or bad for humans. Even though modern-day humans might have more risk for certain diseases, it was also very beneficial for them to be able to better cope with the harsh affects of Ice Age climate change.
The team who conducted the Nature study, led by Sriram Sankararaman, looked for strings of human genetic code that were present in their Neanderthal genome sequence, but were not present in sequences from West Africa, which should theoretically not have had any Neanderthal DNA in them. This helped them to be able to identify which ones likely came from interbreeding between the two species.
Akey’s team took a different approach, using statistical methods to sort out which areas of the modern-day genome probably came from Neanderthals. Then then checked those DNA strings against the Neanderthal genome to see if they matched.
Taken together, the results from these two different techniques can be used to study how interbreeding with Neanderthals, as well as our other ancient ancestors, has influenced our modern-day genome. From this data, scientists can also sort out how interbreeding has affected modern human health.
By Nancy Schimelpfening