Interbreeding with Neanderthals was important for Human Immune System Evolution

While popular depictions of human evolution show a straight progression from ape to hominid to modern Homo sapiens, it has long been known that human evolution was actually a branching tree. Rival species of hominids coexisted and competed until modern humans outlived or exterminated their rivals by about 30,000 years ago. New data suggests that some of those branches of the human family tree live on in our DNA, specifically in certain genes in the immune system which may have been key to our success as a species. The study was published in this week's Science magazine by a group of researchers from Stanford and other universities. In it they report that an important set of genes in the human immune system preserves evidence of ancient interbreeding between humans and two other species of hominids, Neanderthals and Denisovans. Using gene sequencing, the researchers were able to identify an allele known as HLA-B*73 which is far more distantly removed from other typed of HLA genes than those genes are from each other. In essence, using a form of statistical analysis paired with comparisons to living human relatives (chimpanzees and gorillas), they could determine that the allele evolved in isolation from other human genes before being reintroduced by interbreeding sometime in our ancient past. Similar analyses of other alleles in the same region of the human genome reveals another likely cross-breeding event, with Neanderthals, at a different geographical location. Human Family Tree - Nature West Asians are the most likely population to exhibit this gene, with a smaller prevalence in parts of Africa and token appearance in other populations around the world. This pattern suggests that the interbreeding event between humans and Denisovans took place in West Asia and spread to Africa and elsewhere over the following generations. Another interbreeding event between humans and Neanderthals happened somewhere in Eurasia, probably in Northern Europe where the Neanderthals were most established. Previous work has suggested that Neanderthal genes make up between 1 and 6% of the human genome thanks to past interbreeding, but up to 50% of the genes examined by this study may have been inherited from those distant cousins. Such a high proportion suggests that these genes were important to the evolution of our immune systems and to our species' success in the face of constant threat from viral, bacterial, and parasitic infections.