Thank You, Cousin

Homo sapiens is the sole human-like critter to have survived millions of years of primate evolution. One by one, the other hominid species died out—either because they were less fit for their niche or because we may have out-competed them and drove them extinct. The last of our hominid cousins (Neanderthals) disappeared tens of thousands of years ago—leaving us to dominate the planet.

DNA analysis has gotten increasingly accurate in recent years, to the extent that we now know that at some point we interbred with those most recent cousins of ours. Some 4% of our DNA comes from Neanderthals (who left Africa far sooner than our human ancestors did, headed for west Asia and Europe). When we followed them to Eurasia, we coexisted with them for a few thousand years and even had sex with them. Similarly, the recently-discovered Denisovians also departed Africa ahead of us and migrated into East Asia. Since 6% of our DNA comes from them, some of our ancestors must also have mixed it up with the Denisovians.

It’s interesting to contemplate why this interbreeding may have occurred, especially when these two closely-related species looked so different from us humans. (No one yet knows what a Denisovian may have looked like, since we know about their existence only by the discovery of a finger of one of them. The DNA analysis was able to show that they were a separate species.) Our hunter-gatherer ancestors traveled in small bands and tended to look upon other bands of humans as alien and not quite human. So ancient humans must have regarded Neanderthals and Denisovians as even further from allowing any possible relationship, and yet the evidence suggests they did get it on.

Maybe on those long, cold nights during glacial periods, it may have been any port in an ice storm. I could see the temptation to snuggle up with a hairy, warm body as just too great to resist. A warm and fuzzy dog or cat can be a blessing when the temperature drops. It’s also possible that individuals of one species were captured by another species, and found some attraction coming over them, in time. I’m thinking of white people being captured by American Indians (or vice versa) 300 years ago, and then later mating. (I have intentionally avoided considering the possibility of forced sexual relations, although that may well have happened. I’d like to stay upbeat and focus on consensual coupling here.)

So what may have been the result of a little cross-species hanky-panky on the part of our deep ancestors? For one, it may have fostered a little peaceful interaction between these different hominid species—think of Pocahontas’s supposed intervention to save the life of John Smith and later marrying Virginian Thomas Rolfe. Colonial period liaisons such as this certainly could have saved a little blood from being spilled—which may have also been true for humans and Neanderthals.

Recent scientific research has discovered another benefit from our cross fertilization with Neanderthals and Denisovians: it gave a boost to our human immune system. Those rare interbreeding incidents introduced new variants of immune system genes into us—in fact, the study shows that some of our stronger immune system genes came from our hearty cousins. (One interesting ancillary fact that arose from this work is that people from today’s Africa have fewer immune system genes than Europeans and Asians, because their ancestors stayed in Africa and did not get the opportunity to mate with those other extinct hominids.)

So the next time you see a drawing of a Neanderthal, don’t think about how they seem inferior and uglier than we do… feel a little gratitude for the diseases you haven’t had, thanks to those ancient cross-species romances. Thanks, cuz!