23andMe screens for a relatively small percentage of Neanderthal DNA markers. The most they’ve reported to any customer was 397. I have 299, more than 81 percent of their clients. Their client base doesn’t provide a statistical reference, as it’s self-selected. But still . . .
A couple of years ago, I joked that the discovery that many of us share Neanderthal markers was a new source for ethnic pride, if one were so disposed. Now it appears that the Neanderthal were the first members of the human family to paint images on cave walls. 23andMe didn’t mention that trait, which hadn’t been discovered a year ago. Their insightful analysis suggested tendencies toward straight hair, absence of back hair, and soft ear wax.
Uncle Liam aside, the evidence accumulates that modern humans are hybrids of marvelous complexity. Last I read, Homo sapiens sapiens and Neanderthalensis had a common ancestor a million years ago. That estimate is continually debated. Some scientists put the divergence only half that far back. But the emergent picture suggests that the exodus from Africa has been long and repetitive, and that the subsequent events are complex and not well understood. Given how much can happen in hundreds of thousands of years, that’s not surprising. So far 23andMe doesn’t screen for Homo denisova markers. Maybe, somewhere along the long human line, I have an Aunt Denise, though I gather this is unlikely. The Denisovans, who carried Neanderthal markers as well as those of one of the "ghost" species that have left behind nothing but bits of coding, may have vanished* from the human genome except for DNA sequences carried by the aborigines of Australia and Papua New Guinea.
About a decade ago, I wrote a book called Hominid that conjectured about the strong selection that might occur in island populations. A slightly villainous character who is financing research speculates early on that humans and Neanderthals might have interbred, but he later accepts the view prevailing in the first decade of this century that rejected that notion. By the time Svante Pääbo at the Max Planck Institute sequenced Neanderthal DNA and began looking for distinctive markers, the book was finished. The novel was about something else, in any case, the incredible speed at which strong selection can occur. One example is the spread of lactase persistence mutations through Europe in perhaps ten thousand years. The selection isn’t complete; most of us know people who are lactose intolerant. Another example was how a certain favorable genetic trait might flourish in an isolated population, in the novel a gift of longevity.
Those who wish for diversity have it: we have diverged, and rejoined in a limited way, and diverged again, who knows how many times or with how many archaic humans. Our families have experienced adaptation and selection every step of their migration across the earth. Most of you reading this will never be tested for Tay-Sachs, but European Jews who intend to marry certainly will be. It’s distinctive to that population, as the absence of facial hair is to American Indians, the epithelial fold is to Asians, tyrosinemia is to the Chicoutimi, thalassemia is to some Pacific Islanders, and as the absence of back hair is to Uncle Liam’s descendants. The prophet who said his father’s house had many mansions could have been describing us.
[* Note: Some of the material above is already out of date. Harvard researcher David Reich's Who We Are and How We Got Here is a creditable source on recent findings. I could add, to the oddities of human wandering, that 23andMe reports that my Irish-German mother's haplogroup is found today most often in Iraq. As I recall, there's also an error in the novel; knowledgeable characters accept Darwin's belief that most mutations are harmful, when it seems now that most don't matter, accounting for the enormous within-species variation.]
March 3 2018