Belatedly, here is last week's Mind and Matter column from the Wall Street Journal.
I once had a soft spot for the yeti, known in my youth as the "abominable snowman." As a teenager I avidly devoured stories of hairy bipeds glimpsed through snowstorms, strange cries echoing across glaciers, or enigmatic footprints in the snow. Slowly it dawned on me that the testimony was unreliable, the ecology implausible, the demography impossible and the lack of specimens conclusive.
But a chance remark by an evolutionary geneticist at a recent conference jogged my memories of that youthful enthusiasm. For only a very small part of our history, he said, have we or our ancestors been the only species of human on Earth.
Fully modern human beings emerged 180,000 years ago, but until just the last 12,000 years or so, our lineage always shared at least some of the planet with other, closely related species: first, various "robust" Australopithecines (big-jawed, vegetarian cousins in Africa), then Homo erectus (archaic, omnivore cousins in Asia), Neanderthals (strong, carnivore cousins in Europe) and, most recently, "hobbits" (tiny, small-brained cousins on the island of Flores in Indonesia).
Maybe the existence of "other" people-far more different from us than different races are from each other today, but far more similar to us than chimpanzees are-left legends that survived to feed the folklore of the yeti and similar wild, hairy men, such as the Almas of central Asia.
Last year yet another cousin was added to the list, thanks to the tip of a little finger from a cave in the Altai mountains in Siberia. Svante Pääbo, the evolutionary geneticist who jogged my memory last week and the pioneer of analyzing DNA from extinct species, managed to extract unusually pure hominid DNA from the bone: Some 70% of the DNA was hominid, whereas in the Neanderthal bone from Croatia that he sequenced a few years before most DNA proved to be from soil bacteria and fungi.
The DNA showed beyond almost all doubt the existence of a new species of hominid-now known, after the cave where they were first found, as "Denisovans." Judging by their genes, Denisovans were only slightly more closely related to Neanderthals than to us "moderns." They probably separated from Neanderthals shortly after we did, perhaps 250,000 to 300,000 years ago, and may have looked quite different from both species.
The search is on for fossil Denisovans, among forgotten bones in museums as well as in the field. One hefty tooth, and rumors of others, hint at adult Denisovans being rather large animals, or at least having large jaws. If so, the parallel with the yeti, or the Almas of central Asia, is hard to avoid.
The Denisova cave where the first Denisovan was found is just three days' walk from where genuine Neanderthal remains have been found, so it's possible the ranges of the species overlapped at least for a time. Did they have different habits, or did one displace the other? Since Neanderthal fossils and tools have never been found outside Europe and western Asia, maybe Denisovans were a more eastern, forest-dwelling species, perhaps less dependent on hunting big game than the steppe-dwelling Neanderthals.
Two years ago Dr. Pääbo's team proved that all modern people, except Africans, have about 2.5% Neanderthal DNA in their genes, implying interspecies mating soon after some of the moderns left Africa. Now they've shown that all modern Melanesian people from Papua New Guinea and surrounding lands have Denisovan DNA in their genes, at about 5%. This implies interspecies mating somewhere in eastern Indonesia, which was then a continent rather than an archipelago and must have also been part of the Denisovan range.
And what about Africa? We know that all modern Africans are descended from one species that spread throughout the continent at the same time as spilling into Asia, but they too must surely have encountered other cousin species as they did so, and perhaps mixed their genes, too.