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.
