Thanks to modern science, we're gradually learning one of the greatest stories ever told: how humans evolved.
Going back a few years, we separated from other apes 6 million years ago and evolved through many species of Ardipithecus, Australopithecus, Homo and other genuses. Homo appeared 2 million years ago, evolving through Homo ergaster, Homo habilis, Homo erectus and other species. Homo heidelbergensis evolved in Africa a million years ago, with some groups eventually migrating to Europe and China. By 300,000 years ago in Europe, they evolved into Homo neanderthalensis--the Neanderthals. Meanwhile, back in Africa, Homo heidelbergensis evolved into Homo sapiens by 200,000 years ago. Neanderthals were stronger than Homo sapiens, had a larger cranial capacity, fashioned tools, buried their dead, perhaps cared for the sick, perhaps conversed, and looked a lot like us. They went extinct 30,000 years ago. There's a fine reconstruction of a Neanderthal female in Science, Jan. 31, Page 471.
Groups of Homo sapiens migrated out of Africa 125,000 years ago, arriving in Asia 50,000 years ago and Europe 43,000 years ago. In Europe, they co-existed with Neanderthals for more than 10,000 years, eventually replacing them. It's natural to suppose that Homo sapiens drove Neanderthals to extinction, but according to paleontologist Chris Stringer's book "Lone Survivors: How We Came To Be the Only Humans on Earth," this is probably not true. Thirty-thousand years ago, the planet was in the coldest phase of the most recent Ice Age, with frequent sharp climate shifts. It's thought that such environmental stresses, and perhaps some competition from Homo sapiens, finished off what was already a declining species by the time Homo sapiens arrived.
Studies in 1997 showed that Neanderthal shares 99.7 percent of its DNA with humans. For comparison, Homo sapiens shares 98.8 percent of its DNA with its nearest living relative, the chimpanzee. Were Homo sapiens and Neanderthals really separate species? Was there possible or actual breeding?
Two animals are separate species if they cannot interbreed to produce viable offspring. If they interbreed, then their genes mix, generation after generation, and they remain a single species. If they cannot interbreed, they can evolve in different directions without limit. That's how evolution works.
Now new research shows that, 30,000 years ago, Neanderthals and humans were on the verge of completing this speciation process. Nevertheless, as the Science article mentioned above puts it, some hunky Neanderthal males surely lusted after modern human maidens, and some sleek modern human males surely scored with earthy Neanderthal girls.
Two independent research teams, reporting last month in Science and in Nature, used different methods to study data gathered in the 1000 Genomes Project, the world's largest record of modern human DNA. I was delighted to see that the Arkansas Democrat-Gazette published a nice article on this work on Jan. 30.
A "genome" is the record of all the genes of a single individual. Using a known Neanderthal genome, the teams searched for Neanderthal genes in modern human genomes. One finding was that 60 percent of modern Asian and European genomes contained a Neanderthal version of a gene that affects skin, nails and hair. They speculate it helped the darker humans from Africa cope with the cooler, darker northern climate. On the other hand, regions of the human genome that determine speech and communication remained unaffected by the cross-breeding, implying Homo sapiens' superiority in this regard. Evolution proceeds by just such selection of the most "fit" genes.
Both teams found that 1 to 3 percent of the European genome and East Asian genome comes from Neanderthal. Overall, the teams found similar results--a good check on the validity of both reports.
The most interesting result is the extent to which Neanderthal and Homo sapiens are separate species. Consider horses and donkeys: They can breed to create a mule, but mules are infertile and cannot evolve. So horses and donkeys are different species.
The survival of Neanderthal genes in modern humans shows that the two did produce fertile offspring. But the evidence (from "X" chromosomes) shows that the hybrid males were far less likely to reproduce than purely Neanderthal or purely Homo sapiens males. Apparently, during the 300,000 years that Neanderthal and Homo sapiens spent in Europe and Africa, respectively, they "partially speciated." Their genomes grew apart to the extent that, upon interbreeding, their offspring were nearly, but not quite, infertile.
Thus we find, within our own genes, the marks of time, of our relationship with fellow creatures, and direct evidence of how new species come to exist.
Commentary on 02/21/2014