Human groups apparently did not diverse in straight lines over time, as was supposed. And so far, the mystery ancestor hasn’t turned up.
The just-published DNA results have been described as baffling (Nature, ScienceDaily)perplexing (BBC) 400k-year-old human DNA, that is hard to make sense of (The Scientist , New Scientist) and creating new mysteries (New York Times) instead of neatly clarifying human evolution?
Here’s paleoanthropologist John Hawkes’s interpretation of the findings:
For more than a hundred years, scientists have been drawing straight lines connecting different fossils, to try to understand the human family tree. Those straight lines always diverged over time, leading toward increasing specialization and extinction of fossil groups. And for more than twenty-five years, geneticists have been assuming that the lines connecting the genealogy of mtDNA should be the same as the lines connecting the fossils. When those lines were different, geneticists have been happy to toss the fossils out of the human family tree, content to accept the story that the fossil people had become too specialized, too peripheral to be ancestors of today’s people.
But the last five years have made clear that both groups — the fossil scientists drawing straight lines of diverging fossil populations, and the geneticists drawing straight lines of diverging — were wrong.
Does anyone here know why they thought that the straight lines must diverge over time?
Hawkes offers his own assessment:
Let’s explore an alternative: that the Denisovans we know are in part descendants of an earlier stratum of the western Eurasian population. Although they are on the same mtDNA clade, the difference between Sima and Denisova sequences is about as large as the difference between Neandertal and living human sequences. It would not be fair to say that Denisova and Sima represent a single population, any more than that Neandertals and living people do. But they could share a heritage within the Middle Pleistocene of western Eurasia, deriving their mtDNA from this earlier population.
We know that the Denisovan nuclear genome is much closer to Neandertals than the Denisovan mtDNA. We are still waiting for the long-rumored publication of the idea that Denisovan genomes have a “mystery hominin” element in their ancestry. They could be a mixture of any number of earlier populations. None of these have to be East Asian, and as yet we have no suggestion that this “earlier” element of Denisovan ancestry could be as ancient as the first known habitation of Eurasia, as much as 1.8 million years ago. Maybe the Sima hominins represent this “mystery hominin” population.
Well, we love a mystery, and this is shaping up to be as good as it gets.
See also: Hawkes on the grandmother (yet again) thesis.