But that’s not what has emerged from the new study. The Broad Institute scientists lined up millions of bases of DNA in humans and chimps and measured their differences. Humans and chimpanzees both inherited each segment of DNA from a common ancestor. Over time, the copies of that ancestral segment picked up mutations. The differences between them can offer clues to how long they’ve been evolving along separate paths. It turns out that the ancestors for some of those segments are much older than others. The only way to make sense of these results, according to the scientists, is to conclude that hominids and the ancestors of chimpanzees were interbreeding–to some extent at least–for four million years.
Meanwhile, anthropologist John Hawks provides some reasons for skepticism here:
I’ve read the paper, and I have to say it doesn’t deliver on its promises. It fails to cite previous work on the topic, it discards without explanation the hypothesis supported by most previous studies, and it promotes a “provocative” hypothesis for which there is no good evidence. It doesn’t even show that the speciation of humans and chimpanzees was “complex”.
It’s just a mess.
Hawks goes on to back up his charges in considerable detail.