Telltale X reveals traces of prolonged sex with the ex.
David Reich and colleagues have delivered the latest of a long series of breathtaking blows to our amour propre with their new Article in Nature. This work should elicit pant-hoots of delight from those of us who relish science as the antidote to the prevalent and entirely misplaced view that the position of the human species in the natural world is in any sense “special”.
The period during which humans speciated from chimpanzees was apparently a long and ignominious sexual morass, a messy four million years or so of breaking up, getting back together, producing fertile and infertile offspring and finally getting the knack of making do without one another.
What David and his colleagues at the Broad did was to look at 800 times more primate genomic DNA sequence than previously examined, aligning 87Mb of newly assembled gorilla sequence with the exisiting human and chimpanzee genomes, 18.3Mb from five primates, including 1.2Mb from chromosomes 7 and X. This allowed them to compare the sex chromosome with autosomal sequence. Overall they concluded that the divergence time since human-chimp speciation showed enormous variation across the genome and even included chunks of the genome that were more similar between human and gorilla than between human and chimpanzee.
The human X was the youngest of all, a situation that could only have arisen in a scenario where chimpanzee-human hybrids occurred after the initial speciation event, and directional selection for hybrid fertility imposed some systematic restriction on our pattern of reproduction (the authors suggest backcrossing to chimpanzee males for the purpose of discussion).
All this is reminisiscent of the scene in Nagisa Oshima’s film “Max, mon amour”, where the British diplomat in Paris adapted with admirably civilized sang froid to the passionate sexual relationship between the eponymous chimpanzee and his wife – the immor(t)ally irresistible Charlotte Rampling. Of course he did, we’ve seen it all before.
Hear my thoughts on the chimpanzee genome on ABC’s Science Show.