The always informative Ann Gibbons has a piece in Slate, The Neanderthal in My Family Tree. There is almost nothing new for regular readers of this weblog, but it's rather awesome that Slate is now publishing stuff like this. Many people are simply unaware of the new paleogenomics. Case in point, a good friend who has a doctorate in chemical physics, and was totally unaware a year after the seminal Science paper on Neandertal admixture of the likelihood of Neandertal admixture! Nevertheless, I think it is important for me to be repetitive and highlight a disagreement I have with the Gibbons' piece. She says:
...But the differences in the genomes of Denisovans, Neanderthals, and modern humans are also revealing the genetic traits that set us apart from them—the traits that made us human. “I've been comparing it to the pictures of Earth that came back from Apollo 8. The Neanderthal genome gives us a picture of ourselves, from the outside looking in,” says paleoanthropologist John Hawks of the University of Wisconsin in Madison, in his blog on paleoanthropology, genetics, and evolution. “We can see, and now learn about,
the essential genetic changes that make us human
—the things that made our emergence as a global species possible.”
This line of thinking is not Gibbons' creation. Svante Paabo himself is promoting it. And obviously it's not crazy. We can learn a lot from comparative genomics.
But I want to reiterate that there may be no essential or necessary genetic change in the hominin lineage which led to modern humanity.
I'm not saying that this is a done deal. Rather, we need to move toward a position of more agnosticism as to how humans came about. As humans we think we're very special, so there must have been very special genes which gave rise to our very special traits. This need not be so. Humility is warranted at this point, in light of the major revisions in our understanding of the past due to paleogenomics. As analogy, consider someone who is very tall. Is there an essential genetic change which confers upon them tallness? No. Height seems to be controlled by innumerable variants of small effect, and it is the sum totality of these changes which results in extreme values of height. And importantly different genetic configurations can lead toward the same trait value. Similarly, human encephalization and facility with cultural production may also be due to small but continuous changes on a range of loci. There may have been no specific change which led to humanity. This is all important because I think there's a non-trivial probability that in the near future someone will publish a paper which claims to have found the Allele Which Made Us Human. More realistically, that will be the press release spin, even if the paper is more judicious. We need to be cautious and be on guard against this. There may be many alleles which differentiate Neandertals & Denisovans from modern humans, but that doesn't mean that those alleles are responsible for our differences. If we presuppose a model the genomic evidence is such that we may swallow up false positives wholeheartedly. Please do note that I am not forwarding an unequivocal position. I don't know if there was a point mutation which made humanity, or if humanity is a continuous and quantitative trait. Rather, I think we need to be more careful about the inferences we make because of the robustness of our background assumptions. We're special, and we know. But that doesn't mean that we can let that intrude in the judgments of science.