Nature published the results from the first full-genome sequencing of Gorilla this week. As with any new genome sequencing project, this initial publication is the result of an immense amount of work (the author list on the paper runs to 71 if I counted correctly), but is really only the starting point for asking interesting questions. After going through some of the technical specifications of the sequencing work, the first half of the Nature paper deals with the issue of estimating divergence times between humans, Pan and Gorilla given estimated demographic scenarios and mutation rates. This is an area with much significance for interpretations of the fossil record, but the authors decision to basically fit the data to a given interpretation of the fossil record means that the current interpretation does not add a lot of new value to questions as to where Ardipithecus, Orrorin or Sahelanthropus fit within the hominid lineage, for example (see figure b below).
The Nature News piece that accompanies the paper’s publication draws attention to evidence for positive selection on genes associated with hearing, an issue also discussed by John Hawks.
Personally, what I find most interesting is what the genome reveals about our understanding of speciation processes in large-bodied hominids. With the Gorilla genome joining the genomes of Homo sapiens, Pan troglodytes (chimpanzees), Pongo pygmaeus (Orangs) as well as the Neandertal and Denisovan genomes, we now have a dense assemblage of closely related extant and extinct taxa. And, not surprisingly, speciation in this group is a messy and complex process. This paper estimates that 30% of the Gorilla genome reflects incomplete lineage sorting (ILS), meaning nearly a third of the genome reveals closer evolutionary affinities between gorilla and chimps or gorillas and humans. We know that gorillas are actually an evolutionary outgroup to ourselves, chimpanzees and bonobos, so what the high degree of ILS reflects is the “fuzziness” associated with the process of speciation. Speciation in large-bodied mammals is not instantaneous and true isolating mechanisms are especially slow to develop, allowing for continued periodic or isolated instances of meaningful genetic exchange between diverging lineages. The authors provide a second good example of this in the text by suggesting the divergence between Eastern and Western gorilla species began close to 2 million years ago (a date, by the way, that seems to come up a lot), but with more recent genetic exchange between the lineages.
We also compare the western and eastern gorilla species, estimating an average sequence divergence time 1.75 million years ago, but with evidence for more recent genetic exchange and a population bottleneck in the eastern species.
The fundamental messiness of speciation across this group as evidenced by these genomic data is a big part of why I am not sympathetic to models of human evolution postulating many concurrent hominin taxa.
UPDATE: The study’s lead author, Aylwyn Scally, raises some clarifying points in the comment section.
1. Scally, A. et al. Insights into hominid evolution from the gorilla genome sequence. Nature 483, 169–175 (08 March 2012) doi:10.1038/nature10842