I pass along with little comment this New Scientist article in which David Begun is claiming that the fossil ape Dryopithecus may actually be an extinct member of the gorilla clade: “Ape fossils put the origin of humanity at 10 million years ago”.
“What if Dryopithecus – that looks like a little gorilla – really was a little gorilla that had already branched off from humans?” Begun says. We know the relative times of divergence between gorillas, chimps and humans, he says, so we can use the split of gorillas from the others to recalibrate the fossil clock.
The title and article are pitching the idea that if a 12.5-million-year-old gorilla existed, then the divergence of hominin and gorilla lineages must have been earlier than that time. Which means the divergence of hominin and chimpanzee-bonobo lineages was also fairly early.
But the molecular clock is not actually a story here. We’ve known for several years now that the mutation rate is substantially lower than assumed before 2008. For example, I review the situation in my 2012 article, “A longer timescale for human evolution”).
As Langergraber et al. report (2012), a slower rate places the human–chimpanzee common ancestor at more than 7 Mya and possibly as early as 13 Mya, reopening the case for these and other fossils.
A longer time scale has many other consequences. The 10.5-million-year-old Chororapithecus abyssinicus may really be an early member of the gorilla lineage, as its dental anatomy suggests (Suwa et al. 2007). For the orangutan lineage, the prospect of a much deeper genetic estimate of divergence illuminates the relation between phylogenetics and population genetics. Genetic divergence between two species is a function not only of the time that the species became isolated, but also of the genetic variation within their ancient common ancestral population. Whole-genome analysis of apes and humans has uncovered abundant evidence of complex population structure in the common ancestors of living species (Siepel 2009). Hobolth et al. (2011) assessed incomplete lineage sorting of orangutan similarity in human and chimpanzee genomes, showing that the ancestral population of the orangutans and African apes must have been large and diverse. A fast mutation rate and this complex ancient structure made the origin of the orangutan branch uncomfortably recent, only 9 to 13 Mya, barely old enough to accommodate the earliest known orangutan-like fossil evidence, the 12.5-million-year-old Sivapithecus indicus. A slower mutation rate appears to be a better fit to fossil evidence and the genetic structure of this ancient population.
Gorillas splitting from us 13 million years ago is really not a problem with our current understanding of mutation rates. If Dryopithecus is part of the gorilla clade, it is just an incrementally earlier fossil gorilla than already exists in Chororapithecus from Ethiopia. (I wrote about the import of that discovery in 2007: “Did Gen Suwa just save paleoanthropology?”).
So maybe we’re ripe for an expansion of the gorilla clade to include some European fossil apes as well—assuming that there is a reasonable morphological case for it. Living gorillas are morphologically more generalized in some respects than chimpanzees, yet they have a long evolutionary history after the existence of forms like Dryopithecus. Gorillas and Dryopithecus may share many primitive characters that were lost in the hominin and chimpanzee-bonobo lineages. Any shared derived features will be very difficult to distinguish from the background of primitive characters, especially if we look beyond the cranium and teeth and try to examine postcranial morphology, which is varied in Dryopithecus.
I’ll be interested to see Begun’s morphological case. Begun’s views from around 10 years ago are well expressed in his 2005 article, “Sivapithecus is east and Dryopithecus is west, and never the twain shall meet”. He then favored the view that Dryopithecus could be attributed to the clade of African apes including chimpanzees, bonobos, gorillas and hominins, but not particularly as a gorilla relative. The alternative view up until now has been that these European Miocene apes were stem hominoids, branching from our evolutionary history before the divergence of African apes from orangutan ancestors. So placing one of the European apes into the gorilla lineage looks like something of a surprise.
Begun, D. R. (2005). Sivapithecus is east and Dryopithecus is west, and never the twain shall meet. Anthropological Science, 113(1), 53-64. doi:10.1537/ase.04S008
Hawks, J. (2012). Longer time scale for human evolution. Proceedings of the National Academy of Sciences, 109(39), 15531-15532. doi:10.1073/pnas.1212718109
Hobolth, A., Dutheil, J. Y., Hawks, J., Schierup, M. H., & Mailund, T. (2011). Incomplete lineage sorting patterns among human, chimpanzee, and orangutan suggest recent orangutan speciation and widespread selection. Genome research, 21(3), 349-356. doi:10.1101/gr.114751.110
Langergraber, K. E., Prüfer, K., Rowney, C., Boesch, C., Crockford, C., Fawcett, K., ... & Vigilant, L. (2012). Generation times in wild chimpanzees and gorillas suggest earlier divergence times in great ape and human evolution. Proceedings of the National Academy of Sciences, 109(39), 15716-15721. doi:10.1073/pnas.1211740109
Siepel, A. (2009). Phylogenomics of primates and their ancestral populations. Genome research, 19(11), 1929-1941. doi:10.1101/gr.084228.108
Suwa, G., Kono, R. T., Katoh, S., Asfaw, B., & Beyene, Y. (2007). A new species of great ape from the late Miocene epoch in Ethiopia. Nature, 448(7156), 921-924. doi:10.1038/nature06113