When the first molecular clock studies were performed on the apes what surprised everyone was just how recently the human line had split from the apes – prior to that first study in the 1970s everyone had thought the human line went back 20 million years. Then the first result shattered that old view by dating the split to just 4.5 million years ago. In the 30 years since the dating has gradually risen to somewhere between 5 – 7 million years ago, about the same age as the oldest “definite” (in some minds, at least) human ancestors.
A new study has thrown a monkey-wrench into the works. The Open Access Journal, the Public Library of Science – Genetics, has just published this paper:
Genomic Relationships and Speciation Times of Human, Chimpanzee, and Gorilla Inferred from a Coalescent Hidden Markov Model Hobolth A, Christensen OF, Mailund T, Schierup MH PLoS Genetics Vol. 3, No. 2, e7 doi:10.1371/journal.pgen.0030007
…a hidden Markov model tries to account for all the different ways a particular gene lineage can differ from a species lineage. Through the vagaries of inheritance two members of the same species can have genes with lineages that date back before the species split from its sister species. You and I could have genes that diverged before our ancestors became bipedal apes, if not further back. Because the genome of a species is represented by at least thousands of individuals there’s a finite chance this can happen. Only if the original population crashes to just a handful of members can gene lineages be forced into a single common time of origin – and this leaves a distinctive genetic signature in the population. Cheetahs, for example, show signs of being reduced to a single litter at a point about 13,000 years ago. As a population their genetic variation is very, very low.
Humans are more varied, but less so than chimps, so we’ve come closer to extinction than they have at some point in the past. The eruption of the super-volcano, Toba, about 72,000 years ago is considered a likely suspect in causing that population bottle-neck, and just possibly causing our species to have its current genetic make-up. Our ancestors might have come from a group of about 10,000 at that point, preserving some old gene lines and eliminating many others.
Once all the weird and wonderful genetic trails have been accounted for the new study indicates that chimps and humans diverged a mere 4 million years ago (mya). If so then all the Australopithecine species before 4 mya are actually possible ancestors of both chimps and humans. The recent tool use – hammer stones and now spears for hunting monkeys – seen in chimps could mean our common ancestors had similar capabilities.
That very late date for the human-chimp split means chimps once walked upright like humans, then became specialised knuckle-walkers. Knuckle-walking is as odd as bipedalism, and isn’t seen in any apes other than gorillas and chimps. Could gorillas have come from an upright ancestor too? The dates might be just right, especially if Ororrin (6 mya) and Sahelanthropus (6-7 mya) are both confirmed as bipedal. Interestingly one specialist who examined Sahelanthropus was of the opinion that that hominid was most like a young gorilla.
So could Ardipithecus, Australopithecus anamensis and the other hominids just mentioned really be ancestors to both chimps and humans?