Study moves chimp-human split to 4 million years ago
quote:
Chimpanzees and humans split from a common ancestor just 4 million years ago -- a much shorter time than current estimates of 5 million to 7 million years ago, according to a study published on Friday.
The researchers compared the DNA of chimpanzees, humans and our next-closest ancestor, the gorilla, as well as orangutans.
They used a well-known type of calculation that had not been previously applied to genetics to come up with their own "molecular clock" estimate of when humans became uniquely human.
"Assuming orangutan divergence 18 million years ago, speciation time of human and chimpanzee is consistently around 4 million years ago," they wrote in their study, published in the Public Library of Science journal PLoS Genetics, available online at
http://genetics.plosjournals.org/ ... /journal.pgen.0030007 (note - I've shortened the link)
Just last May, David Reich of the Broad Institute at the Massachusetts Institute of Technology and Harvard Medical School's Department of Genetics found evidence that the split probably took 4 million years to occur, although his team put the final divergence at just 5.4 million years ago.
"I don't think it really contradicts our paper," Reich said in an e-mail exchange.
"We were focusing on a maximum time for the common ancestor of humans and chimpanzees, while they were focusing on a best estimate," added Reich, who reviewed Hobolth's paper before it was published.
From PLoS link:
quote:
To estimate speciation times and ancestral population sizes we have developed a new methodology that explicitly utilizes the spatial information in contiguous genome alignments. Furthermore, we have applied this methodology to four long autosomal human-chimp-gorilla-orangutan alignments and estimated a very recent speciation time of human and chimp (around 4 million years) and ancestral population sizes much larger than the present-day human effective population size. We also analyzed X-chromosome sequence data and found that the X chromosome has experienced a different history from that of autosomes, possibly because of selection.
After using a statistical correction for substitution rate heterogeneity, Patterson et al. [2] found that the variance in coalescence times is too large to be accounted for by instant speciation and a large ancestral effective population size, and that the speciation process therefore must have been complex. Particularly, the X chromosome shows a deviant pattern, which also led them to conclude that HC gene flow ceased and final speciation occurred as recently as 4 Myr ago. This date is generally believed to be the most recent time compatible with the fossil record, if the Millennium man and Sahelanthropus are not on the human lineage.
That would put australopithicus very close to the split.
This is still a mathematical model of split time based on some kind of uniform rates of mutation ... or at best an average rate.
I'm not sure that such models are valid for periods of speciation, when there can be more impetus for divergence to reduce direct competition.
Enjoy
ps - Human Origins forum ...
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