Science Literature

Paleoanthropologists have always had plenty to debate, and a new genome comparison study is no exception. Whereas the majority view suggests that hominid line can be traced back to between 5 Ma and 7 Ma, the new study gives a more recent figure. "We apply the HMM [a statistical method] to four autosomal contiguous human-chimp-gorilla-orangutan alignments comprising a total of 1.9 million base pairs. We find a very recent speciation time of human-chimp (4.1 ± 0.4 million years)." This is deemed by many as too late. Blair Hedges considers that the figure "is hard to defend because fossils practically reject it". Ian Tattersall says "it's inconceivable that you could have a common ancestor to both at 4 million years ago when you already have evidence in the hominid lineage that there were bipeds already around at that time."
The study raises again the whole question of what the genome comparisons are telling us. It gives particular relevance to the recent critique of Schwartz and Maresca noted here.

It would be a big step forward if the presuppositions and assumptions of every discipline involved in anthropology were more transparent, because there is often considerable difficulty distinguishing between data and interpretations of data.

Genomic Relationships and Speciation Times of Human, Chimpanzee, and Gorilla Inferred from a Coalescent Hidden Markov Model
Asger Hobolth, Ole F. Christensen, Thomas Mailund, Mikkel H. Schierup
PLoS Genetics, 3(2): e7 doi:10.1371/journal.pgen.0030007

Abstract: The genealogical relationship of human, chimpanzee, and gorilla varies along the genome. We develop a hidden Markov model (HMM) that incorporates this variation and relate the model parameters to population genetics quantities such as speciation times and ancestral population sizes. Our HMM is an analytically tractable approximation to the coalescent process with recombination, and in simulations we see no apparent bias in the HMM estimates. We apply the HMM to four autosomal contiguous human-chimp-gorilla-orangutan alignments comprising a total of 1.9 million base pairs. We find a very recent speciation time of human-chimp (4.1 ± 0.4 million years), and fairly large ancestral effective population sizes (65,000 ± 30,000 for the human-chimp ancestor and 45,000 ± 10,000 for the human-chimp-gorilla ancestor). Furthermore, around 50% of the human genome coalesces with chimpanzee after speciation with gorilla. We also consider 250,000 base pairs of X-chromosome alignments and find an effective population size much smaller than 75% of the autosomal effective population sizes. Finally, we find that the rate of transitions between different genealogies correlates well with the region-wide present-day human recombination rate, but does not correlate with the fine-scale recombination rates and recombination hot spots, suggesting that the latter are evolutionarily transient.

See also:

Gibbons, A., A Recent Split of Humans and Chimps? ScienceNOW Daily News, 27 February 2007.

Lloyd, R., First Humans: Time of Origin Pinned Down, LiveScience, 23 February 2007.