The evolutionary split between human and chimpanzee is much more recent -- and more complicated -- than previously thought, according to a new study by scientists at the Broad Institute of MIT and Harvard and at Harvard Medical School published in the May 17 online edition of Nature.
The results show that the two species split no more than 6.3 million years ago and probably less than 5.4 million years ago. Moreover, the speciation process was unusual -- possibly involving an initial split followed by later hybridization before a final separation.
"The study gave unexpected results about how we separated from our closest relatives, the chimpanzees. We found that the population structure that existed around the time of human-chimpanzee speciation was unlike any modern ape population. Something very unusual happened at the time of speciation", said David Reich, the senior author of the Nature paper, and an associate member of the Broad Institute and assistant professor at Harvard Medical School's Department of Genetics.
Previous molecular genetic studies have focused on the average genetic difference between human and chimpanzee. By contrast, the new study exploits the information in the complete genome sequence to reveal the variation in evolutionary history across the human genome. In theory, scientists have long known that some genomic regions must be 'older' than others, meaning that they trace back to different times in the common ancestral population that gave rise to both humans and chimps (see Graphic). But, the new study is the first to actually measure the range of ages. It gave three surprising results:
The time of from the beginning to the completion of divergence between the two species ranges over more than 4 million years across different parts of the genome. This range is much larger than expected.
The youngest regions are unexpectedly recent -- being no more than 6.3 million years old and probably no more than 5.4 million years old. This finding implies that human-chimp speciation itself is far more recent than previously thought.
If one looks only at the X chromosome, it almost entirely falls at the lower end of the time frame. In fact, the average age of the X chromosome is ~1.2 million years "younger" than the average across the 22 autosomal (non-sex) chromosomes.
"The genome analysis revealed big surprises, with major implications for human evolution," said Eric Lander, Director of the Broad Institute and co-author of the Nature paper. "First, human-chimp speciation occurred more recently than previous estimates. Second, the speciation itself occurred in an unusual manner that left a striking impact across chromosome X. The young age of chromosome X is an evolutionary 'smoking gun.'"