A very interesting problem in our understanding of human evolution has recently popped up. Exactly when did the divergence of the human and chimpanzee ancestral lines occur?
Most estimates from both the fossil record and genetic analysis range from between five and seven million years ago.
Last year however, David Reich of Harvard Medical School in Boston, Massachusetts published a paper suggesting that the split may not have been cut and dry, and that interbreeding of human and chimpanzee ancestors may have continued until as recently as four million years ago.
Last month, Asger Hobolth of North Carolina State University (Go Pack!) published a statistical study of the genetic makeup of several primates that seems to support Reich’s contention.
In a report published online in the February issue of PLoS Genetics, Danish postdoctoral researcher Asger Hobolth of North Carolina State University in Raleigh and his colleagues compared 1.9 million basepairs of DNA in four regions of the genomes of humans, chimpanzees, gorillas, and orangutans. They then used a well-known statistical method called the hidden Markov model, which was developed in the 1960s for speech recognition, to help them identify subtle patterns in the genomes of apes and humans. The researchers used the method to quantify how closely humans are related to chimpanzees, gorillas, and orangutans. They also used it to spot how humans and chimps inherited different segments of noncoding DNA, such as tracing stretches of the genome that humans inherited from their last common ancestor with chimps or from earlier ancestors shared with gorillas or orangutans.
More to the point, the researchers could then calculate the order–and relative timing–in which various lineages split apart on the primate family tree, with orangutans appearing first, followed by gorillas, chimps, and then humans. They dated the branching points by using fossils of orangutan ancestors, which were 18 million years old, to set a starting time at the base of the tree for a “molecular clock.” Although molecules mutate at various rates, the average is relatively constant if enough time passes–and those mutations can be used like a clock to date how long ago two species split. The team ended up with a date of 4.1 million plus or minus 400,000 years for the human-chimp split. It was so recent it even surprised the authors, says Hobolth.
How could the date be so different than currently accepted models? Some scientists are suggesting that the statistical model these researchers used is flawed or at least not entirely appropriate for this kind of genetic modeling without some adjustment.
Another option is that even after the main split between humans and chimpanzees, the two branches continued to interbreed, and thus continued to swap and share genetic material.
In either case, it seems each new discovery in our evolutionary history presents our heritage as less of a tree and more as a bush, with tangles and twists.
It’ll be interesting to see how this plays out.