In recent years, paleoanthropologists have been closing in on the exact time and place where the ancestors of humans and chimpanzees went their separate ways. As they uncovered several types of fossils from the dawn of humankind, they proposed that these early hominids lived between 5 million and 7 million years ago--dates that match up nicely with molecular studies. Now, however, this satisfying consensus is being challenged by a new study that proposes a surprisingly recent separation.
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.
Some researchers say the date is so early, something must be wrong with this application of the Markov methodology. It would bump all the earliest fossils out of the human tree--including a 4.1-million-year-old fossil from Kenya called Australopithecus anamensis, which was already well on its way toward becoming human; it already walked upright, which is a defining character of being ancestral to humans, but not apes. "A 4.1-million-year split for humans and chimps ... is hard to defend because fossils practically reject it," says evolutionary biologist Blair Hedges of Pennsylvania State University in State College.
Others, such as geneticist David Reich of Harvard Medical School in Boston, Massachusetts, say that despite the date, the method shows that the genetic makeup of the last common ancestor of humans and chimpanzees--and the last common ancestor they share with gorillas--was more diverse than expected. The findings also support Reich's controversial proposal last year (Science, 19 May 2006:) that the ancestors of chimps and humans might have interbred more recently than previously believed, even after they had begun to head down separate evolutionary paths.