Cousins--5 million years removed

Humans and chimpanzees probably last shared a common ancestor between 5 million and 7 million years ago. That is the time frame proposed by a team of researchers from Arizona State and Penn State universities.

Chimpanzees are the closest ape relatives to humans today. Previously, scientists have placed the time of divergence from a common ancestor anywhere from 3 million to 13 million years ago. Those numbers were based on several molecular biology and fossil studies.

The scientists analyzed the largest data set yet of genes that code for proteins. They also used an improved computational approach that they developed. The technique takes into account more of the variability—or statistical error—in the data than any other previous study.

Gene studies are needed to address this problem because the interpretation of the earliest fossils of humans at the ape/human boundary is controversial. In addition, almost no fossils of chimpanzees have been discovered.

“There is considerable interest in knowing when we diverged from our closest relative among animal species,” says Sudhir Kumar, director of the Center for Evolutionary Functional Genomics in the Biodesign Institute at ASU.

“This divergence time also has considerable importance,” he adds. “Scientists use it to establish how fast genes mutate in humans and to date the historical spread of our species around the globe.”

Penn State evolutionary biologists Alan Walker and Blair Hedges also took part in the project. The team examined 167 different gene sequence sets from humans, chimpanzees, macaques, and mice.

The scientists estimated the time of divergence between species by studying the sequential arrangement of nucleotides that make up the chain-like DNA molecules of each species. The number of mutations in the DNA sequence of a species, compared with other species, is a gauge of its rate of evolutionary change. The team’s conclusion does not exclude younger or older dates as being possible. “It says they are less likely to be correct,” explains Blair Hedges, an astrobiologist at Penn State.

“Knowing the timescale of human evolution, and how we changed through time in relation to our environment, is important,” he adds. “It could provide valuable clues for understanding—in a more general sense—the evolution of intelligent life.”

What then is the next step? Some additional improvement is possible by including more genes and more species, says Walker, a Penn State paleoanthropologist.

“The greatest opportunity now for further narrowing this estimate of 5-to-7-million years will be the discovery of new fossils,” Walker adds. “So will the improvement in geologic dating of existing fossils.”—Joe Caspermeyer