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. By calibrating this rate with the known time of divergence of a species on another branch of the tree-like diagram that shows relationships among species, scientists can estimate the time when the species they are studying evolved. In this case, the calibration time the scientists used was the split of Old World monkeys -- including baboons, macaques, and others -- from the branch of the phylogenetic tree that led to humans and apes, which fossil studies have shown is at least 24 million years ago. Using this calibration time, the team estimated that the human-chimp divergence occurred at least 5 million years ago, proportionally about one-fifth of the calibration time.
This time is consistent with the findings of several research groups that have used the molecular-clock method to estimate the split of humans and chimpanzees since the first attempt in 1967. But this is only a minimum estimate, because it was based on a minimum calibration time. To obtain a maximum limit on the human-chimp divergence, the team used as a calibration point the maximum estimate, based on fossil studies, of the divergence of Old-World monkeys and the branch leading to humans -- 35 million years ago. Calculations using this date yielded a time for the human-chimp split of approximately 7 million years ago, which again was proportionally about one-fifth of the calibration time.
"We can conclude that humans and chimpanzees probably