OXFORD, ENGLAND—A new study of the rate of gene mutations in three generations of western chimpanzees agrees with recent findings that the human mutation rate is half as fast as had been previously thought. This new understanding pushes back the most recent common ancestor of humans and chimps to at least 12 million years ago. “Our results add substance to the idea that the human-chimpanzee split was considerably older than has been recently thought,” geneticist Gil McVean of the Wellcome Trust Center for Human Genetics told Live Science.