Miocene

Drawing on the most recent fossil evidence, he says that chimps and humans began diverging between 9 million and 8 million years ago. If so, he says, Stedman's estimated timing of the mutation at 2.4 million years ago is much too recent.

Scientists did just that when they analyzed the fossilized remains of the world's largest known rodents, which browsed on the riverbanks of Venezuela about 8 million years ago. The front and rear limbs of a nearly complete skeleton suggest that the 740-kilogram creature rested on its haunches and manipulated food with its front paws (164: 179 *).

He had seen this isotopic shift time and again in fossil horses and other ancient mammals that populated the planet between 6 million and 8 million years ago. During this interval, many types of mammals started to consume plants relatively rich in carbon-13.

Arm, leg, and foot bones of Kenyapithecus contain features in common with later African apes and humans but not fossils of Asian apes dating to around 12 million to 8 million years ago, McCrossin contends.

"I'm sure there was ice in Antarctica 8 million years ago," he says.