We Are a Million and a Half Years Younger than We Thought

Did early humans interbreed with chimps? A 7 million year old fossil, which was previously considered the earliest member of a human family, now appears to have probably been a human-chimp hybrid. The skull, called "Toumai", was hailed as one of the most important recent fossil discoveries because it was thought to belong to an ancient ancestor of modern humans.

"It is possible that the Toumai fossil is more recent than previously thought," said Nick Patterson, of the Broad Institute and a co-author of the study. "But if the dating is correct, the Toumai fossil would precede the human-chimp split. The fact that it has human-like features suggests that human-chimp speciation (separation) may have occurred over a long period with episodes of hybridisation between the emerging species."

Scientists reached this stunning conclusion after comparing the genomes of humans and chimpanzees and found out that the initial split between us and them took place no more than 6.3 million years ago and probably less than 5.4 million years ago. But this separation didn't happen suddenly - scientists estimate that the entire process may have taken about 4 million years. These are 4 million years of possible human and chimp inter-breeding before the final break.

"The study gave unexpected results about how we separated from our closest relatives, the chimpanzees," said David Reich of the Broad Institute and Harvard Medical School's Department of Genetics in Massachusetts.

The human and the chimpanzee genomes are identical in proportion of 99 percent and, in order to deduce how long ago the separation between the two happened, researchers focused on the genetic differences between humans and chimpanzees. They studied and compared two regions of the genomes.

Reich and his colleagues showed that some regions in the human genome are older than others. The full separation between the two species occurred when all these genetic differences accumulated. The species became so different that they couldn't inter-breed anymore. But scientists have now found that the youngest regions are unexpectedly young, which means the separation between the two species is more recent than previously thought.

"A hybridization event between human and chimpanzee ancestors could help explain both the wide range of divergence times seen across our genomes, as well as the relatively similar X chromosomes," Reich explained.