Human Evolution Timeline

Human evolution timeline

Thanks to DNA the human evolution picture is getting clearer. Humans and Chimpanzees diverged from a single ancestral gene pool through a complex process that took around 4 million years. Researchers from the Broad Institute of MIT and Harvard were able to learn when and a little bit about how the two species spilt apart.

“For the first time we’re able to see the details written out in the DNA,” said Eric Lander, founding director of the Broad Institute. “What they tell us at the least is that the human-chimp speciation was very unusual.”

Researchers hypothesize that our ancestral ape species split into two isolated populations about 10 million years ago, then got back together after a few thousand years. At that time the two groups, though somewhat genetically different, would have mated to form a third, hybrid population. That population could have interbred with one or both of its parent populations. Then, at some point after 6.3 million years ago, two distinct lines arose.

Some experts in human evolution are skeptical of that precise scenario, but nevertheless impressed with the study. “It’s a totally cool and extremely clever analysis,” said Daniel Lieberman, a professor of biological anthropology at Harvard. “My problem is imagining what it would be like to have a bipedal hominid and a chimpanzee viewing each other as appropriate mates, not to put it too crudely.”

Since the completion of the chimpanzee genome mapping in September it is possible to look at how specific sections of the genetic code has evolved. The new data suggest the human split was much closer to the present than the 7 million year date that fossils and previous studies indicate, certainly no earlier than 6.3 million years ago, and more likely in the neighborhood of 5.4 million.

This data also indicates that human evolution probably took millions of years. That’s because in some parts of the DNA sequence the genetic difference between humans and chimps is so large that those genes must have been isolated from each other nearly 10 million years ago. But in other places the human and chimp genes are so close that they appear to have still been swapping genetic material at least until 6.3 million years ago.

One of those areas is the X-chromosome, which is intriguing. “The genes that are a barrier to speciation tend to be on the X-chromosome,” said David Reich, the main author of the study.

X-chromosome defined: Of the 23 pairs of chromosomes that humans carry in every cell of their body, genes on a single pair of chromosomes (called X and Y) determine the gender of the fetus. A female is only capable of transmitting the X chromosome to her offspring since females always have two X chromosomes (XX), while males can transmit either an X or a Y chromosome to an offspring since males always have one X and one Y chromosome (XY). Importantly, the X- chromosome also contains many other genes that have nothing to do with determination of gender. Mutations in these genes on the X-chromosome result in diseases that are transmitted to male offspring (who have a single X-chromosome) through the maternal line. These are called X-linked diseases. Females can be carriers of the mutated genes but seldom show signs of the disease since they bear another X chromosome that does not have the same mutation and thus produces a normal protein.