Reich is senior author of a paper detailing the findings, published in the May 17 online edition of the research journal Nature.

“Something very unusual happened at the time of speciation,” he added.

Speciation, the evolutionary branching off of new species from existing ones, is the key mechanism that creates new species, according to evolutionary theory. Since chimps are our closest relatives, our speciation from them would be the pivotal event that put us on the road to humanhood.

Previous genetic studies have focused on the average genetic difference between human and chimpanzee across their genomes.

By contrast, the new study scrutinized the variation in evolutionary history across the whole human genome. In theory, some regions of the genome should be “older” than others, the researchers explained. That is, different regions should have characteristics traceable to different times in the evolutionary history of the common ancestors of humans and chimps.

This analysis led to three surprising conclusions, the scientists said: 

• The time from the beginning to the end of the splitup ranges over more than 4 million years across of the genome. In other words, the date of the divergence seems different depending on where in the genome you look—suggesting the process may have been gradual, and marked by interbreeding.
  
• The youngest genomic regions are surprisingly recent, no more than 6.3 million and probably no more than 5.4 million years old. This would suggest the final speciation itself occurred on the same time frame, more recently than scientists previously thought. 
  
• The X chromosome, which contributes to sexual characteristics, falls almost completely at the more recent end of the time frame.