Retroviruses are viruses that embed themselves in the chromosomes/DNA of a species. When they infect the sperm/egg cells, they also affect the future generations of the species. A retrovirus struck the ancestors of modern day chimpanzees and gorillas some 4 million years ago, but somehow the ancestors of humans (and other apes such as orangutans and gibbons) escaped from its effects. Researchers from University of Washington say that the virus might have affected the evolution of chimps and gorillas. The research appears in the April issue of the journal Public Library of Science-Biology, which is available online on March 1.

Primate Tree (Courtesy: Northern State University)The researchers were studying portions of the so-called junk DNA (DNA that is not used to encode proteins or do any other useful function), when they found several similarities between the chimpanzee and gorilla DNAs. The human DNA lacks those portions, and the scientists were interested to know why. They compared the junk DNA with the database of virus genomes, and discovered that it was the remnant of a retrovirus, a type of virus that copies its genetic information into the host's genome. The virus probably infected the ancestors of the gorilla and chimpanzee. During this time, ancestors of humans were also living in Africa, but either they were far away from the affected region, or were not susceptible to the virus. Either way, it was perhaps a close call! Since retroviruses have the tendency to alter genetic sequences, if that had happened to our direct ancestor, there might not have been any of us today!
Several copies of the virus was found in the chimp/gorilla DNA, suggesting that the virus was at least partly successful in wiping out parts of these populations. Somehow the populations survived, but might have gone through a major period of culling, which perhaps reduced the genetic diversity in both species.