( he virus affected the ancestors of chim...)Retrovirus struck ancestors of chimpanzees and gorillas millions of years ago, but did not affect ancestral humans

he virus affected the ancestors of chimps, gorillas, and Old World monkeys, but didn't affect the ancestors of humans or of Asian apes like orangutans and gibbons. The infections took place independently, and did not originate in a common ancestor of humans and apes. The event also took place between three and four million years ago, well after the separation of humans from apes. That split is estimated to have occurred five to seven million years ago. During that period, ancestral humans were likely to be living in the same area of Africa as great apes. African apes may have been susceptibile to the virus, or ancestral humans and Asian apes may have been resistant to it. Another possibility is that some early humans may have carried the virus, but eventually died off.

Researchers also don't know the impact the virus had on the primate species it did affect. They found many copies of the virus in the genomes of both species, but only a tiny fraction of those copies landed in or near a gene, where it would have the greatest impact. Other studies have shown that most retroviruses typically land near or within genes. This difference may mean the animals that had the virus taken up in or near a gene didn't survive long. Because of that natural selection, researchers believe that the virus may have had major impacts on the formation of the species we now call chimps and gorillas. The virus struck when each of the primate groups was still an incipient species with widely varying populations.

If the virus had killed off much of the population of both species, it may have created what evolutionary biologists call a population bottleneck. This much smaller group of surviving animals would then sort out most of its genetic variation in relatively fewer generations than would a larger group. This would lead to a higher probability of rare genetic variants becoming fixed in a short time. Before long, a genetically disparate population, possibly with wide variati
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Source:University of Washington

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