Early Immunity to Chimp Virus Leaves Humans Open to HIV

Humans are more susceptible to HIV than other primates because our ancestors evolved a protein that could fight off a different retrovirus that infected chimps, says Scientific American. The most conspicuous difference between the chimpanzee genome sequenced in 2005 and the human one, says a Seattle virologist, was 130 copies of a retrovirus that inserted its DNA into cells, as HIV does today.

Scientists then looked for the reason chimps were infected and humans weren't, which led to the modified protein. Sleuthing has also determined that no version of that tricky protein could fend off both the 4-million-year-old virus and HIV. (More AIDS stories.)