About 4 million years ago, humans had split from apes. At this time, Chimpanzees had to go through some tough times. They were infected with a virus. Scientists discovered this when they found traces of this virus in the DNA of Chimps. This meant that this virus, called PtERV1, infected the Chimp population. Interestingly, the traces of the virus were not found in human genome.
Emerman and his team had an explanation. The humans immune to PtERV1 while Chimps had tough time fighting it. The team of researchers identified TRIM5 as the factor that helped humans protect us from the virus. Removing this protein from the cells made them vulnerable to PtVER1, which were reconstructed from Chimpanzees DNA.
Curiously, these cells were found to be less susceptible to HIV. It seems that in the process of developing resistance to one virus, the immune system of our ancestors had us vulnerable to another deadly virus, the HIV.