Retroviruses insert their genetic material into an organism’s DNA to replicate. Over time, the viral DNA can inactivate and remain as a “fossil” relic the DNA passed to future generations. Paleovirologists search for these ancient viral traces in the DNA of present-day organisms to study the evolution of immunity.

Scientists at the Fred Hutchinson Cancer Research Center in Seattle, Washington, discovered that modern primates such as baboons, chimpanzees, and gorillas have traces of a 4 million-year-old virus in their DNA. Modern human DNA does not have these traces. To find out why humans were immune, the researchers resurrected the ancient virus and infected cells with it. They learned that the human version of a primate protein called TRIM5α blocks the ancient retrovirus. Yet unlike other primate forms of TRIM5α, human TRIM5α cannot block HIV-1, one of the retroviruses that causes AIDS. The research suggests that our species’ ancient advantage over other primates four million years ago may have left us more susceptible to this modern-day pathogen.