California ? Viruses that plagued our ancestors some 4 million years ago may have affected how susceptible humans are to HIV today, researchers at the Fred Hutchinson Cancer Research Center in Seattle have found.

By resurrecting a key part of the virus, researchers showed that humans have a protein that almost completely protects against infection by the ancient virus. The research, published this week in Science, showed, however, that humans with the same protein would be less protected against HIV, the virus that causes AIDS.

Could this human antiviral protein work poorly against HIV because it evolved to fight a 4-million-year-old virus?

“Infectious diseases have shaped human history and continue to shape human history,” said Michael Emerman of the Hutchinson center’s division of human biology and senior author of the study. “There is an ongoing arms race between pathogens and our own defenses. The result of a battle 4 million years ago, the study suggests, may have shaped a defense that worked well back then, but has left us vulnerable to modern-day viruses such as HIV.”