The virus plagued our primate ancestors for a staggering 25 million years. Like HIV, it was a retrovirus that hijacked part of its host’s very DNA to aid its survival and replication. But sometime around 11 million years ago, our forebears made a crucial adaptation to wrest the function of a crucial gene back – blocking the virus and casting it into extinction.

The molecular evidence of the crucial genetic war is right there in the “fossil” record of modern animals, reports a team of scientists from Rockefeller University in New York in the journal eLife.

“Broadly speaking, this study shows how analyzing viral fossils can provide a wealth of insight into events that occurred in the distance past,” said Paul Bieniasz, senior author, and Rockefeller professor of retrovirology. “In particular, it represents an example of how viruses themselves can provide the genetic material that animals use to combat them, sometimes leading to their own extinction.”

About eight percent of our modern human genome is virus, left by the traces of invaders and plagues of the past.

The HERV-T left its fingerprints throughout the DNA of the primate family, during its millions of years of infecting our ancestors. The researchers believe, based on the traces that remain, that it used a cell-surface protein named MCT-1 to bind to animal cells.

The gene they found from HERV-T encodes a well-preserved envelope protein – and it was integrated into hominid DNA between 13 and 19 million years ago. That timeframe was just before the virus traces disappeared from the genetic fossil record, and apparently went extinct.

The team even reconstructed the outer protein layer of the long-dead virus, to understand its workings.

The DNA defense of our ancestors was a cunning biological response to the virus, they write.

“(It) apparently functioned by interacting with MCT-1 , either at the cell surface promoting its internalization, or in the secretory pathway, blocking its transport to the plasma membrane, leading to MCT-1 degradation and its depletion from the cell surface,” they contend.

Retroviruses continue to adapt themselves, however. Some studies have placed the genesis of human-borne HIV and other germs to transmission from our ape cousins, and other viruses like Ebola and Marburg have been attributed to a jump from other primate species.