It turns out that about 8 percent of the human genome is made up of viruses that once attacked our ancestors. The viruses lost. What remains are the molecular equivalents of mounted trophies, insects preserved in genomic amber, DNA fossils.

The thousands of human endogenous retroviruses, or HERVs, sketch a history of rough times during the 550 million years of vertebrate evolution. The best-preserved one, HERV-K113, probably arrived less than 200,000 years ago, long after human beings and chimpanzees diverged from a common ancestor.

But these retroviruses are more than just curiosities. They are some of the most important enemies we ever had. They helped mold the immune system that is one of the evolutionary marvels of life on Earth.