10% of the human genome is made up of viruses. Here’s how they helped shape our species’ evolution

If Charles Darwin reappeared today, he might be surprised to learn that humans are descended from viruses as well as from apes. Some 8% of human DNA represents fossil retroviral genomes, and that is not counting the LINE elements and other retrotransposons that are scattered so liberally across our genome. Darwin might be reassured that we share most though not all of these insertions with chimpanzees. But how did endogenous viruses first come to light?

Viruses that insert their own genetic material into the host’s genome are called retroviruses. Their nature was first understood in the 1960s and 1970s, although some had been isolated decades before.

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If a retrovirus infects a cell in a person’s lungs or skin, that may be bad news for the person, but it has limited consequences for our species’ evolution as this DNA is not passed onto the next generation. However, sometimes a retrovirus gets into the germline: the cells that give rise to eggs and sperm, where viral DNA can be passed onto our offspring. These pieces of viral DNA are called endogenous retroviruses or ERVs. It is these heritable bits of viral DNA that can change the course of evolution.

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