In earlier episodes, you heard that trillions of viruses live peacefully, even beneficially, within each of us today.

And that our ancestors battled now-extinct “fossil viruses,” which are preserved in our own genetic code.

Turns out, that’s been beneficial too.

For instance, viruses helped drive our evolution.

100 million years ago, proto-mammals were infected with a virus that encoded into their DNA the ability to produce syncytin, a protein that viruses use to fuse cells together so they can move between them.

But these early mammals evolved to use this protein in a different way: to fuse the placenta to the uterus, allowing nutrients to pass from mother to fetus and making live birth possible.

Since humans diverged from apes, our proteins have adapted frequently to viral threats, even if they’re not part of the immune response. In fact, virus interactions now make up a third of all our protein adaptations.

Finally, researchers found that ancient viral DNA in our genetic code prompts a more vigorous response to new viral threats.

In experiments, when they removed fossil virus DNA from a cell, its immunity was severely weakened. Restoring the viral DNA restored the immune response.

While a rare few viruses have been quite harmful to humans, the vast majority—over millions of years—have helped us to become what we are and then to remain healthy.