When scientists mapped the human genome, they found that just 2% of it controlled our own functioning. Fully 8% was the DNA of fossil viruses.

Paleovirologists who study fossil viruses have found that all viruses share a common single-cell ancestor from more than 3 billion years ago.

About 1.5 billion years ago, viruses changed their protein coating, allowing them to penetrate the cells of their hosts.

As multicellular life became increasingly complex, viruses got simpler, eventually giving up independent life. They threw out genes they didn’t use and discarded their means of reproduction until they were totally reliant on cell hosts.

When a virus enters a cell, part of its RNA is converted to DNA within the cell’s gene code.

If that cell is an egg or sperm, that DNA could be passed to the next host individual.

However, that can only happen if the virus does not kill the host.

So, if the virus is beneficial or benign—or if the host’s immune system is able to defeat it and become healthy enough to reproduce—then the viral DNA can be passed down through generations.

This process has occurred enough times over millions of years for viral DNA to make up that 8% of our gene code.

And fossil viral DNA has been very beneficial to us and all other organisms, as we’ll discuss in another EarthDate.