8% of Your DNA Is Ancient Viruses

Okay so. About 8% of your DNA doesn't come from your human ancestors. It comes from viruses.

Retroviruses — like HIV — infect cells by inserting their DNA directly into your genome. Most of the time that stays in body cells and doesn't get passed on. But occasionally one would infect a sperm or egg cell, and then it becomes permanent. It gets inherited. For millions of years.

Those ancient viral sequences are still in our genome right now. In every cell. They're called endogenous retroviruses and we've been carrying them since before our species existed.

The part that really got me: some of them aren't just sitting there doing nothing. A protein called syncytin, which is essential for forming the placenta in mammals, originally came from one of these ancient viruses. A viral sequence that got repurposed by evolution into something we literally need to reproduce.

I don't know how to feel about this. Fascinated, definitely. A little unsettled. Mostly just amazed that we're walking around carrying millions of years of evolutionary scars inside every single cell.