To Weyrich’s surprise, one of the teeth from El Sidrón contained the genetic signature of a bacteria-like microorganism, Methanobrevibacter oralis, which is still found in our mouths to this day. By comparing the Neanderthal version with the modern human version, she was able to estimate that the two had drifted apart around 120,000 years ago.

If Neanderthals and present-day humans had always shared the same oral companions, you would expect this to have happened much, much earlier – at least 450,000 years ago, when the two subspecies took different paths. “What this means is that the microorganism has been transferred since then,” says Weyrich.

It’s impossible to know for sure how this happened, but it could be linked to something else that occurred 120,000 years ago. “For me what’s fascinating is this is also one of the first time periods where we have described interbreeding between humans and Neanderthals,” says Weyrich. “So it’s wonderful to see a microbe sort of being wrapped up in that interaction.”

Weyrich explains that one possible route for the transfer is kissing.