Newborn human babies probably should be left in the womb for a little longer to cook some more, but alas, they’re born in nine months, coming out way more helpless than, say, a newborn giraffe, which comes out of the womb already knowing how to walk, or a primate that comes out covered in fur that keeps it warm.

That last bit is something some researchers who published their findings in the journal Evolutionary Human Sciences were interested in. Their findings suggest that the human body may have engineered a built-in solution for keeping newborns warm. It’s something all mothers are naturally equipped with and already use to raise their children, but no one really thought about its alternate use.

I’m talking about the warmth provided by a mother’s breasts.

Researchers at the University of Oulu in Finland recently discovered that the breast tissue of lactating women resists cooling at roughly twice the rate of non-breastfeeding women and men. In a climate-controlled study, nursing mothers lost only surface temperature when exposed to cold, while other groups lost nearly double that.

This suggests that the permanent breast tissue unique to humans may have evolved as a kind of thermal support system to prevent newborn hypothermia during skin-to-skin contact.

Human Breasts Are Still Largely a Mystery to Us

In the world of evolutionary biology, human breasts are still a bit of a mystery. While other primates only develop noticeable breast tissue while actively nursing, humans develop them at puberty and keep them for life. For years, scientists have suspected that it probably had something to do with sexual selection, which would position breasts as a way to entice mates.

The findings of the Oulu team suggest that there might be a more practical, survival-based origin for breasts that extends well beyond them being something that titillates a mate. In this case, a mother’s chest could be the only reliable heat source available to keep a baby warm.

The research says that the specific shape and fat content of the breasts maximize the surface area for heat transfer, while the increased blood flow in the breasts during lactation keeps that heater burning just hot enough to warm a cold baby. It’s an interesting argument, one that finds add yet another practical use for breasts outside of lactation.

Should this idea become commonly accepted, it would mean that one of the most well-known and renowned features of the human anatomy might be one of our oldest survival technologies.