If you were to run into our ancient ancestor Homo heidelbergensis on the street, the first thing you might notice is their small forehead and heavy brow.

You and I have lofty domed foreheads. But our great-great-great-etc. grandparents had a skull that could hardly hold on to a hat, with a thick, protruding brow.

You can see the difference easily when comparing skulls.

Heidelbergensis lived around 700,000 to 200,000 years ago and is suspected to be a possible shared ancestor of Neanderthals, the recently discovered Denisovans, and modern humans.

But one place where modern human anatomy diverged from these and other ancestors’ anatomy was in the evolution of our smooth, long foreheads and our agile eyebrows. “The brow ridge is one of the most distinctive features that mark out the difference between archaic and modern humans,” says Penny Spikins, an evolutionary anthropologist at the University of York.

So why did our foreheads become so distinct? The science on this question isn’t settled, but Spikins and her colleagues offer an intriguing new hypothesis published Monday in the journal Nature Ecology and Evolution.

It’s this: Our foreheads facilitate empathy. They are a canvas upon which our eyebrows can paint emotions. And as we became an increasingly social species engaged in increasingly sophisticated communication, they helped us survive.

How your eyebrows “speak”

Spikins and her colleagues came to favor this hypothesis after an analysis of the H. heidelbergensis skull — specifically a skull called Kabwe 1, which was discovered in 1921 and is currently in the Smithsonian. They asked: Why did it have such a thick brow ridge in the first place? The common explanation is that the large brow gave the face additional stiffness and was useful in chewing tough meats.

Using a three-dimensional computer model of a heidelbergensis skull, they manipulated the size of the brow ridge. A smaller ridge ought to increase stresses on the skull. But this wasn’t the case.

It turns out the heidelbergensis brow ridge is overbuilt. “We reduced the brow ridge to the absolute minimum size possible for it to hold the face together,” says Paul O’Higgins, a University of York archeologist and a co-author of the paper. “And still it made no difference in how the face was biting.”

Simply put, the brow ridges of our ancestors did not seem to serve a mechanical function. They were possibly a social signal, a sign of strength and dominance. For the heidelbergensis, prominent brows were just plain sexy, a desirable trait in a mate.

But as time passed, the need to assert dominance was less important and the need to empathize and communicate with others became more important. And that’s why the heavy brow ridges of our ancestors may have morphed into the lofty foreheads of today.

Whereas the strong brow ridge on the heidelbergensis signaled strength, our relatively massive foreheads allow for empathy. It’s an instance where evolution of our physical anatomy reveals more about our minds than our environments.

It’s also possible that as our foreheads grew larger, the muscles used to control our eyes and eyebrows became more complex and capable of subtle movement. Think of all the ways our eyebrows can subtly tell those around us what we’re thinking. Furrowed brows indicate worry or concentration. A cocked eyebrow says we’re skeptical.

Ultimately, expressive eyebrows are “a biological mechanism to demonstrate to other people what we’re genuinely feeling,” Spikins says, somewhat similar to blushing.

It’s very hard to come to firm conclusions about ancient human history

When talking about ancient history, it’s hard to be definitive. There are few samples to study in the fossil records and huge gaps in our understanding of how human anatomy changed, and when. And so Spikins and her co-authors’ hypothesis is far from ironclad.

For instance, it’s possible that heidelbergensis’s huge brow was related to higher levels of testosterone, Ashley Hammond, a paleoanthropologist at the George Washington University, says. “The skeleton as a whole is a lot more robust and thicker,” she says. The thick brow ridge isn’t necessarily designed for social signaling. It could just be a consequence of other differences in body chemistry.

So this paper is far from the final word. “The authors’ suggestion is an intriguing one, [but] I think it remains speculative,” Ian Tattersall, the curator emeritus of human origins at the American Museum of Natural History, writes in an email.

There are some important limitations to this study too. This work is the result of a computer manipulation of one skull that was missing a lower jaw. So the authors took some assumptions as to what the jaw would look and move like.

What else might explain our lofty foreheads?

It’s still possible that our increasing forehead height was the secondary result of other physical changes, such as our faces becoming flatter overall, and our brains shifting forward, and our brains’ frontal lobes growing larger. Or perhaps our ancestors just found higher foreheads more aesthetically pleasing.

Or, even more simply, maybe a higher forehead just helped keep hair out of our eyes, thus ensuring our vigilance and survival. A higher forehead could also better show off our eyes (which have the curious property of being whiter than those of other primates. They help others see where we are looking.)

“It was probably a suite of changes that happened together,” Hammond says. For instance, a larger brain could allow us to better intuit the emotions of others, and at the same time provide for a larger forehead to better display emotions.

Charting our evolutionary history is a hard task. But overall, Spikins says, it’s important to keep considering how human cooperation played a guiding role. Though we’re not always great at it, human cooperation and communication is the key survival mechanism of our species. It shouldn’t be surprising to consider how our anatomies reflect that fact.

“There’s a growing recognition that being able to get on with other people, showing affiliative emotions, is more important in our evolution than” previously recognized, she says. Our bodies may have changed to allow us to be more empathic creatures. And that’s pretty cool.