As humans evolved from their prehistoric forebears, one noticeable change was that they had prominent, bony browlines, and modern humans don't. Instead, we have flatter foreheads and emotive eyebrows.

In a new study published in Nature Ecology & Evolution, a trio of researchers say they now know why: it all comes down to making friends.

Believe it or not, scientists have puzzled over this change for years -- proposing that early man needed the beefier brow for protection, or to aid their larger jaw muscles with chewing tougher grub.

Ricardo Miguel Godinho, Penny Spikins and Paul O’Higgins tested -- and tossed -- those long-held hypotheses and instead say they've gotten to the bottom of the strips of hair atop our eyes.

"Conversion of the large browridges of our immediate ancestors to a more vertical frontal bone in modern humans allowed highly mobile eyebrows to display subtle affiliative emotions."

Simply, our flatter foreheads acted as a billboard to display our more mobile eyebrows -- which were needed to express more subtle emotions than early man needed to convey. Non-verbal communication is essential to social interaction.

Being able to communicate between others that way gave humans the chance to get to know, at a glance, what others are thinking, even if they've just met, instead of just clubbing the new guy.