The prominent ridges underlying the eyebrows of early humans may have signalled social status or aggression - and gave way to the more changeable, expressive eyebrows of modern humans - suggests a paper published online this week in Nature Ecology & Evolution. This is an about-face on previous hypotheses, which argued that browridges played a structural role within skulls.
Modern humans have smooth, vertical foreheads with communicative eyebrows. In contrast, early humans sported thick, bony browridges. Previous studies suggested that browridges protected against the stresses of biting and chewing, or arose through the meeting of two distinct parts of the skull - the eye sockets and braincase.
Ricardo Godinho and colleagues digitally recreated a fossil Homo heidelbergensis skull from what is now Zambia - thought to be between 300,000 and 125,000 years old - and experimented with changing the browridge size and applying different biting pressures to the skull. They found that the browridge in the fossil is much larger than needed to account for the disjunction between the eye sockets and braincase, and that a larger browridge does little to protect the skull when eating.
Instead, browridges may have played a social, not physical, role, the authors propose. Similar skull growths are used for signalling in other primates. The baboon-like mandrills, for example, have bony, colourful muzzles that signal dominance in males and reproductive status in females. Early human browridges may have played a similar role, the authors argue, perhaps as a permanent signal of social dominance or aggression. As humans became more social, brow flattening allowed instead for the development of more visible and mobile eyebrows capable of subtler, changing displays of emotion.