In response to this answer:
http://www.straightdope.com/classics/a2_302.html
I would suggest that a more interesting theory is the aquatic ape theory. Here is a pubmed abstract of a relevant article:
The Aquatic Ape Hypothesis is not thought much of around here.
Has anyone advanced the theory that sinuses are, well, an accident? They don’t really help, and they don’t hurt all that much (in the grand scheme of things), so there was just never any selective pressure at all.
Of course, to the extent that they influence the shape of the face, there would be selective pressure, due to sexual selection. But sexual selection is a positive-feedback system, which means it eventually leads to essentially random traits. In other words, sinuses could have started as a neutral fluke trait, but once sexual selection got ahold of them, they were reinforced.
I thought they dated back to the Cold War and the famous treaty between sinuses and the Communist Russians, calledthe Sinus-Soviet alliance… ?
fair enough. While the theory sounded pretty hokey to me, so did most of the stuff in the original answer. If we’re grabbing at unproven theories we may as well choose some fun ones, right? Here is some more interesting stuff:
I propose “sinai” as an alternative plural.
And let’s not forget that most of the physical features of Homo sapiens are neotenic – that is, they retain their juvenille character well into adulthood. (Adult male humans don’t grow crests on their chests, for example, while all of the other adult male great apes do.)
It may be that the sinuses allow the human face to retain its juvenille appearance as it grows. I guess you could call us Baby-Faced Apes.
What is a chest crest?
As man began walking upright, the Foramen Magnum (where the spinal column enters the skull) migrated from the back of, to underneath, the head. As there is no efficient way to obtain muscular support for a long and heavy face, this created the need to balance the head on the backbone. This is why homo sapiens have a flattened face, sinuses, impacted wisdom teeth and (many of us) crowded teeth.
rwj
My mistake. I meant the sagittal crest, which is on top of the head and is very pronounced in adult male gorillas.