There's a new book out called "Laughter in Ancient Rome" by Mary Beard, University of California Press, in which she suggests that Romans didn't smile.

There was no Roman word for smiling, no mention of it anywhere. Absence of evidence is, of course, not evidence of absence. But it would contradict the hardwired idea.

Incidentally, she was taking the idea from Jacques Le Goff who theorised that smiling was invented in the Middle Ages.

Paul Ekman has shown, fairly conclusively in my opinion, that the expression of emotion in the face is universal. In fact, facial expressions evolved for communication purposes. Despite the regular pleading of, 'I am not a mind reader,' human beings evolved to be able to read expressions at a distance. The reason we have white sclera is so that we can see where another person is looking with relative ease.

I think there is an argument to be made about 'display rules' in ancient cultures, but when Romans were reading Catullus one can only guess that they were gut-laughing about a senator brushing his teeth with donkey piss.

This is definitely a topic where you need to look at the science of expression rather than the opinions of historians. We know something about the evolution of expression and so are more likely to find conclusive answers there.

Out of curiosity, is that thing about westerners and eyebrows really true? Do you have a source or link?

I suppose that in the early years of humanity we showed our teeth to express anger/fear, like most of others mammals.