As you can see from the illustration below, primate faces are extremely variable. Sharlene Santana, Jessica Alfaro and Michael Alfaro of the University of California, Los Angeles were interested in how that diversity came about. They found that both local environments and population size were driving factors in facial evolution.
Eleven species of Neotropical (Central and South American) primates were compared. In each case, two to ten photographs of adult males were studied. Each face was divided into fourteen sections which were graded for colors and patterns of hair and skin.
Primates from Central and South America. (1) Cacajao calvus, (2) Callicebus hoffmansi, (3) Ateles belzebuth, (4) Alouatta caraya, (5) Aotus trivirgatus, (6) Cebus nigritus, (7) Saimiri boliviensis, (8) Leontopithecus rosalia, (9) Callithrix kuhli, (10) Saguinus martinsi and (11) Saguinus imperator.
Illustrations by Stephen Nash, courtesy of University of California - Los Angeles.
Interestingly, they found that primates that live in small groups have more complex facial patterns (varying colors, alternating areas of hair and skin) than those that live in large social groups. As Santana says:
We found very strong support for the idea that as species live in larger groups, their faces become more simple, more plain.
In case you’re wondering what this says about humans, by the criteria of this study, we have extremely plain faces. But don’t feel bad. The authors suggest that plainness correlates with expressiveness. Emotional content can be read more easily on a plain face, which is much more important for individuals living in large groups.
Among the additional findings, species that live near other members of the same genus had more complex facial features than primates with no closely related neighbor species. Presumably, this helps to avoid hybridization between similar species. Also, colorization of specific facial regions correlated well with latitude and forestation. Monkeys living at higher latitudes had lighter crowns but darker nose and mouth patches than those at lower latitudes, those in forested regions had darker crowns and eye patches than those in open areas.
Taken together, the data suggest that facial differences did not arise by random chance but are selected for based on behavior (group size) and location.
You can see a slide show of monkey faces (not necessarily the ones in this study) presented by BBC Nature here.