Yesterday's Science Daily contains a report based on a new paper by Lynne Isbell in the Journal of Human Evolution. In it, she argues that "that primates developed good close-up eyesight to avoid a dangerous predator -- the snake." In Isbell's words: "A snake is the only predator you really need to see close up. If it's a long way away it's not dangerous."
I haven't yet had time to read the paper thoroughly, but the basic idea is certainly an interesting one, and seems concordant with at least some aspects of the fossil record.
If this is true, it may also explain in part why so many people are absolutely terrifed of snakes... it may also help explain the current buzz (which sometimes borders on obession) surrounding the upcoming releason of the movie "Snakes on a Plane" starring Samuel L. Jackson!
Isbell, L. A. 2006. Snakes as agents of evolutionary change in primate brains. Journal of Human Evolution 51:1-35.