Primates are distinguished from many other mammals by a suite of features of the visual system, among them convergent orbits, postorbital bars, increased density of photoreceptors and ganglion cells in the central retina, and increased representation of the central retina in the visual structures of the brain. These features appear to be part of a suite of changes related to visually guided manual predation of insects in the fine shrub layer of tropical rainforests. Comparative analyses of eye size and shape across mammals suggest that nocturnal primate eyes are not differently shaped than those of other mammals, but they are larger. This suggests that the origin of primates was associated with increased visual acuity, but not necessarily increased brightness of the retinal image. Claims that early primates were diurnal have been made on the basis of relative orbit size in the fossil primate Teilhardina asiatica. This early fossil primate falls outside the size range of living primates, raising the question of how to infer activity pattern in fossil primates that are smaller then living primates.