Blythe Williams, Richard Kay, and Christopher Kirk have published a new article in the PNAS which does a very nice job in synthesizing some new fossils and new genetic data with current hypotheses for the origins of anthropoids, the group which includes old world monkeys (catarrhines), new world monkeys (platyrrhines) and apes (hominoids).
One of the key events that seems to have happened after the origins of anthropoids is an increase in body size, coupled with a corresponding shift in diet. Many of the earliest anthropoids were teeny tiny- no more than a few hundred grams. An animal that small has no problem finding enough bugs to eat. However, if a primate lineage evolves a body size of more than 500 g (“Kay’s Threshold”), that lineage usually needs to supplement that diet of insects with a substantial portion of leaves, and many end up switching to leaves altogether. Eating insects and eating leaves are fundamentally different enterprises, and many adaptations for bug-hunting won’t do a leaf-eating primate any good.
So it seems that in anthropoid origins, there was an increase in body size, followed by adaptations to eating the leaves that this larger body size requires.
What kinds of adaptations do leaf-eaters need that bug-hunters don’t have? Since leaf-eaters are active during the day, their eyes don’t have to be quite as large. However, they still require a great deal of visual acuity, which is relflected in the size of the cornea, a retinal fovea, and a macula lutea.
In addition, they also need to see in color. At night, there is no light to reflect off of a certain object, so color is not as important. In the daytime, different fruits and leaves can be judged on their ripeness by how bright in color they are.
As a result of the eyes converging to the midline of the face, the chewing muscles had a much greater propensity to distort the visual field of a primate who was chewing its food. The masseter (which forms a “sling” around the lower jaw and then runs along the side of the head) and the other chewing muscles put some pressure on the eye from behind and the from the side, so a bony cup- the postorbital septum- helps to keep them separate from one another.
One of the more interesting things that recent research in primate genetics and primate paleontology has taught us is that New World Monkeys and Old World Monkeys both have enlarged brains, but that their brains enlarged separately, after they split from one another. This occurred after both of them enjoyed an increase in visual acuity, so it probably means that the structures which became enlarged are distinct from the visual structures of the brain. There are many hypotheses for why having an enlarged brain would be adaptive, including ecological models (it’s easier to find patchy food resources), social models (it’s easier to manipulate your buddies if you can think what they’re thinking), and others.
The article is a nice summary of recent work in anthropoid paleontology and genetics. I am always a little cautious when attributing an adaptive cause to so many traits, though. There are definite adaptations here (stereoscopic vision, for instance), but I think it’s good to exercise caution when talking about other things, like the reduction of olfactory systems. They also discuss why it’s difficult to pinpoint the biogeography of the early anthropoids. Asia and Africa are both pretty promising as the site of the first anthropoids, and it was much easier to go back and forth between the continents back then.
Williams, B., Kay, R., & Kirk, E. (2010). New perspectives on anthropoid origins Proceedings of the National Academy of Sciences, 107 (11), 4797-4804 DOI: 10.1073/pnas.0908320107
I can see leaves as an early fallback, but IMO fruits are likely to be more important in driving eyesight since they can co-evolve faster (not having the constraints leaves have). Moreover, they are dedicated to cooperative reproduction (bribes for spreading seeds).
As for enlarged brains, the limiting factor here may well be energy, with an increased proportion of fruits and (oily) nuts offering the necessary energy while insect diets provided the protein for growth. IIRC there’s an early Jurassic mammal (Hadrocodium wui?) with an abnormally large brain, and it seems possible to me that this was a fluke produced by co-evolution with some plant. (AFAIK flowering plants weren’t around yet, but conifers and other gymnosperms could conceivably have produced an oily or sugary/starchy seed casing. Or perhaps there was a cooperative relationship with an angiosperm precursor drinking proto-nectar while hunting insects that performed pollination: AFAIK we don’t really know how or when the whole flower thing kicked off.)