Earliest primate ancestor had surprisingly tiny brain

From New Scientist

The earliest ancestors of old-world monkeys, apes and humans had surprisingly small brains, a new study shows. …

This implies that higher primates, or anthropoids, must have still had small brains when Aegyptopithecus lived, about 29 million years ago — which is after old-world anthropoids diverged from their new-world cousins.

The large brains of modern monkeys and apes in the two regions must therefore have evolved independently sometime after that, says Simons.

That seems fairly significant. Why did brains in two different regions of the world both grow so large? Is it something about how monkeys lived that led to a need for larger brains?

The new skull hints at several … features of Aegyptopithecus' lifestyle. The relatively small eyes suggest it was active during the daytime (diurnal), and the well developed visual region of its brain indicates it had acute vision.

The new skull, that of a female, is also smaller and has more delicate canine teeth than an earlier, male skull fragment from the same species - indicating that males were much larger and fiercer than females. Such size disparities only arise in primates that live in groups, where evolution favours larger males who can better compete for mates and defend the group against threats.

All three of these characteristics, however — diurnality, acute vision, and group living — have often been advanced as reasons why primates evolved their large brains. However, Aegyptopithecus, which has all three while still having a tiny brain, argues against these theories, says Simons.

I don't see why Aegyptopithecus's small brain argues against these theories. It suggests that living that sort of lifestyle was possible without large brains. But the parallel evolution of large brains in separate parts of the world also suggests that diurnality, acute vision, and group living encourage the development of large brains.