From New Scientist:
The earliest ancestors of old-world monkeys, apes and humans had surprisingly small brains, a new study shows.
This finding – based on a newly described fossil skull – means that large brains evolved independently in new- and old-world primates. It also suggests that evolutionary anthropologists may have to rethink some cherished theories about why such big, powerful brains evolved.
The skull in question, which belongs to a roughly cat-sized primate called Aegyptopithecus zeuxis, is remarkable because it is so well preserved.
The brain turned out to be much smaller than they expected – in fact, no larger in proportion to the body than the brain of lower primates such as lemurs. 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.
Update: The study has now been published in the Proceedings of the National Academy of Sciences.