Our earliest ancestors had surprisingly small brains particularly about 29 million years ago, says a new study.
Elwyn Simons, a physical anthropologist at Duke University in Durham, North Carolina, US and team measured a preserved skull of a roughly cat sized primate Aegyptopithecus zeuxis also known as Dawn Ape found near Cairo, Egypt.
The earliest ancestors of old-world monkeys, apes and humans had surprisingly small brains and large brains evolved independently in new - and old - world primates, the researchers said.
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, reported online edition of New Scientist.
The large brains of modern monkeys and apes in the two regions must therefore have evolved independently sometime after that, says Simons.
"Without a doubt, basal anthropoids had small brains. It confirms that absolutely," says Callum Ross, an evolutionary anthropologist at the University of Chicago who was not involved in the study.
The new skull hints at several other features of Aegyptopithecus' lifestyle. The relatively small eyes suggest it was active during the daytime (diurnality), 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 - diurnality, acute vision and group living - however, have often been advanced as reasons why primates evolved their large brains.