PNAS

No brainer.
The skull shape of a female A. zeuxis reflects well-developed visual system (back of the head), but not much thinking power.

Human Ancestors Were No Brainiacs

A 30-million-year-old skull discovered in Egypt indicates that the brains of humans' earliest ancestors were not as advanced as some have thought.

The Old World anthropoid primates made their appearance on Earth more than 35 million years ago. These so-called catarrhines are named for their downturned nostrils. But they had other characteristics that have carried through the hominid line: they were diurnal, for example. In addition, they had flat fingernails, not particularly useful tails, and they were sexually dimorphic--the females were significantly smaller than the males.

But what were their brains like? Some researchers have long assumed that the increased visual processing that goes with the demands of daylight living also boosted brain power in these primates. Now an exceptionally well preserved 30-million-year-old skull from a female catarrhine suggests that expansion of the neocortex--associated with the ultimate development of humanlike intelligence and problem-solving--did not necessarily evolve in tandem with the visual system.

The authors, led by Elwyn Simons of Duke University in Durham, North Carolina, report online this week in the Proceedings of the National Academy of Sciences that although the skull is of a female not quite fully grown, it still shows that the brain cases were smaller than had been thought--and only 70% the size of a male of the same species, Aegyptopithecus zeuxis. From both the teeth and skull, A. zeuxis may be the "earliest evidence" for extreme sexual dimorphism in primates "in or near our direct ancestry," says Simons. Males tend to be larger than females among primates that live in large groups, he says.

But the authors also contend that earlier estimates of the males' brain size--from fragmentary samples--were also too high. The latest work indicates that even though the animals had an expanded primary visual cortex, in keeping with a diurnal existence that entails more complex social interactions, their cortices remained small. That, the authors say, suggests that cortical expansion was not just the result of the hours the monkeys kept, their diets, and their social lives. Rather, they say, the brain growth that ultimately led to Homo sapiens was probably pushed by more general "selection for higher intelligence" spurred by such circumstances as the invasion of new competitors and predators.

The work is "a refreshing take on the problem of brain size evolution" in primates, says anthropologist Chet Sherwood of George Washington University in Washington, D.C. "Those of us who study living species" need to take more account of the ancient developments underlying "the diversity in brain organization we see today," he says.

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Posted in Evolution, Archaeology