Nut-Size Ancient Skull Explains Our Brains’ Bigness?

(NationalGeographic)   By scanning a 54-million-year-old skull roughly the size of a walnut, scientists have created the first virtual 3-D model of an early primate brain, a new study says. Surprisingly, the model suggests that primates (such as lemurs, monkeys, apes, and humans, among others) might have evolved larger brains as a result of the need to move quickly from tree to tree—not, as commonly assumed, to hunt for fruit or navigate within a single tree.

Charles Q. Choi
for National Geographic News
June 30, 2009

By scanning a 54-million-year-old skull roughly the size of a walnut, scientists have created the first virtual 3-D model of an early primate brain, a new study says.

Surprisingly, the model suggests that primates (such as lemurs, monkeys, apes, and humans, among others) might have evolved larger brains as a result of the need to move quickly from tree to tree—not, as commonly assumed, to hunt for fruit or navigate within a single tree.

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The 1.5-inch-long (4-centimeter-long) skull belongs to the long-gone Ignacius graybullianus—described as a cousin of our earliest ancestors—which arose less than ten million years after the dinosaurs vanished.

Discovered in Wyoming roughly 25 years ago, the fossil “is the most complete early primate skull known,” said study co-author Jonathan Bloch, a vertebrate paleontologist at the University of Florida.

Due to its completeness and age, the skull gives us the clearest idea yet what early primates were like, the researchers argue. Even so, they say more early-primate fossils are needed to test the study’s conclusions.

Doing More With Less

After taking more than 1,200 detailed X-ray images of the skull, researchers combined them to help create a 3-D model of Ignacius’ brain.

The model showed a brain just one-half to two-thirds the size of the smallest modern primate brain, the study says.

And yet that seems to have been enough for tree dwelling and fruit seeking. Ignacius’ teeth, for example, suggest it had a fruit diet, while the animal’s claws and flexible joints hint at tree dwelling.

The finding therefore reopens the question of what triggered the evolution of large brains in later primate species, if not branch living or fruit eating?

The Eyes Didn’t Have It

One activity Ignacius seems unsuited for is jumping from tree to tree, as opposed to simply climbing branches. In primates, this type of leaping generally requires long hind limbs, large inner-ear organs linked to balance—and strong visual processing.

Instead of a robust center of vision, Ignacius’ brain had large lobes dedicated to smelling, the model suggests.

The prehistoric primate “was mostly a nose-first animal that relied on smell instead of sight, unlike modern primates, which have far more developed visual processing areas,” explained lead study author Mary Silcox, a vertebrate paleontologist at the University of Winnipeg.

For primates, “stepping up vision would have been key for leaping safely,” she surmised. But that apparently would have to wait until the time—and the brain—was right.

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