Old World Monkeys Had Complex Brain Structure, Reveals Scan

A fossilized skull from a 15-million-year-old monkey was analyzed researchers by from Duke University, with surprising results.

The ancient monkey, with the scientific name Victoriapithecus, made headlines when its fossilized skull was discovered in 1997 on an island in Kenya’s Lake Victoria, where it lived 15 million years ago. More recently, high-resolution X-ray imaging was used to gaze inside its cranial cavity and a a three-dimensional computer model of what the animal’s brain likely looked like was created.

The Micro-CT scans of the skull show that Victoriapithecus had a tiny brain relative to its body.

But the animal’s brain turned out to be surprisingly complex.

Study paper co-authors Fred Spoor, of the Max Planck Institute for Evolutionary Anthropology, and Lauren Gonzales, of Duke University, calculated its brain volume to be about 36 cubic centimeters. That is less than half the volume of monkeys with the same body size living today.

If similar-sized monkeys have brains the size of oranges, the brain of this particular male was closer to a plum. Co-author Brenda Benefit of New Mexico State University, who first discovered the skull with NMSU co-author Monte McCrossin, said:

“When Lauren finished analyzing the scans she called me and said, ‘You won’t believe what the brain looks like.'”

The CT scans showed the researchers multiple unique wrinkles and folds. The olfactory bulb, the portion of the brain used to perceive and analyze smells, was three times larger than would be expected.

“It probably had a better sense of smell than many monkeys and apes living today,” Gonzales said. “In living higher primates you find the opposite: the brain is very big, and the olfactory bulb is very small, presumably because as their vision got better their sense of smell got worse.”

“But instead of a tradeoff between smell and sight, Victoriapithecus might have retained both capabilities,” Gonzales said.

The finding is meaningful since it offers new clues to how primate brains evolved over time, and is from a period from which there are very few fossils.

“This is the oldest skull researchers have found for Old World monkeys, so it’s one of the only clues we have to their early brain evolution,” Benefit said. “Brain size and brain complexity can evolve independently; they don’t have to evolve together at the same time.”

The finding also gives support to theories that the small brain of the human ancestor Homo floresiensis, whose 18,000-year-old skull was discovered on a remote Indonesian island in 2003, isn’t as remarkable as it might seem. In spite of their small brains, Homo floresiensis was able to make fire and use stone tools to kill and butcher large animals.

Lauren A. Gonzales, Brenda R. Benefit, Monte L. McCrossin & Fred Spoor
Cerebral complexity preceded enlarged brain size and reduced olfactory bulbs in Old World monkeys
Nature Communications 6, Article number: 7580 doi:10.1038/ncomms8580

Illustration: Credit: Fred Spoor of the Max Planck Institute for Evolutionary Anthropology.