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The oldest known Old World monkey, Victoriapithecus, first made headlines in 1997 when its fossilized skull was discovered on an island in Kenya's Lake Victoria, where it lived 15 million years ago. An international team led by Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and University College London (UCL), UK, has now visualized this monkey's brain for the first time: The creature's tiny but remarkably wrinkled brain supports the idea that brain complexity can evolve before brain size in the primate family tree. Especially surprising was the size of the monkey's olfactory bulb: It was three times larger in Victoriapithecus than in present-day monkeys of comparable body size.
Fred Spoor and co-author Lauren Gonzales of Duke University in Durham, USA, created a three-dimensional computer model of the animal's brain and calculated its brain volume to be about 36 cubic centimeters, which is less than half the volume of monkeys of the same body size living today. If similar-sized monkeys have brains the size of oranges, the brain of this particular male was more akin to a plum.
"When Lauren Gonzalez finished analyzing the scans she called me and said, 'You won't believe what the brain looks like,'" says co-author Brenda Benefit of New Mexico State University, who first discovered the skull with NMSU co-author Monte McCrossin. "This study shows the power of modern digital imaging methods to get essential information out of precious fossils that would otherwise remain unknown", says Fred Spoor, an expert in studying fossils with computed tomography and 3D visualization.
Despite its puny proportions, the animal's brain was surprisingly complex. The CT scans revealed numerous distinctive wrinkles and folds, and the olfactory bulb - the part of the brain used to perceive and analyze smells - was three times larger than expected. "Victoriapithecus probably had a better sense of smell than many monkeys and apes living today," Gonzales explains. "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 says.
The findings offer new clues to how primate brains changed over time, and during 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," adds Benefit.
In the absence of fossil evidence, previous researchers have disagreed over whether primate brains got bigger first, and then more folded and complex, or vice versa. "In the part of the primate family tree that includes apes and humans, the thinking is that brains got bigger and then they get more folded and complex," Gonzales says. "But this study is some of the hardest proof that in monkeys, the order of events was reversed - complexity came first and bigger brains came later."
The findings also lend support to claims 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, is not as remarkable as it might seem. In spite of their pint-sized brains, Homo floresiensis were able to make fire and use stone tools to kill and butcher large animals. "Brain size and brain complexity can evolve independently; they don't have to evolve together at the same time," says Benefit.
L. Gonzales, B. Benefit, M. McCrossin and F. Spoor
Cerebral Complexity Preceded Enlarged Brain Size and Reduced Olfactory Bulbs in Old World Monkeys
Nature Communications, 3 July 2015 (DOI: 10.1038/ncomms8580)