The skull of Victoriapithecus, an ancient Old World monkey, grabbed headlines back in 1997 when anthropologists from New Mexico State University discovered it on an island in Kenya’s Lake Victoria. The skull’s owner lived some 15 million years ago, making it the oldest known cranium belonging to an Old World monkey, a group that includes baboons and macaques. Now, thanks to high-resolution X-ray imaging, researchers have discovered the skull is surprisingly tiny compared to those of modern monkeys. It’s also surprisingly complex, suggesting that—contrary to prevailing scientific wisdom—brain complexity in primates can evolve before brain size.

Old World monkeys, one of the two main monkey groups, belong to the family Cercopithecidae, which is related to apes and humans. They are distinguished from New World monkeys by the form of the nose: Like apes and humans, Old World monkeys have narrow noses with downward-facing nostrils; they are classified as catarrhines (Latin for “downward-nosed”). New World monkeys, by contrast, have broader noses with wide septums and nostrils that face outward, and are known as platyrrhines (“flat-nosed”). New World monkeys, such as capuchins, marmosets and spider monkeys, live primarily in the tropical climates of South America, while Old World monkeys are native to Africa and Asia.

Now, researchers from Duke University and the Max Planck Institute for Evolutionary Anthropology have used high-resolution X-ray imaging to delve inside the 15-million-year-old Old World monkey skull found in Kenya nearly two decades ago. Using these images, they created a three-dimensional model of what the brain of Victoriapithecus probably looked like. According to the study’s findings, published last week in the journal Nature Communications, the brain had a volume of around 36 cubic centimeters—which is markedly small in comparison with the animal’s body size, and less than half the volume of the brain of a modern monkey around the same size.

The micro-CT scans of the brain revealed that despite its tiny size, it was surprisingly complex, with many distinctive wrinkles and folds in its surface.
The olfactory bulb, the part that controls how the brain perceives and processes smells, was also three times larger than the researchers expected, indicating that the ancient animal might have had better sense of smell than living monkeys. As study co-author Lauren Gonzalez explained in a press release: “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.”

As the Victoriapithecus skull dates to a period from which few fossils are known to exist, the new study’s findings represent a wealth of new information in the debate over how primate brains evolved over time. Without hard fossil evidence, scientists have long gone back and forth over which came first—large brain size or increased brain complexity. The common wisdom regarding the branch of the primate family that includes apes and humans is that brains evolved to be bigger, and only then got more complex, with a greater number of folds.

The new study, however, suggests the opposite, at least for this Old World monkey. The idea that brain size and complexity can evolve independently, and at different times, undoubtedly has implications for other ancient fossil finds, including the 18,000-year-old skull of a human ancestor discovered on the remote Indonesian island of Flores in 2003. Despite a diminutive stature (around 3 feet 6 inches tall) and a strikingly tiny brain, Homo floresiensis used stone tools to hunt and butcher large animals, and may even have used fire.