Scans of one of the oldest and most complete monkey skulls found in South America suggest that the evolution of the primate brain was more complex (and less consistent) than previously realized, say researchers writing in the journal Science Advances.

A team led by scientists from the American Museum of Natural History, the Chinese Academy of Sciences, and the University of California Santa Barbara created a 3D reconstruction of the inside of the skull of Chilecebus carrascoensis – a now-extinct species of New World monkey that lived in Chile some 20 million years ago.

The main takeaway from the study: Anthropoid primate brains have grown and shrunk repeatedly – and independently – over time, and in some cases, their evolution turns out to be far more complex than previously thought. Research such as this could help explain the development of the (uniquely sizable) Homo sapien brain.

"Human beings have exceptionally enlarged brains, but we know very little about how far back this key trait started to develop," lead author Xijun Ni, a research associate at the Museum and a researcher at the Chinese Academy of Sciences, said in a statement.

"This is in part because of the scarcity of well-preserved fossil skulls of much more ancient relatives."

Anthropoid primates encompass a large family that can be split into two types: Old World monkeys and great apes (think: macaques, gorillas, and, of course, humans), or catarrhines, and New World monkeys (tamarins, capuchins, and squirrel monkeys, for example). The 20-million-year-old Chilecebus belongs to the latter – a group also known as the platyrrhines. 

Although the two types are thought to have diverged from a common ancestor around 36 million years ago, they share many traits and features that appear to have evolved or developed in parallel after the split. 

Second, the researchers built on previous research to determine the size of Chilecebus' brain in relation to its body size, confirming it is relatively small for a primate: its phylogenetic encephalization quotient (PEQ) works out at just 0.79. In comparison, most primates alive today have a PEQ between 0.86 to 3.39, whereas humans have an extraordinarily large PEQ of 13.46. This shows the brain size of Homo sapiens increased rapidly and drastically over a small timeframe evolutionarily speaking (7 million years).

Comparing the Chilecebus' PEQ to other anthropoids (alive and extinct, platyrrhines and catarrhines), the researchers concluded that while there is a general trend that sees PEQ increase over time, there are also cases where the reverse takes place and PEQ drops – particularly within the platyrrhine branch of the family tree. This shows primate brains have grown and shrunk over time.

Speaking about the new study, John Flynn, the Museum's Frick Curator of Fossil Mammals, said: "Chilecebus is one of those rare and truly spectacular fossils, revealing new insights and surprising conclusions every time new analytical methods are applied to studying it."