A rare, palm-size, 20 million-year-old fossil skull from an extinct monkey that was discovered high in the Andes Mountains is helping researchers learn more about the evolution of the human brain.

The fossilized skull is the only known specimen of the extinct Chilecebus carrascoensis, a New World monkey that lived in what is now Chile and had a body mass of roughly a pound.

A study published Wednesday in the journal Science Advances, led by researchers from Manhattan’s American Museum of Natural History, the Chinese Academy of Sciences, and the University of California Santa Barbara, suggests that the brain “enlarged repeatedly and independently over the course of anthropoid history and was more complex in some early members [of anthropoid primates] than previously recognized.”

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

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

John Flynn, the museum’s Frick curator of fossil mammals, called Chilecebus “one of those rare and truly spectacular fossils, revealing new insights and surprising conclusions every time new analytical methods are applied to studying it.”

The use of high-resolution CT scanning and 3D digital reconstruction of the inside of Chilecebus’ skull gave the research team new understanding of the anatomy of its brain.

With Post wires