Chilean monkey (Chilecebus carrascoensis) skull

The two-inch (five-centimeter) long fossil skull of the 20-million-year-old Chilecebus (“Chilean monkey”) is the only one of its kind ever discovered. When Museum paleontologist John Flynn, with research associates Xijun Ni and Andre Wyss, investigated its internal structure, cutting open the skull wasn’t an option. Instead they used a high-resolution CT scanner to peer inside. One finding was the shape of the head’s semicircular canals, highlighted here in blue. These structures help control balance, and they affect how animals walk. Another discovery—the extremely small brain size—proves that the monkeys living today in Central and South America evolved large brains after their ancestors migrated from Africa.