English: An endocast of an adult Homo erectus, on display in the Hall of Human Origins in the Smithsonian Museum of Natural History in Washington, D.C.

Homo erectus lived between 1.8 to 1.3 million years ago. It was discovered on the island of Java in 1891 by Eugène Dubois. (It was initially known as "Java Man.") Their brains varied in size, with earlier specimens and specimens in distinct sub-populations having slightly smaller brains than Homo sapiens, and others being just slightly larger. They were about as tall as modern humans, but much more muscular.

An endocast is where scientists fill the inside of a skull, and make a model of the brain. The brain and its blood vessels leave imprints on the inside of the skull. Because more advanced brains have smaller veins and many more folds and lobes, an endocast is very useful in determing how intelligent a human ancestor might have been, and what portions of its brain were more developed.