English: An endocast of a Homo rudolfensis skull on display in the Hall of Human Origins in the Smithsonian Museum of Natural History in Washington, D.C.

Homo rudolfensis lived about 1.9 million years ago. Homo rudolfensis were discovered by Bernard Ngeneo in 1972 on the east side of Lake Rudolf (now Lake Turkana) in Kenya.

Originally, it was thought that the brain of Homo rudolfensis was about 752 cubic centimeters in size. But a reconstruction of the skull in 2007 showed it to be just 526 cubic centimeters. A published analysis in 2008 said it was 700 cubic centimeters. (A chimpanzee had a brain that's about 500 ccs in adulthood, while a modern human has a brain that's 1,850 ccs in adulthood.)

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.