Relative Size of Brain

A. anamensis

Small brain, approximately 365-370 cc. Similar to chimpanzees relative to body size. About 1/4 the size of modern human brains.

A. afarensis

Small brain, 380-430 cc. Slightly larger than modern chimps but only about 30% of modern human brain size. Brain-to-body ratio similar to great apes.

A. africanus

Slightly larger brain, 420-500 cc. Shows modest increase over A. afarensis but still only about 35% of human brain size. Indicates beginning of gradual encephalization trend.

A. sediba

Brain size around 420-450 cc, but with some derived Homo-like features in brain organization (based on endocasts). Shows mosaic of primitive size with potentially more advanced neural organization.

P. aethiopicus

Small brain, approximately 410 cc. No increase despite later chronological appearance - indicates brain expansion was not priority in robust lineage.

P. robustus

Small brain, 500-530 cc. Despite living contemporaneously with larger-brained Homo species, shows no trend toward encephalization. Evolutionary investment went to chewing apparatus rather than brain.

P. boisei

Small brain, 480-545 cc. Similar to P. robustus - shows that robust australopithecines did not follow the brain expansion trend seen in Homo lineage. Resources devoted to massive chewing system rather than brain tissue.