today. All others have vanished. These dates are approximate and
subject to alteration as more is discovered from the fossil and archeo-
logical record. Even the names may change. The field of hominin pale-
ontology is dynamic, and much presumably still lies out there, buried,
waiting to be uncovered and interpreted.
Examination of the fossil fragments of ancient skulls allows us to
estimate the brain sizes of these early hominins:
Ardipithecus ramidus ~ 350 cubic centimeters (cc)
Australopithecus africanus ~500cc
Homo habilis ~650cc
Homo erectus ~ 1200 cc
Homo neanderthalensis ~ 1400 cc
Homo sapiens ~ 1400 ccThere has been a substantial increase in the size of the brain over the
several million years of hominin evolution. Part of this increase can
be attributed to an increase in the size of the body; the typical Homo
sapiens is larger than the typical Australopithecus, and larger animals
tend to have larger brains. However, there is more to this bigger brain
than increased body size. It appears that the size of the hominin brain
expanded rapidly during the last two million years of our evolution-
ary history. Accompanying the expansion of the brain has been the
development of ever more sophisticated behaviors: tool use, nuanced
social interaction, language, mathematical skill, complex problem-
solving abilities, and a capacity to construct elaborate explanatory
frameworks to aid in understanding our world.
This book is about our brain and its connection with our behavior.
One of those connections is our capacity to kill our fellow humans,