Humans have the largest brain to body size ratio among terrestrial mammals, rivaled only by the smaller odontocete whales. The modern human brain has nearly tripled in size since the origins of the subfamily Homininae. The brain reaches its modern size relative to body size at approximately 300,000 years ago, which is late in human evolutionary history. Brain size reaches its apogee among the Neanderthals, where the average cranial capacity was about 300 cc more than that of the average for living humans (1,200 cc).
Using other primates for comparison, many researchers argue that human brain size increase is associated with social intelligence, driven by complex social interactions and the ability to predict and manipulate the behavior of other members of the social group (Machiavellian intelligence). However, tool behavior also must be a factor that contributes to human technical intelligence and innovation. Furthermore, humans have an ability to understand and manipulate the behavioral ecology of other species, and understand the physical properties of inanimate objects. This ability distinguishes humans from other primates, whose intelligence is oriented towards conspecifics.
Humans have the ability to use symbols and engage in symbolic behavior. In living humans, this powerfully affects all social and economic interactions. Artifacts can have symbolic properties. Archaeologists have tried to study the beginnings of symbolic behavior by investigating symmetry and other properties of stone tools. Art and bodily ornamentation are widely considered to signal the unequivocal beginning of human symbolic behavior. Pigments like red ochre and signs of pigment processing are found in archaeological sites dating to 250,000 years ago. Representational art and ornaments occur much later, and do not become abundant until about 40,000 years ago.
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