The brains of a monkey, chimpanzee, and human, shown here to scale, differ dramatically in size and in surface appearance. With an encephalization quotient of 2.0, the monkey’s brain is just over one-quarter the size of a human’s (EQ 7.0). The chimp’s brain, EQ 2.5, is a bit more than one-third as large (see Figure 1-15).

Reproduced or adapted from our websites at http://www.brains.rad.msu. edu and http://brainmuseum.org, supported by the U. S. National Science Foundation.