About 27 humanoid species roamed the Earth since splitting off
from their ape ancestors 7 million years ago; more are turning
up, but only one remains. Science journalist and former CNN
bureau chief Walter (Thumbs, Toes, and Tears: And Other Traits
That Make Us Human, 2006) delivers a mixture of fact, research
and conjecture that describes how this happened. Anthropologists
define humans as primates who walk upright, but the author
focuses on another fundamental, if bizarre, difference—human
species deliver children prematurely. Traditionally, teachers
explain that this allows passage of our increasingly large skull
and brain through the birth canal, but there is more to it. We
are not only born as fetuses, but we keep infantile ape features
(near-hairlessness, flatter faces, higher foreheads, a straight
big toe) into adult life. Carrying the analogy further, Walter
emphasizes that humans not only have an extended physical
childhood (gorillas reach adulthood at 11), but we preserve the
mental qualities of chi