THE EVOLUTIONARY STORY OF HOMO SAPIENS 559
can choke to death on food––as you may know if you
have ever used the Heimlich maneuver. This fitness
cost of our reshaped vocal tract testifies to a strong
fitness advantage of spoken language [42].
There is no clear picture of when and how human
language evolved. No other primate uses verbal
language in the wild, but other species have been
taught to use sign language and symbols. The most
extraordinary case is a bonobo named Kanzi, stud-
ied by Sue Savage-Rumbaugh and colleagues [67].
The researchers did not try to teach Kanzi to associ-
ate symbols with objects when he was being reared
by his mother, but they did try to teach his mother.
Kanzi spontaneously learned the symbols’ mean-
ing by observation. He later learned about 200 other
symbols, how to associate English words with the
symbols, and most important, how to create mean-
ingful combinations of symbols (FIGURE 21.16).
Chimpanzees can learn simple requests, such as
“Give banana,” in which the person addressed is the
giver and the chimpanzee is the recipient. But Kanzi
formed more complex requests in which he was nei-
ther the giver nor recipient, but referred instead to
other individuals. The exact criteria for what constitutes language are imprecise,
so there is little point in debating whether or not bonobos and chimpanzees are
capable of true language. But at least rudimentary cognitive abilities for language—
though not the physical apparatus for speech—must date back to our common
ancestor that lived some 7 Mya.
Tool using and tool making were once claimed to be unique to humans, but they
no longer are. Several species of birds, including one of the Galápagos finches, use
twigs and spines to extricate insects from crevices. The New Caledonian crow (Cor-
vus moneduloides) fashions hooked and barbed tools from twigs and leaves [32], and
in experiments it can use three tools in the required sequence to get a reward [71].
Orangutans, chimpanzees, and bonobos can accomplish similar tasks [45]. All the
great apes use tools, both in the wild and in captivity (see Figure 21.3). Kanzi and
other bonobos learned how to strike stones together in order to make sharp flakes
that they used to cut strings in order to obtain food [75, 81]. This is just what our
ancestors were doing 3.3 Mya. As hominin brains evolved, technology and culture
generally grew in complexity.Diet and Agriculture: A Revolution in Our World
With our large brain come large costs. Our big head makes childbirth difficult
and dangerous—humans are the only species in which helpers assist with birth.
Even more important is the vast quantity of energy that our brain uses [41]. It con-
sumes about 20 percent of an adult’s basal metabolism, and up to 60 percent of an
infant’s. Growing the large adult brain requires about 18 years, longer than in any
other primate. During that time, neural connections are formed, cognitive abilities
increase, and social skills are shaped.
Humans are paradoxical primates. At first look, our life histories do not follow
the trade-offs one might expect (see Chapter 11). We reproduce more often and
have offspring that are larger at birth than other primates. Yet despite those higher
reproductive costs, we have a longer life span. To pay the energetic price of a bigFIGURE 21.16 The bonobo Kanzi and researcher Sue Savage-Rumbaugh
having a conversation using a set of plastic symbols. Kanzi can form simple
requests that refer to other individuals and can make statements about
what he will then do.21_EVOL4E_CH21.indd 559 3/22/17 1:51 PM