Evolution, 4th Edition

558 CHAPTER 21

The terms “race” and “ethnic group” are often used in common speech. From

a biological perspective, however, they are not useful. Those terms suggest that

phenotypic variation falls into discrete categories. In most cases, however, there is

a continuous range of genetic and phenotypic variation linking different popula-

tions. Although Dutch people tend to be taller and lighter skinned than Spaniards,

there are smooth clines of body height and skin color that connect those extremes.

Populations that are truly distinctive, such as the very small Biaka hunter-gather-

ers of central Africa, are uncommon. For that reason, in this book we refer to the

people living in a given region as a “population.” Using that word connects our

discussions of humans with those of other species, and it avoids the emotional and

political baggage that comes with the terms “race” and “ethnic group.”

Brain and Language

The most extraordinary physical characteristic of modern humans is our enor-

mous brain. Relative to our body mass, our brain is three times larger than that

of other primates (see Figure 21.5). Our brain gives us cognitive abilities (“intelli-

gence”) that far surpass those of any other species. These cognitive abilities enable

humans to make and understand language, which in humans is vastly more com-

plex than in any other species.

Two hypotheses have been proposed for the evolution of our unique brain. The

first is ecological. This hypothesis suggests that selection favored learning how to

function in complex environments, for example while hunting. A second, proposed

by anthropologist Robin Dunbar, is the social brain hypothesis [15]. Dunbar rea-

soned that living in complex social groups selected for large brains, particularly

enlargement of a region called the neocortex, which is responsible for learning,

memory, and cognition. In early human societies, individuals formed alliances

with key social partners for help in hunting and in resolving conflicts. In support of

this idea, Dunbar showed that primate species with a larger neocortex live in larger

social groups. The size of our brain corresponds to that of a species that lived in

groups with 100–200 members (FIGURE 21.15), a plausible group size for humans

early in our evolutionary history.

Homo sapiens has a broader geographic distribution, inhabits a greater variety of

environments, and consumes a greater variety of foods than any other species on

Earth. This ecological success depends on elaborate social cooperation, on causal

reasoning, and on accumulated knowledge. In turn, all of those abilities depend on

language. Language originally may have been advantageous because it mediated

social interactions, but it also enabled humans to transmit and receive information

that was important in many other contexts.

The key elements of language, according to the evolutionary psychologist and

linguist Steven Pinker, are metaphorical abstraction and combinatorial structuring

[55, 56]. Words are abstract symbols associated with objects, actions, and concepts.

Meaning is conveyed by words and how they are arranged. Language transmits

information efficiently and at low cost to the transmitter, and it allows information

to be pooled among individuals and accumulated across generations. An individu-

al’s ability to use language can increase fitness, and this was very likely important

in the evolution of our large brains.

Some of the great complexity of human language is made possible by the great

variety of sounds made in human speech. Our vocal versatility is possible only

because of changes in our vocal tract that evolved after humans diverged from

other apes. Our larynx is deeper in the throat, and the tongue curves down into the

throat. This produces an L-shaped vocal tract that enables us to produce a remark-

able diversity of sounds. An unfortunate side effect of this arrangement is that we

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_21.15.ai Date 02-02-2017

Mean group size

1 10 100

5

Neocortex ratio

3

2

1

4

Apes

Monkeys

Humans

FIGURE 21.15 Living in large social groups

may have caused the natural selection that

favored the evolution of the very large hu-

man brain. Across species of monkeys and

apes, there is a strong correlation between

the mean size of social groups and the rela-

tive size of the neocortex, a part of the brain

important to learning, memory, and cogni-

tion. The relative size of the human neo-

cortex suggests that our species may have

evolved in groups with 100–200 individuals.

(After [15].)

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