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32 Chapter 2
Figure 2.12 Vertebrate evolution.
Fishes without Amphibians Reptiles
backbones
Million
Years
Ago
Bony fishes Birds Mammals
6 HOMININS EMERGE
160 BIRDS EMERGE
180 MAMMALS EMERGE
300 REPTILES EMERGE
400 AMPHIBIANS EMERGE
425 VERTEBRATES EMERGE
450 CHORDATES EMERGE
FIRST COMPLEX MULTI-
600
CELLULAR ORGANISMS
as they migrated out of Africa (see Gibbons, 2014; of the brains of acclaimed intellectuals (e.g., Einstein) were
Hammer, 2013; Wong, 2015). These findings change found to be unremarkable, certainly no match for their gi-
the way we see our origins: We are not the product gantic intellects. It is now clear that, although healthy adult
of a single ancestral population originating in Africa; human brains vary greatly in size—between about 1,000
rather, we are the offspring of many Homo populations and 2,000 grams—there is no clear relationship between
that once coexisted. overall human brain size and intelligence.
One obvious problem in relating brain size to intel-
Evolution of the Human Brain ligence is the fact that larger animals tend to have larger
brains, presumably because larger bodies require more
LO 2.7 describe how research on the evolution of the
brain tissue to control and regulate them. Thus, the facts
human brain has changed over time.
that large men tend to have larger brains than small men,
Early research on the evolution of the human brain focused that men tend to have larger brains than women, and that
on size. This research was stimulated by the assumption elephants have larger brains than humans do not suggest
that brain size and intellectual capacity are closely related— anything about the relative intelligence of these popula-
an assumption that quickly ran into two problems. First, tions. This problem led to the proposal that brain weight
it was shown that modern humans, whom modern hu- expressed as a percentage of total body weight might be
mans believe to be the most intelligent of all creatures, a better measure of intellectual capacity. This measure
do not have the biggest brains. With brains weighing allows humans (2.33 percent) to take their rightful place
about 1,350 grams, humans rank far behind whales and ahead of elephants (0.20 percent), but it also allows both
elephants, whose brains weigh between 5,000 and 8,000 humans and elephants to be surpassed by that intellectual
grams (Manger, 2013; Patzke et al., 2014). Second, the sizes giant of the animal kingdom, the shrew (3.33 percent).
Evolution, Genetics, and Experience 33
A more reasonable approach to the study of brain evo-
Figure 2.13 The brains of animals of different
lution has been to compare the evolution of different brain evolutionary ages—cerebrums are shown in pink;
regions. For example, it has been informative to consider brain stems are shown in orange.
the evolution of the brain stem separately from the evolu-
tion of the cerebrum (cerebral hemispheres). In general, the
brain stem regulates reflex activities that are critical for sur-
vival (e.g., heart rate, respiration, and blood glucose level),
whereas the cerebrum is involved in more complex adaptive
processes such as learning, perception, and motivation.
Figure 2.13 is a schematic representation of the relative
size of the brain stems and cerebrums of several species
that are living descendants of species from which humans
evolved. This figure makes three important points about
the evolution of the human brain:
• The brain has increased in size during evolution.
• Most of the increase in size has occurred in the
cerebrum.
• An increase in the number of convolutions—folds on
Human
the cerebral surface—has greatly increased the sur-
face area of the cerebral cortex, the outermost layer of
cerebral tissue (see Geschwind & Rakic, 2013; Zilles,
Palermo-Gallagher, & Amunts, 2013).
Although the brains of related species differ, there are
fundamental similarities: All brains are constructed of neu-
rons, and the neural structures in the brains of one species
can usually be found in the same locations in the brains
of related species (e.g., Goulas et al., 2014). For example,
the brains of humans, monkeys, rats, and mice contain
the same major structures connected in similar ways, Chimpanzee
and similar structures tend to perform similar functions
(see Cole et al., 2009). The human brain appears to have
evolved from the brains of our closest primate relatives
(see Hofman, 2014; Matsuzawa, 2013).
Evolutionary Psychology:
Understanding Mate Bonding
LO 2.8 discuss the field of evolutionary psychology Cat
and the study of mate bonding.
The evolutionary approach has been embraced by many
psychologists. Indeed, a field of psychology, termed evolu-
tionary psychology, has coalesced around it. Evolutionary psy-
chologists try to understand human behaviors Rat
through a consideration of the pressures that
led to their evolution (see Burke, 2014). Some of
the most interesting and controversial work in
this field has focused on questions of sex differences in mate
bonding, questions you may be dealing with in your own life.
In most vertebrate species, mating is totally promis-
cuous—promiscuity is a mating arrangement in which the
members of both sexes indiscriminately copulate with Bass
many different partners during each mating period.
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