THE EVOLUTIONARY STORY OF HOMO SAPIENS 549
the hominins, which diverged from the chimpanzee
lineage about 7 Mya^1 [1, 36]. The hominin lineage
gave rise to many more species, all of which went
extinct save for one: Homo sapiens.
We have learned much about the last few million
years of human evolution by comparing hominins
with our closest living relatives [48]. The other apes
have arms that are longer than their legs. All can
walk more or less upright for short distances. Their
feet are like hands, with opposable first (big) toes,
but their thumbs are not as opposable as ours. The
African apes are highly social, and males are consid-
erably larger than females, a consequence of sexual
selection. Bonobos use sexual interactions, in all pos-
sible heterosexual and homosexual combinations, to
resolve conflicts and maintain bonds. Chimpanzees
have more conflict-ridden societies. They use tools,
cracking open nuts on stone anvils and using twigs to
fish termites out of their mounds (FIGURE 21.3). They
hunt cooperatively for monkeys. Some investigators
think chimpanzees have cognitive abilities and emo-
tions like those of humans, although less developed.
They may have a rudimentary “theory of mind”—the ability to infer the intentions
and emotions of others [10]. Although chimpanzees have a great variety of vocaliza-
tions, they do not use language in nature. Captive apes, however, can learn to use
sign language or sets of symbolic objects to express rudimentary language abilities
[67], suggesting these abilities were present in our common ancestor.
Humans and chimpanzees differ by less than 2 percent in the DNA sequences
of our protein-coding genes^2 [72]. The two species are so closely related that at
several loci they share polymorphic alleles that have persisted since our common
ancestor because of balancing selection, perhaps related to resistance to pathogens
[40]. Both the human and chimpanzee lineages have evolved in many ways since
they diverged [14]. Our common ancestor surely shared many features with liv-
ing chimpanzees, such as an opposable big toe, longer arms than legs, a project-
ing lower face, large canine teeth, plentiful body hair, and a relatively small brain.
Because chimpanzees and bonobos are our closest relatives, they provide critical
insights into how humans evolved.How humans differ from other apes
One of the most conspicuous differences between modern humans and other
living apes is that we are fully bipedal. We are well adapted for walking and run-
ning [41]. Our pelvis has a different shape, anchoring muscles that stabilize the
body. Our legs are relatively longer than in other apes, and are angled inward
so that they are directly below our center of gravity. This improves balance and(^1) Several factors contribute to uncertainty about the ages of speciation events in apes, including
the hominins. The mutation rates used to estimate the dates from molecular data are not known
with great accuracy. Hybridization between emerging species may have continued for long
periods after the lineages began to diverge. Last, the geological ages of fossils tell us when their
lineages were living, but not when they diverged from other lineages. For simplicity, we use 7
Mya as the approximate date for the split between the human and chimpanzee lineages.
2
This number is based on the genes that humans and chimpanzees share in common. If we take
into account the genes that have been deleted or duplicated since the lineages diverged, the dif-
ference between the two species increases to about 6 percent [11].
FIGURE 21.3 Chimpanzees (Pan troglodytes) learn how to use tools by
imitation. As a female and her infant watch, a male cracks nuts using a rock
as a hammer.
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