THE EVOLUTIONARY STORY OF HOMO SAPIENS 551
Most important is our enormous brain. Relative to body mass, the human
brain is three times the size of other primate brains, and five times the size of
most mammalian brains (FIGURE 21.5).
Human babies are larger than those of other primates, but they are
unusually helpless. Infancy is followed by a long period of childhood that
requires continued parental care. In contrast to humans, female chimpan-
zees care for their offspring for about 5 years and do not ovulate or have
more offspring during that time. But human females can give birth to more
children while their older children are still dependent on them. The potential
growth rate of human populations is therefore much greater than that of
other apes. That may help explain the demise of our competitors, as you will
see shor t ly.Our ancestry: Hominins through time
At least 7 My of evolution separate the single living species of hominin,
Homo sapiens, from the other living species of apes. Critical clues to the story
of how our physical differences evolved come from the fossil record. There
are few fossils of other ape lineages, but fortunately there are many hominin
fossils. Some hominin species are known from only a few fragments, such
as a jaw. Few are represented by enough specimens to determine whether
one or several species were alive at the same time, or what the evolution-
ary relationships were among fossils from different times. But while there is
uncertainty about some of those details, there is broad agreement about the
major features of hominin evolution.
In testimony to the predictive power of evolutionary science, Darwin wrote in
The Descent of Man [9]:In each great region of the world the living mammals are closely related to
the extinct species of the same region. It is therefore probable that Africa was
formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee;
and as these two species are now man’s nearest allies, it is somewhat more
probable that our early progenitors lived on the African continent than
elsewhere.More than 50 years later, the first hominin fossils were found in South Africa. The
entire history of paleoanthropology since then has shown that Darwin’s predic-
tion was right: the origin and most of the later evolution of hominins, including
Homo sapiens, played out in Africa.
Fossils show that after diverging from the chimpanzee lineage, hominins prolif-
erated into several species. Most of them were not our direct ancestors, but instead
were on closely related lineages that later became extinct. They give important
clues to human evolution, however, because those extinct species are more closely
related to us than to any living species.
The species of hominins that are generally agreed on are shown in FIGURE 21.6.
Most anthropologists are confident that Sahelanthropus, Orrorin, and Ardipithecus
are members of the hominin clade. If so, the 6- to 7-My-old Sahelanthropus marks
the minimal age of the split between hominins and the chimpanzee lineage. A key
link between hominins and their common ancestor with other apes may be Ardipi-
thecus ramidus (FIGURE 21.7A), from 4.4-My-old deposits in Ethiopia [80]. It had
many apelike features, such as a brain the size of a chimpanzee’s and adaptations
for climbing such as an opposable big toe. But it also had hominin features, such
as small canine teeth (which are enlarged in male apes for fighting) and a pelvis
adapted for walking upright.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_21.05.ai Date 02-02-2017Body mass (kg)Other primatesHuman3xChimpanzee Gorilla0 25 50 75 100 12512501500Brain mass (g)25050007501000FIGURE 21.5 Plot of average brain mass against
average body mass for primates. Relative to
body mass, the human brain is three times larg-
er than the average primate brain. (After [41].)21_EVOL4E_CH21.indd 551 3/22/17 1:51 PM