Human Evolution

Human Evolution, the biological and cultural development of the species Homo
sapiens, or human beings. A large number of fossil bones and teeth have been
found at various places throughout Africa, Europe, and Asia. Tools of stone,
bone, and wood, as well as fire hearths, campsites, and burials, also have been
discovered and excavated. As a result of these discoveries, a picture of human
evolution during the past 4 to 5 million years has emerged.

Human Physical Traits Humans are classified in the mammalian order Primates;
within this order, humans, along with our extinct close ancestors, and our
nearest living relatives, the African apes, are sometimes placed together in the
family Hominidae because of genetic similarities, although classification
systems more commonly still place great apes in a separate family, Pongidae. If
the single grouping, Hominidae, is used, the separate human line in the hominid
family is distinguished by being placed in a subfamily, Homininae, whose members
are then called hominines—the practice that is followed in this article. An
examination of the fossil record of the hominines reveals several biological and
behavioral trends characteristic of the hominine subfamily.

Bipedalism Two-legged walking, or bipedalism, seems to be one of the earliest of
the major hominine characteristics to have evolved. This form of locomotion led
to a number of skeletal modifications in the lower spinal column, pelvis, and
legs. Because these changes can be documented in fossil bone, bipedalism usually
is seen as the defining trait of the subfamily Homininae.

Brain Size and Body Size Much of the human ability to make and use tools and
other objects stems from the large size and complexity of the human brain. Most
modern humans have a braincase volume of between 1300 and 1500 cc (between 79.3
and 91.5 cu in). In the course of human evolution the size of the brain has more
than tripled. The increase in brain size may be related to changes in hominine
behavior. Over time, stone tools and other artifacts became increasingly
numerous and sophisticated. Archaeological sites, too, show more intense
occupation in later phases of human biological history. In addition, the
geographic areas occupied by our ancestors expanded during the course of human
evolution. Earliest known from eastern and southern Africa, they began to move
into the tropical and subtropical areas of Eurasia sometime after a million
years ago, and into the temperate parts of these continents about 500,000 years
ago. Much later (perhaps 50,000 years ago) hominines were able to cross the
water barrier into Australia. Only after the appearance of modern humans did
people move into the New World, some 30,000 years ago. It is likely that the
increase in human brain size took place as part of a complex interrelationship
that included the elaboration of tool use and toolmaking, as well as other
learned skills, which permitted our ancestors to be increasingly able to live in
a variety of environments. The earliest hominine fossils show evidence of marked
differences in body size, which may reflect a pattern of sexual dimorphism in
our early ancestors. The bones suggest that females may have been 0.9 to 1.2 m
(3 to 4 ft) in height and about 27 to 32 kg (about 60 to 70 lb) in weight, while
males may have been somewhat more than 1.5 m (about 5 ft) tall, weighing about
68 kg (about 150 lb). The reasons for this body size difference are disputed,
but may be related to specialized patterns of behavior in early hominine social
groups. This extreme dimorphism appears to disappear gradually sometime after a
million years ago.

Face and Teeth The third major trend in hominine development is the gradual
decrease in the size of the face and teeth. All the great apes are equipped with
large, tusklike canine teeth that project well beyond the level of the other
teeth. The earliest hominine remains possess canines that project slightly, but
those of all later hominines show a marked reduction in size. Also, the chewing
teeth—premolars and molars—have decreased in size over time. Associated with
these changes is a gradual reduction in the size of the face and jaws. In early
hominines, the face was large and positioned in front of the braincase. As the
teeth became smaller and the brain expanded, the face became smaller and its
position changed; thus, the relatively small face of modern humans is located
below, rather than in front of, the large, expanded braincase.

Human Origins The fossil evidence for immediate ancestors of modern humans is
divided into the genera Australopithecus and Homo, and begins about 5 million
years ago. The nature of the hominine evolutionary tree before that