The History of 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 is uncertain.
Between 7 and 20 million years ago, primitive apelike animals were widely distributed on
the African and, later, on the Eurasian continents. Although many fossil bones and teeth
have been found, the way of life of these creatures, and their evolutionary relationships
to the living apes and humans, remain matters of active debate among scientists. One of
these fossil apes, known as Sivapithecus, appears to share many distinguishing features
with the living Asian great ape, the orangutan, whose direct ancestor it may well be.
None of these fossils, however, offers convincing evidence of being on the evolutionary
line leading to the hominid family generally or to the human subfamily in particular.
Comparisons of blood proteins and the DNA of the African great apes with that of humans
indicates that the line leading to modern people did not split off from that of
chimpanzees and gorillas until comparatively late in evolution. Based on these
comparisons, many scientists believe a reasonable time for this evolutionary split is 6
to 8 million years ago. It is, therefore, quite possible that the known hominine fossil
record, which begins about 5 million years ago, extends back virtually to the beginnings
of the human line. Future fossil discoveries may permit a more precise placement of the
time when the direct ancestors of the modern African ape split off from those leading to
modern people and human evolution can be said to begin.
Australopithecus
The fossil evidence for human evolution begins with Australopithecus. Fossils of this
genus have been discovered in a number of sites in eastern and southern Africa. Dating
from more than 4 million years ago (fragmentary remains are tentatively identified from
about 5 million years ago), the genus seems to have become extinct about 1.5 million
years ago. All the australopithecines were efficiently bipedal and therefore indisputable
hominines. In details of their teeth, jaws, and brain size, however, they differ
sufficiently among themselves to warrant division into four species: A. afarensis, A.
africanus, A. robustus, and A. boisei.
The earliest australopithecine is A. afarensis, which lived in eastern Africa between 3
and 4 million years ago. Found in the Afar region of Ethiopia and in Tanzania, A.
afarensis had a brain size a little larger than those of chimpanzees (about 400 to 500
cc/about 24 to 33.6 cu in). Some individuals possessed canine teeth somewhat more
projecting than those of later hominines. No tools of any kind have been found with A.
afarensis fossils.
Between about 2.5 and 3 million years ago, A. afarensis apparently evolved into a later
australopithecine, A. africanus. Known primarily from sites in southern Africa, A.
africanus possessed a brain similar to that of its predecessor. However, although the
size of the chewing teeth remained large, the canines, instead of projecting, grew only
to the level of the other teeth. As with A. afarensis, no stone tools have been found in
association with A. africanus fossils.
By about 2.6 million years ago, the fossil evidence reveals the presence of at least two,
and perhaps as many as four, separate species of hominines. An evolutionary split seems
to have occurred in the hominine line, with one segment evolving toward the genus Homo,
and finally to modern humans, and the others developing into australopithecine species
that eventually became extinct. The latter include the robust australopithecines, A.
robustus, limited to southern Africa, and A. boisei, found only in eastern Africa. The
robust australopithecines represent a specialized adaptation because their principal
difference from other australopithecines lies in the large size of their chewing teeth,
jaws, and jaw muscles. The robust australopithecines became extinct about 1.5 million
years ago.
The Genus Homo
Although scientists do not agree, many believe that after the evolutionary split that led
to the robust australopithecines, A. africanus evolved into the genus Homo. If so, this
evolutionary transition occurred between 1.5 and 2 million years ago. Fossils dating from
this period display a curious mixture of traits. Some possess relatively large
brains-several almost 800 cc (about 49 cu in)-and large, australopithecine-sized teeth.
Others have small, Homo-sized teeth but also small, australopithecine-sized brains. A
number of fossil skulls and jaws from this period, found in Tanzania and Kenya in eastern
Africa, have been placed in the category H. habilis, meaning "handy man," because some of
the fossils were found associated with stone tools. H. habilis possessed many traits that
link it both with the earlier australopithecines and with later members of the genus
Homo. It seems likely that this species represents the evolutionary transition between
the australopithecines and later hominines.
The earliest evidence of stone tools comes from sites in Africa dated to about 2.5
million years ago. These tools have not been found in association with a particular
hominine species. By 1.5 to 2 million years ago, sites in various parts of eastern Africa
include not only many stone tools, but also animal bones with scratch marks that
experiments have shown could only be left by humanlike cutting actions. These remains
constitute evidence that by this time early hominines were eating meat, but whether this
food was obtained by hunting or by scavenging is not known. Also unknown at present is
how much of their diet came from gathered vegetable foods and insects (dietary items that
do not preserve well), and how much came from animal tissue. It is also not known whether
these sites represent activities by members of the line leading to Homo, or if the robust
australopithecines were also making tools and eating meat.
Fossil evidence of a large-brained, small-toothed form, known earliest from north Kenya
and dating from 1.5 to 1.6 million years ago, has been placed in the species H. erectus.
The first part of the time span of H. erectus, like that of the earlier-in-time
hominines, is limited to southern and eastern Africa. Later-between 700,000 and a million
years ago-H. erectus expands into the tropical areas of the Old World, and finally at the
close of its evolution, into the temperate parts of Asia. A number of archaeological
sites dating from the time of H. erectus reveal a greater sophistication in toolmaking
than was found at the earlier sites. At the cave site of Peking man in north China, there
is evidence that fire was used; the animal fossils that have been found are sometimes of
large mammals such as elephants. These data suggest that hominine behavior was becoming
more complex and efficient.
Throughout the time of H. erectus the major trends in human evolution continued. The
brain sizes of early H. erectus fossils are not much larger than those of previous
hominines, ranging from 750 to 800 cc (45.8 to 48.8 cu in). Later H. erectus skulls
possess brain sizes in the range of 1100 to 1300 cc (67.1 to 79.3 cu in), within the size
variation of Homo sapiens.
Early Homo sapiens
Between 200,000 and 300,000 years ago, H. erectus evolved into H. sapiens. Because of the
gradual nature of human evolution at this time, it is difficult to identify precisely
when this evolutionary transition occurred, and certain fossils from this period are
classified as late H. erectus by some scientists and as early H. sapiens by others.
Although placed in the same genus and species, these early H. sapiens are not identical
in appearance with modern humans. New fossil evidence suggests that modern man, H.
sapiens sapiens, first appeared more than 90,000 years ago. There is some disagreement
among scientists on whether the hominine fossil record shows a continuous evolutionary
development from the first appearance of H. sapiens to modern humans. This disagreement
has especially focused on the place of Neandertals (or Neandertals), often classified as
H. sapiens neanderthalis, in the chain of human evolution. The Neandertals (named for the
Neander Valley in Germany, where one of the earliest skulls was found) occupied parts of
Europe and the Middle East from 100,000 years ago until about 35,000 to 40,000 years ago,
when they disappeared from the fossil record. Fossils of additional varieties of early H.
sapiens have been discovered in other parts of the Old World.
The dispute over the Neandertals also involves the question of the evolutionary origins
of modern human populations, or races. Although a precise definition of the term race is
not possible (because modern humans show continuous variation from one geographic area to
another), widely separate human populations are marked by a number of physical
differences. The majority of these differences represent adaptations to local
environmental conditions, a process that some scientists believe began with the spread of
H. erectus to all parts of the Old World sometime after a million years ago. In their
view, human development since H. erectus has been one continuous, in-position evolution;
that is, local populations have remained, changing in appearance over time. The
Neandertals and other early H. sapiens are seen as descending from H. erectus and are
ancestral to modern humans.
Other scientists view racial differentiation as a relatively recent phenomenon. In their
opinion, the features of the Neandertals-a low, sloping forehead, large brow ridge, and a
large face without a chin-are too primitive for them to be considered the ancestors of
modern humans. They place the Neandertals on a side branch of the human evolutionary tree
that became extinct. According to this theory, the origins of modern humans can be found
in southern Africa or the Middle East. Evolving perhaps 90,000 to 200,000 years ago,
these humans then spread to all parts of the world, supplanting the local, earlier H.
sapiens populations. In addition to some fragmentary fossil finds from southern Africa,
support for this theory comes from comparisons of mitochondrial DNA, a DNA form inherited
only from the mother, taken from women representing a worldwide distribution of
ancestors. These studies suggest that humans derived from a single generation in
sub-Saharan Africa or southeastern Asia. Because of the tracing through the material
line, this work has come to be called the "Eve" hypothesis; its results are not accepted
by most anthropologists, who consider the human race to be much older. See also RACES,
CLASSIFICATION OF.
Whatever the outcome of this scientific disagreement, the evidence shows that early H.
sapiens groups were highly efficient at exploiting the sometimes harsh climates of Ice
Age Europe. Further, for the first time in human evolution, hominines began to bury their
dead deliberately, the bodies sometimes being accompanied by stone tools, by animal
bones, and even by flowers.
Modern Humans
Although the evolutionary appearance of biologically modern peoples did not dramatically
change the basic pattern of adaptation that had characterized the earlier stages of human
history, some innovations did take place. In addition to the first appearance of the
great cave art of France and Spain See CAVE DWELLERS, some anthropologists have argued
that it was during this time that human language originated, a development that would
have had profound implications for all aspects of human activity. About 10,000 years ago,
one of the most important events in human history took place-plants were domesticated,
and soon after, animals as well. This agricultural revolution set the stage for the
events in human history that eventually led to civilization.
Modern understanding of human evolution rests on known fossils, but the picture is far
from complete. Only future fossil discoveries will enable scientists to fill many of the
blanks in the present picture of human evolution. Employing sophisticated technological
devices as well as the accumulated knowledge of the patterns of geological deposition,
anthropologists are now able to pinpoint the most promising locations for fossil hunting
more accurately. In the years ahead this will result in an enormous increase in the
understanding of human biological history.
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