Essay/Term paper: Human evolution

Essay, term paper, research paper:  Society

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.

Daniel Mokari