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 several million years
has emerged that is clear in many details, but sketchy in others.
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 great apes—are
commonly placed together in the family Hominidae because of genetic
similarities. Within this family, H. sapiens is often grouped with
the African great apes—chimpanzees
and gorillas—in the subfamily
Homininae. Chimpanzees and humans are particularly close genetically,
and many scientists therefore place them together in a common group
known as the tribe Hominini. A few specialists have even suggested
that chimpanzees and humans should be classified in the same genus.
Meanwhile, however, some writers continue to follow older classification
schemes that typically restrict the family Hominidae (or alternatively
the subfamily Homininae) to modern humans and their extinct forebears,
and put the great apes in a family called Pongidae (or the subfamily
Ponginae).
Bipedalism.
Two-legged walking, or bipedalism, as a primary means of locomotion seems
to be one of the earliest of the major human 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, habitual bipedalism
usually is seen as a defining trait of humans and their close (nonape)
relatives and ancestors.
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 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
central, eastern, and southern Africa, they began to move into the
tropical and subtropical areas of Eurasia well over a million years
ago, and into the temperate parts of these continents more than
500,000 years ago. Much later (perhaps 50,000 years ago) anatomically
modern humans were able to cross the water barrier into Australia.
Only after the appearance of modern humans did people move into
the New World, probably less than 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.
While fossil remains are often scanty for the predecessors
of Homo sapiens, at least some early species appear to show evidence
of marked differences in body size, which may reflect a pattern
of sexual dimorphism in our early ancestors. In a typical case,
if 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, then 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 social groups. This extreme dimorphism
appears to disappear gradually sometime after a million years ago.
Face and Teeth.
The third major trend in human 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. (Canine teeth are located next to the front
incisors and are used for cutting and tearing food.) The earliest
remains of our forerunners possess canines that project slightly, but
those of all later species show on the whole 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 and relatives
of modern humans is incomplete, but finds made so far indicate the
existence of several species belonging to the genus Homo and to
an earlier genus called Australopithecus. Fragmentary
fossils point to the existence of other genera apparently related
or directly ancestral to modern apes and humans during the Miocene
epoch, which lasted from about 23 million years ago to 5.3 million
years ago. Early in the Miocene, for example, there lived in Africa primitive
apelike animals dubbed Proconsul (named for a chimpanzee called
Consul at the Folies Bergère in Paris), which lacked a
tail and climbed trees. From a slightly later period, such African
genera as Morotopithecus (named for the fossil site, Moroto in Uganda),
Afropithecus, and Kenyapithecus have been identified. Some of the
early apelike animals spread to Eurasia, reflected in fossil finds
from such genera as Griphopithecus and, dating from 8 to 14 million
years ago, Dryopithecus (“ape of the oak forests”)
and Pierolapithecus (named for a fossil site in Spain), which are
suspected by some scientists to be near to humans' and
great apes' common forebear. Another fossil apelike creature,
known as Sivapithecus (named for fossil sites in the Siwalik Hills
of India and Pakistan), appears to share many distinguishing features
with the living Asian great ape, the orangutan,
whose direct ancestor it may be. 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.
Comparisons of blood proteins and the DNA
of the African great apes with that of humans indicates that the
line leading to humans and chimpanzees did not split off from that
leading to gorillas until comparatively late in the Miocene. Based
on these comparisons, many scientists believe a reasonable time
for this evolutionary split is 6 to 8 million years ago. The division
between the line leading to chimpanzees and the one leading to humans
may have occurred perhaps a million years later. Future fossil discoveries
may permit a more precise placement of the time when the direct
ancestors of the modern gorilla split off from those leading to
modern people and chimpanzees, as well as the time when the separation between
the human and chimp lines occurred.
Australopiths.
The first major fossil find from a humanlike creature preceding those
now assigned to the genus Homo was made in South Africa in 1924
by South African anatomist Raymond Dart (1893–1988). He
called the creature Australopithecus africanus, meaning “southern
ape from Africa.” Fossils of several species in this genus
were subsequently discovered in a number of sites in eastern as well
as southern Africa, but the name Australopithecus was retained.
The term “australopithecine,” or “australopith” for
short, has also come to be used in an informal way not only for
creatures classified in Australopithecus but for those belonging
to other early genera that seem to share key traits characteristic
of modern humans rather than modern apes, such as bipedalism as
the primary means of locomotion and the presence of relatively small
canine teeth.
As of 2006, the earliest known species possibly classifiable
as an australopith was Sahelanthropus tchadensis, fossils of which
were first discovered in 2001 in Chad in Central Africa. (The name
roughly translates as “Sahel human of Chad,” where
Sahel is the name of the Sahara's semidesert southern border region.)
The fossil remains found, estimated to date from 6 to 7 million
years ago, were insufficient to determine for sure whether the creature
was bipedal, but humanlike features such as small canine teeth and
a substantial brow ridge above the eyes suggested that the evolutionary
lineage leading to modern humans may have already split from that
leading to African great apes.
Another early species was Orrorin tugenensis (“original
man of Tugen”). Fossils dating from some 6 million years
ago were found in the Tugen Hills in western Kenya in 2000. Although
canine teeth appeared to be rather apelike, leg bones showed telltale
signs of habitual bipedalism. A slightly more recent East African species
mixing apelike (such as thin tooth enamel) and humanlike characteristics
was Ardipithecus ramidus (ardi means “floor” or “ground” in
the local language, and ramid means “root”). Fossil
remains uncovered so far, in Ethiopia's Afar region, are
scant. Scientists estimate the species lived about 4.4 million years
ago. Scattered fossils were later found in Ethiopia from an earlier
Ardipithecus species, A. kadabba (kadabba means “base ancestor”),
which lived in Ethiopia 5.2 million to 5.8 million years ago.
Australopithecus, judging by fossil remains, emerged more
than 4 million years ago and seems to have become extinct by about
1.2 million years ago. The various creatures classified in Australopithecus
were efficiently bipedal and therefore may be regarded as indisputable
members of the human lineage. In details of their teeth, jaws, and
brain size, however, they differ sufficiently among themselves to
warrant division into several species.
The earliest known Australopithecus species is A. anamensis
(anam means “lake” in the local language), evidenced
by fossils found in the Turkana Lake region of northern Kenya and
in Ethiopia dating from around 4 million years ago. Somewhat similar,
but much more extensively represented in the fossil record, is A. afarensis, which
lived in eastern Africa between about 3 million and 4 million years
ago. Found in the Afar region of Ethiopia and in Tanzania, A.
afarensis had a brain a little bigger than those of chimpanzees
(about 400 to 500 cc, or 24 to 34 cu in). Some individuals had canine
teeth somewhat more projecting than those of later hominines. Among
notable A. afarensis fossils are a partial skeleton of a female
dubbed Lucy that was discovered at Hadar, Ethiopia, and a partial
skeleton of a three-year-old young girl called Selam found at Dikika,
Ethiopia. The hyoid bone observed at the base of Selam's
tongue resembled less that of a modern human, however, than that
of a great ape. (The hyoid plays an essential role in human speech.)
Also, aspects of Selam's anatomy suggested a capability
for climbing trees. But her legs were suitable for walking upright, and
footprints found at Laetoli, Tanzania, appeared to have been made
by A. afarensis. In addition, study of Selam's brain size
indicated that the species, like later humans, matured to adulthood
more slowly than chimpanzees. Fossil remains suggesting that A.
afarensis was not the only humanlike species living in East Africa
around 3.5 million years ago were discovered in 1999 in the Lake
Turkana region of Kenya. The fossils, pieces of a cranium, showed
an unusual combination of early characteristics, such as a small
brain, and characteristics ordinarily found in much later fossils,
such as the configuration of the cheekbone and the flatness of the
area below the nose. The australopith was named Kenyanthropus platyops
(“flat-faced Kenya human”), although some scientists
remained doubtful that it represented a distinct species.
A. africanus, the species discovered by Dart,
appears to have lived in southern Africa between 3.3 million and
2.5 million years ago. A. africanus had a brain
similar to that of A. afarensis, although the former's braincase
was more globular in form. As in A. afarensis, the chewing teeth
were large, but the canines, instead of projecting, grew only to
the level of the other teeth.
Australopith fossil remains from about 2.5 million years ago
that were found in the Afar region of Ethiopia were given the species
name A. garhi (garhi means “surprise” in the Afar
language). Distinctive features included sizable incisor and molar
teeth along with an extended forearm and thighbone. Discovered with
the fossils were some of the oldest known stone tools, as well as
animal bones that showed signs of having been cracked and cut by
tools, presumably indicating that meat played a role in the creatures' diet.
Scientists commonly divide the australopiths into two groups:
gracile and robust. The gracile species by and large evolved more
than 3 million years ago and tended to have smaller jaws and teeth.
The robust australopiths appeared later and generally had bigger,
flatter faces with big jaws, small incisors, and large molars covered
with thick enamel—features presumably reflecting vigorous
and lengthy chewing of food, as might be expected from a primarily
vegetarian diet. As early as 2.7 million years ago the robust A.
aethiopicus was living in East Africa. Fossils of the species show
some characteristics reminiscent of A. afarensis, raising the possibility
in the minds of some scientists that the latter may have evolved
into A. aethiopicus. Numerous fossils have been found of the robust
A. boisei (originally called Zinjanthropus boisei, or “East
Africa human”), which lived in East Africa for about a
million years beginning about 2.3 million years ago. Another robust
species, A. robustus, lived in southern Africa between 1.8 million
and 1.3 million years ago, according to fossil evidence. Scientists
have not definitively established the relations between the various
australopith species. Nor is it known why they became extinct. Among
the possible reasons often suggested are climate change and competition
from other species.
The Genus Homo.
Climate change has also been proposed as a possible factor
in the development of the genus Homo, which presumably evolved from
one or more australopithecine species, most likely gracile species,
since Homo lacks the large teeth and jaw structure characteristic
of robust australopiths. But little is known for sure about how
the evolution of Homo unfolded. A key distinguishing feature of
the genus Homo is a larger brain than is seen in australopiths,
and while stone tools were apparently used by some late australopiths,
Homo's bigger brain may have provided a competitive advantage by
facilitating toolmaking and tool use.
An early Homo species was H. habilis (“handy
man”), thought to have lived in Africa from about
2.5 million years ago until about 1.5 million years ago. It was
first identified on the basis of fossils found in Olduvai Gorge
in Tanzania. The species name refers to the fact that some of the
fossils were found associated with stone tools. H. habilis had
many traits linking it both with the earlier australopithecines
and with later members of the genus Homo. Another
early species was H. rudolfensis, named for Lake Rudolf (the former name
of Lake Turkana). Fossil remains of the species dating from some
1.9 million years ago have been found in the lake region. They suggest
that H. rudolfensis was larger than H. habilis and had bigger teeth.
The next stage in the development of Homo was introduced by
H. ergaster (“working man”), probably about 2
million years ago. Most of the fossil finds date from about 1.8
million to 1.5 million years ago. H. ergaster possibly coexisted
in Africa with remaining robust australopiths and the Homo species
H. rudolfensis and H. habilis. It shared a number of skull features
with modern humans, such as thin cranial bones. A rather complete
skeleton of an H. ergaster boy found in the Turkana region and dating
from around 1.6 million years ago reveals proportions of limbs and
body similar to those of modern humans rather than to apelike H. habilis.
The Turkana boy was tall and thin, adapted not for tree climbing
but for walking in hot conditions. Members of H. ergaster were the
first representatives of the australopith/Homo group to
leave Africa—or at least the first for which fossil evidence
has been found. Fossil remains discovered at Dmanisi in the southern Caucasus
Mountains in Georgia and dated to about 1.7 million years ago have
been classifed as H. ergaster.The first species to spread widely
in Asia was H. erectus (“upright man”),
which may have developed from H. ergaster around 1.8 million years
ago. (Some scientists, however, prefer to regard H. ergaster as
a subspecies of H. erectus.) H. erectus featured a generally larger
brain than H. ergaster but also a thicker cranium (and other bones.
The first part of the time span of H. erectus is
limited to southern and eastern Africa. Later H. erectus expands
into the tropical areas of the Old World, reaching as far as Java,
some experts believe, by 1.7 million years ago. Toward the close
of its evolution, it moved 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 earlier
sites. At the cave site of Peking (now Beijing) 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 behavior shown by members of Homo was becoming
more complex and efficient.
Throughout the time of H. erectus the major
trends in human evolution continued. The adult brain sizes of early H.
erectus fossils are not much larger than those of H. habilis
and H. rudolfensis, measuring as small as 800 cc (50 cu in). Later H.
erectus skulls possess brain sizes in the range of 1100
to 1300 cc (67 to 79 cu in), within the size variation of Homo
sapiens. H. erectus appears to have survived
for more than 1.5 million years; some researchers date fossils found
in Java to less than 50,000 years ago. Even more recent, estimated to
be as little as 18,000 years old, are remains found on the Indonesian
island of Flores in 2003. These were regarded by their discoverers
as a dwarf species, H. floresiensis, which may have developed from
H. erectus as a result of the evolutionary tendency toward dwarfism
that can occur on small isolated islands with limited food and no
predators. A rather complete skeleton that was recovered was believed
to be from an adult female with a height of about 1 m (3.3 ft).
Some scientists, however, doubt that the find represents a new species,
arguing that the small size of the specimen's skull could
have been a result of a disease such as microcephaly.
Early migrants to Europe, which some experts believe may have
arrived as long as 800,000 years ago, are often classified as H.
heidelbergensis, named for a fossil jaw discovered near Heidelberg,
Germany. This large-boned, big-brained species has traits in some
regards midway between H. ergaster and H. sapiens. Some authorities
attribute a number of fossils found in Africa to it.
Early Homo
sapiens.
Perhaps 200,000 or more years ago, H. sapiens is
thought to have evolved from a population of some earlier Homo species.
Because of the gradual nature of human evolution at this time, it
is difficult to identify precisely when the transition occurred,
and certain fossils from this period are classified differently
by different scientists.
Although placed in the same genus and species, early H.
sapiens were not identical in appearance with modern humans.
Interpretation of the fossil record is complicated by the question
of the place of Neanderthals (or Neandertals) in the chain of human
evolution. They have sometimes been regarded as a subspecies of
H. sapiens: H. s. neanderthalis. They have also often been
treated as a separate species: H. neanderthalis. Genetic
studies lend support to the latter approach. Distinguishing features
of the Neanderthals (named for the Neander Valley in Germany, where
one of the earliest skulls was first found in 1856) included a large
and low-lying braincase, a protruding face with a low, sloping forehead,
sizable brow ridges, a large nose, lack of a prominent chin, and
a big-boned skeleton resulting in a short, stocky build useful for conserving
body heat during the Ice Age. Neanderthals were found in much of
Europe, Central Asia, and the Middle East from about 300,000 years
ago until about 28,000 years ago, when they disappeared from the fossil
record. Fossils of additional varieties of early H. sapiens have
been found in various parts of the world.
The dispute over the Neanderthals 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. In their view, human
development since H. erectus has been one continuous,
in-position evolution; that is, local populations have essentially
remained, changing in appearance in some regards over time. The
Neanderthals along with mainline early H. sapiens are
seen as descending from H. erectus and as ancestral
to modern humans. This approach suggests an explanation for the
fact that H. erectus fossil skulls found in East Asia show certain
features also seen in people living in the region today.
Other scientists view racial differentiation as a relatively
recent phenomenon. In their opinion, the features of the Neanderthals
are too primitive for them to be considered the ancestors of modern
humans. They place the Neanderthals on a side branch of the human
evolutionary tree that became extinct. According to this theory,
modern humans first evolved perhaps 200,000 years ago in Africa.
These people then spread to all parts of the world, supplanting
the local, earlier populations, such as Neanderthals and H. erectus.
In addition to fragmentary fossil finds from Africa, support for
this “out of Africa” theory comes from comparisons
of mitochondrial DNA—a 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. Because of the tracing through the maternal
line, this work has come to be called the “Eve” hypothesis;
it is not accepted by some anthropologists, who consider the human
race to be much older. See also Races,
Classification of.
A number of scientists prefer a compromise approach making
use of selected features from both theories. Whatever the outcome
of this scientific disagreement, the evidence shows that early humans
were highly efficient at exploiting the harsh climates of Ice Age
Europe. Further, Neanderthals were apparently the first group in
human evolution to bury their dead deliberately, the bodies often
being buried with stone tools, animal bones, and even 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
12,000 years ago, one of the most important events in human history
took place—plants were domesticated, and 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
and genetic studies, but the picture is far from complete. 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 should result in an enormous increase
in the understanding of human biological history.
See also Anthropology.
For further information on this topic, see the Bibliography, sections 69.
Creation, 138. Human Evolution–139.
Race and ethnicity, 447.
Evolution.
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