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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).
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.
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.
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.
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.