Human Evolution, the biological and cultural development of the species Homosapiens, 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.
3and 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 hominin 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 about68 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 hominins, 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 indicate 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. a farensis, which lived in eastern Africa between 3 and 4million 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 400to 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 kindhave been found with A. a farensis 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 tothe 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 almost800 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 “handyman,”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 hominins. The earliest evidence of stone tools comes from sites in Africa datedto 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 human-like 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 donot 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 hominin 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, 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 neanderthals, 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 from100,000 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 isnot 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 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 are 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.
