Human EvolutionHuman Evolution, the biological and cultural development of the species Homosapiens, or human beings. A large number of fossil bones and teeth have beenfound at various places throughout Africa, Europe, and Asia.
Tools of stone,bone, and wood, as well as fire hearths, campsites, and burials, also have beendiscovered and excavated. As a result of these discoveries, a picture of humanevolution 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 ournearest living relatives, the African apes, are sometimes placed together in thefamily Hominidae because of genetic similarities, although classificationsystems more commonly still place great apes in a separate family, Pongidae. Ifthe single grouping, Hominidae, is used, the separate human line in the hominidfamily is distinguished by being placed in a subfamily, Homininae, whose membersare then called homininesthe practice that is followed in this article. Anexamination of the fossil record of the hominines reveals several biological andbehavioral trends characteristic of the hominine subfamily.Order now
Bipedalism Two-legged walking, or bipedalism, seems to be one of the earliest ofthe major hominine characteristics to have evolved. This form of locomotion ledto a number of skeletal modifications in the lower spinal column, pelvis, andlegs. Because these changes can be documented in fossil bone, bipedalism usuallyis seen as the defining trait of the subfamily Homininae. Brain Size and Body Size Much of the human ability to make and use tools andother objects stems from the large size and complexity of the human brain. Mostmodern 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 morethan tripled. The increase in brain size may be related to changes in homininebehavior. Over time, stone tools and other artifacts became increasinglynumerous and sophisticated.
Archaeological sites, too, show more intenseoccupation in later phases of human biological history. In addition, thegeographic areas occupied by our ancestors expanded during the course of humanevolution. Earliest known from eastern and southern Africa, they began to moveinto the tropical and subtropical areas of Eurasia sometime after a millionyears ago, and into the temperate parts of these continents about 500,000 yearsago. Much later (perhaps 50,000 years ago) hominines were able to cross thewater barrier into Australia. Only after the appearance of modern humans didpeople move into the New World, some 30,000 years ago. It is likely that theincrease in human brain size took place as part of a complex interrelationshipthat included the elaboration of tool use and toolmaking, as well as otherlearned skills, which permitted our ancestors to be increasingly able to live ina variety of environments.
The earliest hominine fossils show evidence of markeddifferences in body size, which may reflect a pattern of sexual dimorphism inour 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, whilemales 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 socialgroups. This extreme dimorphism appears to disappear gradually sometime after amillion years ago. Face and Teeth The third major trend in hominine development is the gradualdecrease in the size of the face and teeth. All the great apes are equipped withlarge, tusklike canine teeth that project well beyond the level of the otherteeth. The earliest hominine remains possess canines that project slightly, butthose of all later hominines show a marked reduction in size.
Also, the chewingteethpremolars and molarshave decreased in size over time. Associated withthese changes is a gradual reduction in the size of the face and jaws. In earlyhominines, the face was large and positioned in front of the braincase. As theteeth became smaller and the brain expanded, the face became smaller and itsposition changed; thus, the relatively small face of modern humans is locatedbelow, rather than in front of, the large, expanded braincase. Human Origins The fossil evidence for immediate ancestors of modern humans isdivided into the genera Australopithecus and Homo, and begins about 5 millionyears ago.
The nature of the hominine evolutionary tree before that is uncertain. Between 7 and 20 million years ago, primitive apelike animals were widelydistributed on the African and, later, on the Eurasian continents. Although manyfossil bones and teeth have been found, the way of life of these creatures, andtheir evolutionary relationships to the living apes and humans, remain mattersof active debate among scientists. One of these fossil apes, known asSivapithecus, appears to share many distinguishing features with the livingAsian great ape, the orangutan, whose direct ancestor it may well be. None ofthese fossils, however, offers convincing evidence of being on the evolutionaryline leading to the hominid family generally or to the human subfamily inparticular. Comparisons of blood proteins and the DNA of the African great apeswith that of humans indicates that the line leading to modern people did notsplit off from that of chimpanzees and gorillas until comparatively late inevolution.
Based on these comparisons, many scientists believe a reasonable timefor this evolutionary split is 6 to 8 million years ago. It is, therefore, quitepossible that the known hominine fossil record, which begins about 5 millionyears ago, extends back virtually to the beginnings of the human line. Futurefossil discoveries may permit a more precise placement of the time when thedirect ancestors of the modern African ape split off from those leading tomodern people and human evolution can be said to begin. Australopithecus The fossil evidence for human evolution begins withAustralopithecus. Fossils of this genus have been discovered in a number ofsites 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 theaustralopithecines were efficiently bipedal and therefore indisputable hominines. In details of their teeth, jaws, and brain size, however, they differsufficiently among themselves to warrant division into four species: A. afarensis, A. africanus, A.
robustus, and A. boisei. The earliestaustralopithecine is A. afarensis, 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 teethsomewhat more projecting than those of later hominines. No tools of any kindhave been found with A. afarensis fossils.
Between about 2. 5 and 3 million yearsago, A. afarensis apparently evolved into a later australopithecine, A. africanus. Known primarily from sites in southern Africa, A.
africanus possesseda brain similar to that of its predecessor. However, although the size of thechewing teeth remained large, the canines, instead of projecting, grew only tothe level of the other teeth. As with A. afarensis, no stone tools have beenfound 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 asfour, separate species of hominines. An evolutionary split seems to haveoccurred in the hominine line, with one segment evolving toward the genus Homo,and finally to modern humans, and the others developing into australopithecinespecies that eventually became extinct. The latter include the robustaustralopithecines, A. robustus, limited to southern Africa, and A. boisei,found only in eastern Africa.
The robust australopithecines represent aspecialized adaptation because their principal difference from otheraustralopithecines lies in the large size of their chewing teeth, jaws, and jawmuscles. The robust australopithecines became extinct about 1. 5 million yearsago. The Genus Homo Although scientists do not agree, many believe that after theevolutionary split that led to the robust australopithecines, A. africanusevolved into the genus Homo.
If so, this evolutionary transition occurredbetween 1. 5 and 2 million years ago. Fossils dating from this period display acurious mixture of traits. Some possess relatively large brainsseveral almost800 cc (about 49 cu in)and large, australopithecine-sized teeth. Others havesmall, Homo-sized teeth but also small, australopithecine-sized brains. A numberof fossil skulls and jaws from this period, found in Tanzania and Kenya ineastern Africa, have been placed in the category H.
habilis, meaning “handy man,”because some of the fossils were found associated with stone tools. H. habilispossessed many traits that link it both with the earlier australopithecines andwith later members of the genus Homo. It seems likely that this speciesrepresents the evolutionary transition between the australopithecines and laterhominines. 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 associationwith a particular hominine species. By 1. 5 to 2 million years ago, sites invarious parts of eastern Africa include not only many stone tools, but alsoanimal bones with scratch marks that experiments have shown could only be leftby humanlike cutting actions. These remains constitute evidence that by thistime early hominines were eating meat, but whether this food was obtained byhunting or by scavenging is not known. Also unknown at present is how much oftheir 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 knownwhether 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 fromnorth Kenya and dating from 1. 5 to 1. 6 million years ago, has been placed in thespecies H. erectus. The first part of the time span of H.
erectus, like that ofthe earlier-in-time hominines, is limited to southern and eastern Africa. Laterbetween 700,000 and a million years agoH. erectus expands into the tropicalareas of the Old World, and finally at the close of its evolution, into thetemperate parts of Asia. A number of archaeological sites dating from the timeof H.
erectus reveal a greater sophistication in toolmaking than was found atthe earlier sites. At the cave site of Peking man in north China, there isevidence that fire was used; the animal fossils that have been found aresometimes of large mammals such as elephants. These data suggest that homininebehavior was becoming more complex and efficient. Throughout the time of H.
erectus the major trends in human evolution continued. The brain sizes of earlyH. erectus fossils are not much larger than those of previous hominines, rangingfrom 750 to 800 cc (45. 8 to 48. 8 cu in).
Later H. erectus skulls possess brainsizes in the range of 1100 to 1300 cc (67. 1 to 79. 3 cu in), within the sizevariation of Homo sapiens.
Early Homo sapiens Between 200,000 and 300,000 years ago, H. erectus evolvedinto 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 somescientists and as early H.
sapiens by others. Although placed in the same genusand species, these early H. sapiens are not identical in appearance with modernhumans. New fossil evidence suggests that modern man, H. sapiens sapiens, firstappeared more than 90,000 years ago. There is some disagreement among scientistson whether the hominine fossil record shows a continuous evolutionarydevelopment from the first appearance of H.
sapiens to modern humans. Thisdisagreement 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 theearliest 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 disappearedfrom the fossil record.
Fossils of additional varieties of early H. sapiens havebeen discovered in other parts of the Old World. The dispute over theNeandertals also involves the question of the evolutionary origins of modernhuman populations, or races. Although a precise definition of the term race isnot possible (because modern humans show continuous variation from onegeographic area to another), widely separate human populations are marked by anumber of physical differences. The majority of these differences representadaptations to local environmental conditions, a process that some scientistsbelieve began with the spread of H. erectus to all parts of the Old Worldsometime after a million years ago.
In their view, human development since H. erectus has been one continuous, in-position evolution; that is, localpopulations have remained, changing in appearance over time. The Neandertals andother early H. sapiens are seen as descending from H. erectus and are ancestralto modern humans. Other scientists view racial differentiation as a relativelyrecent phenomenon.
In their opinion, the features of the Neandertalsa low,sloping forehead, large brow ridge, and a large face without a chinare tooprimitive for them to be considered the ancestors of modern humans. They placethe Neandertals on a side branch of the human evolutionary tree that becameextinct. According to this theory, the origins of modern humans can be found insouthern 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 findsfrom southern Africa, support for this theory comes from comparisons ofmitochondrial DNA, a DNA form inherited only from the mother, taken from womenrepresenting a worldwide distribution of ancestors. These studies suggest thathumans derived from a single generation in sub-Saharan Africa or southeasternAsia. Because of the tracing through the material line, this work has come to becalled the “Eve” hypothesis; its results are not accepted by mostanthropologists, who consider the human race to be much older. See also RACES,CLASSIFICATION OF. Whatever the outcome of this scientific disagreement, theevidence shows that early H. sapiens groups were highly efficient at exploitingthe sometimes harsh climates of Ice Age Europe.
Further, for the first time inhuman evolution, hominines began to bury their dead deliberately, the bodiessometimes being accompanied by stone tools, by animal bones, and even by flowers. Modern Humans Although the evolutionary appearance of biologically modernpeoples did not dramatically change the basic pattern of adaptation that hadcharacterized the earlier stages of human history, some innovations did takeplace. In addition to the first appearance of the great cave art of France andSpain See CAVE DWELLERS, some anthropologists have argued that it was duringthis time that human language originated, a development that would have hadprofound implications for all aspects of human activity. About 10,000 years ago,one of the most important events in human history took placeplants weredomesticated, and soon after, animals as well. This agricultural revolution setthe stage for the events in human history that eventually led to civilization.
Modern understanding of human evolution rests on known fossils, but the pictureis far from complete. Only future fossil discoveries will enable scientists tofill many of the blanks in the present picture of human evolution. Employingsophisticated technological devices as well as the accumulated knowledge of thepatterns of geological deposition, anthropologists are now able to pinpoint themost promising locations for fossil hunting more accurately. In the years aheadthis will result in an enormous increase in the understanding of humanbiological history.Daniel Mokari