Evolution and systematics

Humans are members of the primate infraorder Catarrhini. This infraorder encompasses the Old World monkeys (family Cercopithecidae), lesser apes (family Hylobatidae), and great apes and humans (family Hominidae). It has been clear since the 1930s that all of the living catarrhines comprise a closely related group of organisms that is both morphologically and physiologically very similar.

The taxonomy for humans is Homo sapiens Linnaeus, 1758, Uppsala, Sweden. All living humans belong to the subspecies Homo sapiens sapiens.

Humans have 46 chromosomes, in contrast to the 48 chromosomes of pongids. DNA-DNA hybridization studies initially highlighted the close genetic relationship between humans and common chimpanzees. However, in general, there is a high degree of genetic similarity between humans and other mammals. The genetic similarity between human and mouse is approximately 90%. Sequencing of the human genome was completed in 2001. A 2002 comparison of human and mouse genomes showed the existence of about 30,000 genes in both organisms. The same genetic elements can be rearranged, and appear on different chromosomes. The mouse genome has evolved 2-5 times more rapidly than the human genome, probably because the shorter generation length of mice allows for greater rates of change. Mouse genes appear to be more subject to physical reordering, and mouse genes in different locations on the same chromosome can evolve at different rates. About one-third of the genes shared between human and mouse do not encode proteins. Some of these may encode RNA, while others may serve regulatory functions. Studies of evolutionary development in humans and other vertebrates demonstrate the existence of conservative Hox genes that are responsible for establishing the embryonic blueprint.

Hominins (members of the subfamily Homininae) are descendants of an unknown pongid from the late Miocene of Africa. The first hominin may be the late Miocene Sahelan-thropus chadensis, dating to 6-7 million years ago (mya), from Chad, in Central Africa. However, this species is known only from cranial and dental remains. Orrorin tugenensis is a slightly more recent (6 mya) fossil species from western Kenya with postcranial remains. Femurs of Orrorin indicate that it had bipedal locomotion, which is the hallmark of the hominid family. A climatic trigger for hominin origins is often invoked. A period of late Miocene aridity in Africa is thought to have eliminated forests and caused the spread of extensive open-country grasslands, and thus created selection pressures for the origins of terrestrial bipedal hominins. However, Sahe-lanthropus, Orrorin, and later hominins that are well known postcranially are found in environmental mosaics that include

Fossil hominid skeleton (Australopithecus afarensis) known as "Lucy." Lucy was part of a rich find of fossils made in the Afar region of Ethiopia between 1973 and 1977. She dates from 3.3 million years ago and is widely accepted as the earliest link in the human record. The remains comprise 40% of an entire skeleton. (Photo by John Reader/Science Photo Library/Photo Researchers, Inc. Reproduced by permission.)

Fossil hominid skeleton (Australopithecus afarensis) known as "Lucy." Lucy was part of a rich find of fossils made in the Afar region of Ethiopia between 1973 and 1977. She dates from 3.3 million years ago and is widely accepted as the earliest link in the human record. The remains comprise 40% of an entire skeleton. (Photo by John Reader/Science Photo Library/Photo Researchers, Inc. Reproduced by permission.)

forested areas. The origins of terrestrial bipedal locomotion, therefore, cannot be simply linked to the disappearance of forest and the spread of grasslands.

The poorly known species Ardipithecus ramidus occurs between 5.8 mya and 4.4 mya, but the density of hominin fossils increases later, after 4.4 mya. A suite of hominin species appears in East Africa during this time range. Hominin species also occur at South African sites, although these sites lack volcanic materials, and are therefore more difficult to date. However, the South African species Australopithecus africanus and Australopithecus robustus appear to be later in time than East African material. These South African species were the first fossil hominins recognized from Africa, and are now among the most well known fossil hominins from the Plio-Pleistocene.

The genus Australopithecus alone contains eight species of hominin. Members of the genus occur principally in East and South Africa, and date from 4.4-1.2 mya. The longest-lived species (Australopithecus boisei) has a million year span, dating from 2.2-1.2 mya. It is clear that an evolutionary radiation of hominins occurred during the late Miocene through the early Pleistocene. Furthermore, there is definite evidence of sym-patric species, indicating that niche differences allowed species to divide the shared resource space.

Besides the possible hominin Sahelanthropus, there is an additional hominin species recognized from Chad. This is Australopithecus bahrelghazali, dating to about 3 or 3.4 mya. Its principal importance lies in the fact that the site lies 1,550 mi (2,500 km) west of the East African rift. These fossils demonstrate that hominins had a wide geographic distribution, and excellent dispersal abilities even at this early date. This fact might not be obvious from the plethora of human fossils that come from the rift. The richness of the fossil finds from the East African rift is a taphonomic accident, and is caused by the fact that the rift is a sediment trap with the potential for excellent fossil preservation, as well as chronometrically datable volcanic materials. A wide geographical range of ho-minins at this date indicates that intrinsic biological properties are contributing to dispersion, and not necessarily complex sociality or cultural behavior.

The site of Laetoli, in Tanzania, has hominin footprints laid down in trackways dating to 3.6 mya. These footprints were preserved in a gentle fall of volcanic ash that was deposited by rain. The importance of this site lies not only in its unequivocal record of bipedal locomotion, but also in its documentation that three hominins made the trackways—this is the earliest record of hominin sociality. Because fossils of Australopithecus afarensis occur at Laetoli, hominins belonging to this species were apparently responsible for the trackways. This agrees with traits that are unequivocal adaptations for bipedality in the vertebral column, pelvis, and lower limb of this species. Slightly later in time, Australopithecus afarensis is also found at localities in Hadar, Ethiopia. As of 2002, the remains of 17 contemporary individuals of this species have been found at the Hadar locality AL 333. A sudden, unknown event—not a flood—was responsible for the mass mortality. This material is also important in documenting sociality, because these individuals were apparently members of the same social group.

Although a large brain relative to body size was long considered the hallmark of the Homininae, by 2003 it became clear that the earliest hominins had a brain to body size ratio comparable to those of living pongids. Brain size increases only with the appearance of genus Homo. However, because there is a concomitant increase in body size, the relative increase in brain size does not become obvious until the late Pleistocene.

What of archaeology, which is the evidence of hominin behavior? The earliest stone tools, belonging to the Oldowan Industry, appear in Africa at 2.5-2.6 mya. Stone tools thus occur long after hominin origins. Besides the stone tools themselves, animal bones that show hominin modifications, such as cut-marks or percussion marks, yield a record of ho-minin behavior. Some early archeological sites contain no stone tools at all, but only modified bone. The earliest archaeological evidence occurs without substantial brain size increase. For example, the Ethiopian site of Bouri, dating to 2.5 mya, contains the hominin species Australopithecus gahri, along with modified bone. This species has a brain size of 450 cc, which is equivalent to that of a pongid, and smaller than that of most australopithecines. Hominin tool behavior is thus not dependent on brain size.

An illustration showing stages in the evolution of humans. At left, proconsul (23-15 million years ago) is depicted hypothetically as an African ape with both primitive and advanced features. From it, Australopithecus afarensis (>4-2.5 million years ago) evolved and displayed a bipedal, upright gait walking on two legs. Homo habilis (2.5 million years ago) was truly human. About 1.5 million years ago Homo erectus (at center) appeared in Africa and migrated into Eurasia. Homo neanderthalensis (200,000 years ago) lived in Europe and the Middle East and was closely related to modern humans (right). (Photo by David Gifford/Science Photo Library/Photo Researchers, Inc. Reproduced by permission.)

An illustration showing stages in the evolution of humans. At left, proconsul (23-15 million years ago) is depicted hypothetically as an African ape with both primitive and advanced features. From it, Australopithecus afarensis (>4-2.5 million years ago) evolved and displayed a bipedal, upright gait walking on two legs. Homo habilis (2.5 million years ago) was truly human. About 1.5 million years ago Homo erectus (at center) appeared in Africa and migrated into Eurasia. Homo neanderthalensis (200,000 years ago) lived in Europe and the Middle East and was closely related to modern humans (right). (Photo by David Gifford/Science Photo Library/Photo Researchers, Inc. Reproduced by permission.)

In 1999, a major taxonomic revision of Plio-Pleistocene hominins collapsed two early species of genus Homo (H. habilis and H. rudolfensis) into the genus Australopithecus, reserving genus Homo for material that unequivocally showed an increase in body size, had modern human proportions, and had no traits indicating a retention of climbing or arboreal adaptations. Some researchers argue that six or more species of genus Homo coexisted in the early Pleistocene, only to be winnowed out with the advent of Homo sapiens. However, it is unlikely that early genus Homo was speciose. One can assess the species richness of early Homo in contrast to other mammalian genera by examining the species richness of extant mammalian genera with a similar body size. Using this method, one or two hominin species is the number expected for a mammal genus of 66-143 lb (30-65 kg), which is the size range usually estimated for early Homo fossils.

African Homo erectus, appearing at 1.8 mya, is the first unequivocal member of genus Homo. Postcranial fossils indicate that body size has increased in this species. A higher quality or more predictable diet must underlie this increase in body size. Details of tooth enamel formation demonstrate that Homo erectus matured quickly, in an ape-like fashion. Sexual maturity may have been reached by females at 8-9 years, and by males at 10-12 years. This faster maturation may be a major factor in the dispersal abilities of this species, which was the first hominin to emerge from Africa to penetrate other regions of the Old World.

Slow maturation, equivalent to that of modern humans, appears only with the Neanderthals. Neanderthal fossils date from 300,000-28,000 years ago, and occur in Europe, Central Asia, and the Middle East. They are the most well known of fossil humans, because of the completeness of their skeletal remains. Nearly all researchers agree that this completeness results from deliberate burial of remains, rather than accidental preservation. Neanderthals possess a distinctive suite of skeletal traits. These traits (especially traits in the nasal region) appear to be adaptations to extremely cold, dry conditions. Neanderthals had highly carnivorous diets, as established by the bone chemistry of these fossil humans and contemporary animals. The taxonomic status of Neanderthals has been problematic since the discovery of the first Neanderthal fossils in the middle of the nineteenth century. As of 2003, most researchers assign them to a different species (Homo neanderthalensis), but many argue that they are distinct only at a subspecies level (Homo sapiens neanderthalensis). The argument is not trivial, because it affects discussions of whether modern human populations incorporate genetic material from earlier, non-modern humans, or represent de-

A Cro-Magnon skull. (Photo by E. R. Degginger/Photo Researchers, Inc. Reproduced by permission.)

scendants of a completely novel small founding population that completely replaces earlier humans.

Mitochondrial DNA (mtDNA) evidence initially seemed to support the origin of anatomically modern humans from a very small late Pleistocene founding population in sub-Saharan Africa. This idea became a prominent feature in many textbooks, where it was categorized as the "Out of Africa" or "Complete Replacement" model, because it seemed to imply that modern humans completely replaced their predecessors

Neanderthal man's skull. (Photo by E. R. Degginger/Photo Researchers, Inc. Reproduced by permission.)

in the Old World, who went extinct without issue. However, Templeton in 2002, using mtDNA and nuclear DNA from both autosomes and sex chromosomes, demonstrated that the evolutionary picture is substantially more complex, with a series of migrations out of Africa and another migratory vector out of Asia. There was no single small founding population for modern humans, during either the middle or late Pleistocene. Mitochondrial DNA has also failed to elucidate lower level questions about human evolution and dispersal. For example, it is clear in 2003 that mtDNA from Native Americans cannot illuminate crucial questions about the peopling of the Americas, such as the number or timing of migration events, or the source of the founding populations.

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