Eric Delson & Karen L Baab. McGraw-Hill Encyclopedia of Science & Technology.10th edition, Volume 7, McGraw-Hill, 2007.
All prehistoric skeletal remains of humans that are archeologically earlier than the Neolithic (necessarily an imprecise limit), regardless of degree of mineralization or fossilization of bone, and regardless of whether the remains may be classed as Homo sapiens sapiens (anatomically modern humans). In this sense, the term “humans” is used broadly to mean all primates related to living people since the last common ancestor of people and African apes, thus all species currently included in the genera Homo, Australopithecus, Ardipithecus, and Paranthropus (and potentially others discussed below).
Discoveries began early in the nineteenth century, although their meaning and antiquity were not recognized before the finding of the Neanderthal specimen in 1856, combined with Darwin’s demonstration of the reality of evolution soon afterward. Fossil human remains have come principally from Europe, western Asia, China, Java, and Africa. Because of the rather late entry of humans into the New World, all American remains are of relatively recent origin and recognizable as H. sapiens.
Dating Fossils
The human lineage was once thought to have come into existence only at the beginning of the Pleistocene, the geological time interval from about 1.8 million years ago to the present. It is now known to have been fully separate from the ancestry of the apes at least as far back as the Early Pliocene or perhaps even the later Miocene (4-7 million years ago, or Ma). This view was widely held by anthropologists, on theoretical grounds, in the early part of the twentieth century. It was abandoned in the 1940s because of lack of supporting evidence. Discoveries in Pliocene and Miocene deposits since then have led to a reappraisal of the evidence.
Dating within the Pleistocene (and earlier) is accomplished by the methods of conventional paleontology and geology, by association with human implements, by several chemical and physical tests for relative age (such as the fluorine test for accumulation of this element in bone), and by geochronometric methods which provide an age in years. The latter usually involve measuring the presence of radioactive substances such as radiocarbon and radiopotassium as well as other radioactivity-based methods such as electron spin resonance, thermoluminescence, and fission-track dating. All of thesemethods are broadly termed radiometric methods, as opposed to approaches such as amino acid racemization or dendrochronology (tree-ring dating).
One of the most important methods of correlation between dated geological sequences is paleomagnetism, which does not itself provide a date because it is basically a boolean system with two states (so-called normal and reversed polarity of geomagnetism) of which all occurrences are indistinguishable. But if the pattern of reversals at any site can be matched against the global paleomagnetic time scale and even approximately dated by means of geochronology or paleontology, a sequence of dates may result.
Prehuman Ancestry
Humans are catarrhine primates, part of a group including Old World monkeys, apes, and various extinct forms. Most evidence from both comparative morphology and molecular studies of proteins shows that humans’ closest living relatives are the African apes: the chimpanzee and the gorilla. Less close is the Asian orangutan, and most distinctive of all apes are the gibbons. A classification which conforms to these relationships within the Hominoidea (apes, humans, and close extinct relatives) recognizes the family Hylobatidae for the gibbons and Hominidae for humans and great apes. The latter family is divided into Ponginae (orangutans and extinct relatives) and Homininae. Many workers, however, continue to reject this view and place humans in the Hominidae as contrasted with the paraphyletic (multi-origined) apes in Pongidae. A few others have suggested that in fact orangutans are the closest living relatives of humans. No fossils of any modern ape are known, other than Pleistocene gibbon and orangutan teeth from southern Asia, and a few Kenyan teeth identified as chimpanzee which were recognized in 2005.
The oldest certain representatives of the Catarrhini are fossils from the Fayum beds of northern Egypt dated around 34 Ma. The best known is Propliopithecus (=Aegyptopithecus) zeuxis, a species near the common ancestry of apes, humans, and Old World monkeys. Hominoids originated in Africa, probably during the Late Oligocene and Early Miocene (26-17 Ma), as suggested by the genera Kamoyapithecus and Proconsul, placed in the family Proconsulidae. However, some scholars have questioned the view that these forms are already hominoids, proposing instead that they were conservative catarrhines that predate the evolutionary split between hominoids and Old World Monkeys. Between 20 and 15 Ma, there are several more derived (“advanced”) African hominoid genera (for example, Morotopithecus, Afropithecus, and Kenyapithecus) and one Eurasian form (Griphopithecus). If these are more closely related to living great apes than are gibbons, as has been proposed in the past, they may represent early members of the Hominidae. However, if they are conservative (“primitive,” plesiomorphic) by comparison with the gibbons, then they should not be included in the hominid family.
In the latter case, several workers have argued that the origin of modern ape (and human) lineages must have occurred in Eurasia, where a number of derived genera occur between 14 and 8 Ma. These include Dryopithecus and Pierolapithecus (discovered in 2004), which were probably close to the common ancestor of all great apes and humans; Ankarapithecus and Sivapithecus, which appear to represent two early stages in the orangutan lineage (Ponginae); and Graecopithecus (also termed Ouranopithecus), which has been suggested as a member of Homininae, that is on the lineage leading toward African apes and humans. Another early hominine is Samburupithecus, known only from an upper jaw fragment found in the Samburu Hills of Kenya and (like Graecopithecus) dated to about 9.5 Ma. A possible alternative interpretation is that a form such as Morotopithecus, with relatively derived but poorly known postcranial elements, might have given rise both to the early Eurasian hominids and to an as yet unknown African line which culminated in Samburupithecus. In either case, it is not until the latest Miocene (7-5 Ma) that we see the first possible fossil representatives of the Hominini (hominins), the group including modern humans and all our relatives since the last common ancestor shared with chimpanzees. Molecular anthropological studies of DNA sequences and other lines of evidence have suggested that chimpanzees are more closely related to humans than either is to gorillas. This would appear counterintuitive, given the morphological similarity between the two African apes, both of which are also apparently derived in their knuckle-walking locomotion. But similarities may be merely holdovers from the common ancestor of all three forms, while chimps and humans shared an intermediate common ancestor after the gorilla lineage split away. Paleonto-logical evidence is sparse in Africa between 12 and 5 Ma, but it is possible that Samburupithecus (or Graecopithecus) represents a species near the base of that three-way split or already on the gorilla line. Their ages of 10-9 Ma fit reasonably well with the “molecular clock” estimates of 10-7 Ma for that divergence. The human-chimp split is molecularly estimated at 8-5 Ma or even less, but the human fossil record refutes a date younger than perhaps 5 Ma at the minimum.
Identification of Early Hominins
When paleoanthropologists find early fossils that might be on the human lineage, they must identify them as such by locating bony features which distinguish the earliest hominins from their ape relatives or ancestors.
In order to find such features in fossils, we must begin by determining what makes modern humans different from living apes. We can then discern the sequence in which such features appeared in the human fossil record. However, not all changes happened at the same time. This pattern is known as mosaic evolution, referring to mosaic pictures made up of numerous small parts which fit together to make a complete image.
One major difference involves the mode of locomotion used by hominins versus apes. Hominins are habitually bipedal animals (walking on two legs rather than four), whereas our ape ancestor likely included quadrupedal, suspensory and/or knuckle-walking behaviors in its locomotor repertoire. The human skeleton has been modified in many ways to accommodate this change. For example, hominins have a foot with a longer big toe that is in line with the other toes, rather than a grasping apelike foot with a divergent big toe. The foot has also developed two arches, one longitudinal and one transverse (along the long and short axes of the foot, respectively) that act as shock absorbers when walking bipedally. While the spine of apes is C-shaped, hominins evolved thoracic and lumbar curvatures, resulting in an S-shaped spine that acts to balance the torso over the pelvis and legs. This spine also connects to the skull more vertically (and centrally) underneath rather than toward the back of the skull (as in apes), which can be seen by the more forward placement of the foramen magnum (opening where the spinal cord enters the skull). The pelvis and pelvic muscles have also gone through a major reorganization: the pelvis is less tall and more bowl-shaped, and the gluteus maximus muscle is more developed to help extend the leg backward in walking. The femur (bone of the thigh) is more angled in at the knee in order to center the feet beneath the body, and the knee is capable of straightening out completely. Modern humans also have quite long legs and only moderately long arms compared to body size. All of these features aid in more stable and efficient bipedal locomotion in hominins than apes. The fossil record suggests that relatively modern-looking spinal curvature, pelvic shape, and femoral angling appeared early, while foot arching, perhaps big toe position, and especially fully modern limb proportions did not evolve until later in the hominin lineage. The human skull has also undergone changes not directly related to bipedal locomotion, such as an increase in brain size (relative to body size) and changes in the teeth, including a decrease in canine size. While some early hominins have relatively larger brains than chimpanzees of similar body weight, major brain expansion probably did not occur until the earliest member of our own genus Homo evolved.
The next question then is: When in the fossil record do we first see evidence of these changes? Recently discovered fossils from Chad (termed Sahelanthropus) may include the oldest hominin cranium with smaller canines and a forwardly positioned foramen magnum (which may in turn suggest some bipedalism). Limb bone fossils from Kenya (named Orrorin) are said to have a number of characteristics consistent with bipedalism, though some debate surrounds these claims. Both sets of fossils probably date from 7-6 Ma, making them the oldest possible hominins in existence. Younger finds (∼4.4 Ma) from Ethiopia (Ardipithecus)have much smaller canines and may also have a foramen magnum that is positioned more forward than in apes. Canine reduction may have begun even before hominins appeared, as seen in the fossil ape Graecopithecus. Further decreases in size eventually led to the very small canines present in modern humans. It has been suggested that smaller canines (with no significant difference between those of males and females) reflect a different social organization, because large canines are associated in apes (and monkeys) with a high degree of competition among males for mates. Fossil australopiths (see below) show clear evidence of bipedalism both in their skeletons and in fossilized footprints found at the site of Laetoli, Tanzania. These hominins still display many apelike features in their skull, showing that habitual bipedalism evolved before modern skull anatomy.
Miocene Hominini
In 2002, French and Chadian researchers led by Michel Brunet reported the discovery of new fossils from Chad in Central Africa. Based on the associated mammalian fossils, the site of Toros-Menalla was estimated to date about 7-6 Ma. Previously, almost all early human remains had been found in eastern or southern Africa, although a few fragments from Chad dating to about 3.5 Ma had been known since the 1990s. The new fossils (named Sahelanthropus tchadensis) included a fairly complete though crushed cranium (the skull without the lower jaw) of what might be the oldest hominin ever found. Recent computer-based analysis has permitted a “virtual” reconstruction of what the cranium might have looked like before it was crushed. Although the brain size is no bigger than that of a similarly sized ape, its teeth are more like a hominins, especially in having an apparently reduced canine. The relatively anterior position of the foramen magnum suggested to the discoverers that this species may have been at least partly bipedal, but others have questioned this view. Sahelanthropus is likely near to the chimpanzee-human split, based on both its morphology and the molecular divergence date discussed above.
In late 1999, Paris-based Martin Pickford and Brigitte Senut working in Kenya discovered a small collection of fragmentary fossils, mostly limb elements, which they also claimed as the earliest known hominin. The fossils were first termed “Millenium Man,” as they were announced to the public just before 2000, but they have been formally named Orrorin tugenensis. These remains are better dated than those of Sahelanthropus, to about 6 Ma, by radiometric techniques. It has been claimed that the femur shows features indicating bipedalism, but most researchers have questioned this interpretation, and some have suggested that Orrorin is actually the long-sought fossil of an early African ape ancestor. Unfortunately, dental and cranial remains are too fragmentary to be certain either way.
Still younger fossils are known from sites in the Middle Awash Valley in Ethiopia. Recovered between 1997 and 2004 and first described in 2001, these were named Ardipithecus kadabba in 2005. The fossils range in age between 5.8 and 5.2 Ma, at the very end of the Miocene epoch. Most are fragmentary jaws, but a single toe bone has been claimed to indicate bipedalism, although (as usual), some researchers have questioned this interpretation.
Pliocene Hominins: The Australopiths
Pliocene humans have been grouped in various ways, but it now seems that four main types can be distinguished. Three of these, dating from roughly 4.5-1 Ma, have previously been assigned to the genus Australopithecus and can informally be termed australopiths (here they are placed in the genera Ardipithecus, Australopithecus, and Paranthropus). The fourth group includes early species of Homo, beginning about 2.5 Ma. The australopiths have previously been divided into gracile and robust varieties, but the former term really is not accurate and is not used here. All australopith species appear to share a number of basic characteristics (see above) distinguishing them from living and fossil apes and also from later humans, although clearly linking them to the latter. The species of australopith also broadly share a smaller body size than modern humans with surprisingly little difference among the known forms. Estimates of body size based on analysis of weight-bearing joint surfaces and bone lengths suggest average weights around 130 lb (59 kg), with females often under 77 lb (35 kg) and males over 155 lb (70 kg). This quite high sexual dimorphism is also typical of australopiths, as are back teeth that are large for the estimated body size, although the actual sizes and tooth proportions are among the features distinguishing the several species from one another. In light of the diversity now recognized, many researchers accept a division into the genera Australopithecus and Paranthropus, as well as Ardipithecus, but others continue to recognize only one or two of these genera.
The fossils of these early humans were first found in South Africa in 1924, but the most recent major discoveries and the best evidence of their age come from East Africa. The South African fossils come from six main sites, which are the remains of ancient cave systems. The australopiths did not live in caves, but their carcasses may have been washed or dropped into crevices near the ground surface by leopards or other carnivores. In two sites especially, many fossils are known from relatively short spans of time (perhaps less than 200 thousand years at each), but the evidence for dating is not definite.
In 1959 Mary Leakey and L. S. B. Leakey discovered a nearly complete australopith skull at Olduvai Gorge, Tanzania, and colleagues dated it at about 1.75 Ma, far older than previously thought. Since then, American and joint American-French and American-Ethiopian expeditions have found hundreds of human fossils in the Omo and Afar areas of Ethiopia, while a Kenyan team has worked around the shores of Lake Turkana (formerly Lake Rudolf), Kenya, and various studies have continued at Olduvai and nearby Laetoli. These regions have yielded smaller numbers of specimens at many separate subsites, but the age of each site can usually be estimated closely by potassium-argon and paleomagnetic dating. Also, many specimens are more complete and show less distortion than their contemporaries from South Africa. Until 1993, no definite examples of Australopithecus or Paranthropus were known outside these areas, although some claims have been made. The recovery of australopith specimens from Chad (and of Paranthropus and early Homo from Malawi) demonstrates that new productive regions remain to be explored. It is not clear whether australopiths made stone tools, although they were probably collectors of plant foods, using unpreserved wooden sticks and skin bags, and perhaps scavengers of small game animals.
Ardipithecus: Still a Question
The oldest known likely human species, Ardipithecus ramidus, is known from a small group of fossils found at the Aramis locality in the Middle Awash Valley, Ethiopia. (Additional remains of the species have been reported from the nearby Gona locality as well.) The Aramis fossils have been recovered since 1993 from rocks just above a layer dated to 4.4 Ma, which also yielded animal and plant fossils suggesting a relatively wooded environment. The remains of Ard. ramidus include teeth, part of a skull base, and the almost complete long bones of a forelimb, as well as many pieces of a fragmented adult skeleton, including parts of almost all regions of the body. As of early 2006, this skeleton had not yet been formally described or illustrated, but the skull is said to be crushed flat and fragmented into hundreds of tiny pieces which have not yet been reconstructed. The elements described so far document a mosaic pattern combining features similar to those of younger humans (for example, the anterior position of the foramen magnum and nearby structures, implying upright posture and presumably bipedalism; and partial reduction of the canines) with others reflecting retention of apelike conditions (for example, thin enamel covering on molars and incisors; lack of a second cusp, or metaconid, on the lower anterior premolar; relatively large size of canines; and apelike shape of the lower anterior deciduous premolar or “milk molar”). It has been suggested that their presence in a wooded environment implies that australopiths may have differentiated from apelike ancestors in forests rather than open savannahs. But an alternative view suggests that this species represents a “failed” human lineage, one which returned to the forest and secondarily developed thinner enamel convergently with African apes. The remaining bony features of Ard. ramidus are all reasonably interpreted as ancestral conditions, to be expected in an ancient human ancestor. Analysis of the partial skeleton should permit determination of the locomotor abilities of this species. Other specimens from Kenya might be referable to this species but are too fragmentary to be sure yet.
Earliest Definite Humans: Australopithecus
The first habitual bipeds, Australopithecus, appear in the fossil record in quantity around 4.2 Ma, during the Early Pliocene. The earliest representatives of Australopithecus, named Aus. anamensis, have been found since 1994 at sites in the southern Lake Turkana region of Kenya. From Kanapoi, in a layer dated to 4.2-4.1 Ma, come several jaws that differ in shape from those of other Australopithecus species. A partial ankle joint from a younger layer confirms upright walking around 4 Ma. It seems likely that Aus. anamensis may be close to the ancestry of later species and a possible descendant of Ard. ramidus.
Fossils from sites in Ethiopia and Tanzania reveal far more details about a still younger species, Aus. afarensis. The most complete material is known from the Ethiopian site of Hadar, about 50 km (30 mi) north of Aramis, where deposits yielded fossils dating between 3.4 and 2.9 Ma. In 1974 a partial skeleton was found and identified as a female by its pelvic bones (and small size compared to other fossils) and nicknamed Lucy. This individual would have stood only 3.5 ft (106 cm) tall and weighed perhaps 65 lb (30 kg). The leg bones of this skeleton indicate that Lucy’s legs would have been rather short for a modern human of comparable body weight, but they were in proportion for her estimated stature; this observation supports other evidence that most australopiths were more robustly built than modern humans. Larger males, perhaps 130-175 lb (59-80 kg) in weight, are known from more fragmentary remains, although a nearly complete skull (not known for Lucy) was found in the 1990s and analyzed in detail in 2004. Additional specimens, including partial skulls, continue to be found at Hadar. In addition, mandibles and postcranial elements from Maka (across the Awash River from Aramis) and Laetoli (Tanzania) dating around 3.4-3.7 Ma confirm the sexually dimorphic but taxonomically unified nature of the species, while a frontal bone from Belohdelie (3.8 Ma, near Maka) and teeth from the Lake Turkana Basin (3-2.7 Ma) define its temporal range. A fragmentary lower jaw from Chad is estimated to date to a similar time interval by comparing the animal fossils associated with it to those from East Africa; it has been given a new species name but could be a western representative of Aus. afarensis.
Lucy’s pelvis and leg bones, as well as remarkably preserved footprints from Laetoli, clearly demonstrate that upright bipedal walking was well developed by 3.6 Ma, along with a brain somewhat larger than in modern apes of similar body size. Brain size in Aus. afarensis might have been between 350 and 450 ml, in a body weighing some 65-154 lb (30-70 kg) as compared to 365 ml in a 100-lb (45-kg) chimpanzee, 500 ml in a 300-lb (135-kg) gorilla, or 1400 ml in a 150-lb (67-kg) living human. Arguments as to the priority of brain expansion or walking ability in human evolution thus have yet to be fully resolved.
Australopithecus afarensis combines both of these advanced, human characteristics with numerous other features reminiscent of later Miocene hominids and modern apes. The lower face is rather projecting (prognathic), the canines project slightly beyond the level of the neighboring teeth, the anterior lower premolar sometimes has only one major cusp, the tooth row is elongated and nearly parallel-sided, and the forehead is low and retreating. However, the mastoid region (below and behind the bony ear opening in the skull) projects inferiorly more than in either living apes or modern humans. Moreover, the long curved fingers and toes support the idea that it often climbed trees, perhaps to sleep, although there is still some controversy over the degree of posible arboreal behavior in this species.
The geologically youngest of the early australopiths is actually the first to have been recognized. Australopithecus africanus was named by R. Dart in 1925 on the basis of a juvenile specimen from Taung (South Africa). It appears to have lived from 3 to 2.3 Ma, but so far it is known only from four sites in South Africa; by far the most numerous specimens come from the middle levels (Member 4) of Sterkfontein cave, dated about 2.8-2.6 Ma. It may have stood 4-5 ft (120-150 cm) tall, weighed 65-150 lb (30-68 kg), and had an average brain size of some 450 ml. The skull seems more lightly built than in Aus. afarensis, with a rounded vault but more projecting face. The teeth are more humanlike as well, especially in the presence of two cusps on all anterior lower premolars (like all younger hominins) and less projecting canines. Although the absolute tooth size of Aus. africanus is nearly equal to that of small gorillas, the proportions are human, with a smooth decrease in size from molars through incisors. Its postcranial skeleton appears quite similar to that of Australopithecus afarensis, with a fully bipedal (if by no means modern) locomotor adaptation. Thus, it was perhaps little changed from its putative ancestor, but those few changes are roughly in the direction of later humans. Some workers have suggested that it might lie close to the ancestry of the genus Homo, but pending clearer evidence for its phyletic position, that step is not taken here.
In 1995 researchers described bones of a partial foot from the lower levels (Member 2) of Sterkfontein, perhaps contemporaneous with Aus. afarensis. These bones were interpreted as indicating a foot partially adapted to grasping and perhaps tree climbing rather than only to bipedal walking. In late 1998 the find of additional parts of the same skeleton was reported, including both legs, some arm bones, and what appears to be most of the skull. All of these elements must be fully removed from the encasing rock and reconstructed before they can be accurately analyzed, but much more of the skeleton may be preserved, in which case it might surpass Lucy in its completeness. Dates between 4 and 3 Ma have been suggested by different techniques (and some scholars even think it might be younger than 2.5 Ma), and it has not yet been determined if it should be identified as Aus. africanus, Aus. afarensis, or a new species of australopith.
In 1999, Ethiopian and American researchers described Aus. garhi from deposits in the Awash valley of Ethiopia, dated to about 2.5 Ma. A partial face and uncertainly associated limb bones were said to represent a new species close to the origin of Homo, but other workers have suggested alternative interpretations, such as that these fossils represented a late population of Aus. afarensis or female individuals of Par. aethiopicus. Stone tools and animal bones with stone tool cut marks were also found in the vicinity, raising the possibility that australopiths may have manufactured stone tools. On the other hand, early members of our own genus, Homo, first appear in the fossil record at other sites around this time period and may have been responsible for making and using these tools.
Robust Varieties
Until 1986, robust australopiths were known from two forms, usually accepted as distinct species and increasingly given generic status as Paranthropus: P. robustus in South Africa and the more extreme P. boisei in East Africa. These species lived between about 2.3 and 1.4 Ma (the age of P. robustus is known with less certainty, perhaps 1.9-1.6 Ma) and are distinguished from the other australopiths by their larger size and craniodental specializations. They may have been 4 ft, 6 in. to 5 ft, 9 in. (135-175 cm) tall, weighed 80 to 190 lb (36 to 86 kg), had a somewhat heavy and muscular body build, and a brain size of about 525 ml. The skull is robust, with deep cheekbones and thick lower jaw and often a slightly raised sagittal (midline) crest in the middle part of the skull roof from back to front. These features indicate strong chewing muscles and perhaps a diet of tough foods. The teeth themselves are distinctive: the back teeth (molars and premolars) are large to huge; the front teeth (incisors and canines) are quite small and run nearly straight across the front of the mouth. This difference from other hominins, including apes (which generally have large front teeth and small back teeth), combined with a low forehead and a concave, nearly upright face, further suggests adaptation to powerful chewing. The anterior teeth were probably used as much for grinding as for cutting. The scrappy postcranial elements reveal no major adaptive differences from those of Australopithecus.
An even more ancient species, (P. aethiopicus), presents a combination of many of these Paranthropus features plus others that appear to be holdovers from an Aus. afarensis-like ancestry. Here, large molars lie behind sockets for rather large incisors and canines; a concave upper face with low forehead sits above a projecting snout; and the sagittal crest extends back to meet the large nuchal (neck muscle) crest. The brain size was small, near 400 ml. The combination indicates that this species emphasized both large front and back teeth, occupying an evolutionary position intermediate between Aus. afarensis and the two previously known robust australopiths; its large mastoid area also links it to Aus. afarensis. It is also intermediate in time, ranging from 2.7 to 2.3 Ma.
Australopith Relationships
The widely accepted view before 1978 was that Aus. africanus represented the common ancestor of the robust forms and Homo. The discovery of Aus. afarensis led to placement of this species as the basal hominin, with several alternative views of its descendants. In the 1990s the newly recovered Aus. anamensis was usually ranked as a still older common ancestor, with Ard. ramidus held off to the side, in suspense. Early analyses of P. aethiopicus considered it as close to the common ancestor of P. boisei and P. robustus, but some later studies argued that the three robust species were “only” linked by their common possession of a heavily built chewing apparatus, which might have evolved convergently in two or more disparate lineages. These studies proposed (as had earlier workers on other grounds) that Aus. africanus might have been ancestral to P. robustus in South Africa or that P. aethiopicus might have been an early experiment unrelated to later robust species. Such views seem poorly founded, for if the three robust species are in fact linked by the adaptive complex of “heavy chewing” composed of a number of closely similar elements in each form, it is far more likely that they developed from a single common ancestor than that they were convergently comparable but unrelated. Here the robust clade or lineage is recognized as an evolutionary unit, the genus Paranthropus, which was thus long-lived but seems to have had no later descendants.
No known australopith clearly shows features that link it to later human species of the genus Homo, but many workers have suggested that Aus. africanus might represent the closest approach to such an ancestry yet recovered. However, some have thought that several shared features of the skull point to a common ancestry of Homo and Paranthropus to the exclusion of Aus. africanus (see also below). If Aus. afarensis is eventually determined to have been (close to) the common ancestor of both Paranthropus and Homo (plus Aus. africanus?), then it will probably be wise to provide a new genus name for afarensis (and also anamensis), but for the moment all three species are retained in the genus Australopithecus.
The Rise of Homo
The only other genus of the Hominini is Homo, true humans, into which all later forms are placed. To summarize in advance, in this essay we suggest that after the brief occurrence of two or three early species of Homo in Africa, H. erectus evolved about 2 Ma and soon spread into Eurasia east of continental Europe; its far-flung populations were connected by periodic population movements spreading genetic modifications and keeping the species relatively unified. Homo erectus persisted, at least in eastern Asia, until 0.2 Ma or later, but farther west around 1 Ma, a geographically restricted species arose near the Mediterranean, with possible representatives in southern Europe and northern Africa. This new species in turn gave rise to at least two major lineages: one in Europe led to the Neanderthals, while a second in Africa continued with little change for half a million years. By about 200 Ka (Ka = thousand years ago) our own species Homo sapiens arose in Africa and slowly spread across that continent and into Eurasia, where it eventually replaced Neanderthals and all other human forms.
Early Homo
The identification of the earliest specimens of Homo is a subject of debate among paleoanthropologists. In the late 1970s the scientific pendulum had swung back to an idea proposed on less secure grounds by L. S. B. Leakey and colleagues in 1964. They named the species H. habilis, based on several finds from Olduvai. Especially significant was the discovery of the remains of a juvenile’s lower jaw, with teeth much like those of Aus. africanus, and its partial skull, with an estimated cranial capacity of about 685 ml, dated about 1.8 Ma. After much argument over the “reality” and distinctiveness of the new species, it was made clear from additional finds at Olduvai, Lake Turkana, and probably a younger level at Sterkfontein that a relatively small-brained (510-700 ml) and small-toothed Homo was present during 2.0-1.5 Ma. This was thought to be younger than Australopithecus, older than (most) H. erectus, and contemporaneous with P. boisei.A partial skeleton discovered at Olduvai in 1986 has similar teeth and an estimated body size comparable to Lucy; this small size was reported as remarkable but should have been expected, given the similarity in skull size to those of the smaller australopiths.
Several fossils, especially from the Lake Turkana region, appeared to represent a different “morph” or structural pattern. These were typified by skull KNM-ER 1470 (its catalog number in the Kenya National Museum) which has a relatively flat but protruding face, a brain size of about 750 ml, a high rounded vault, and probably large teeth (the crowns are broken off). This (and a few more fragmentary specimens) were at first suggested to represent male individuals, while the smaller individuals were considered females of the same species. Leg bones suggesting a size of 4 ft, 9 in. (155 cm) and 110 lb (50 kg) were originally thought to go with these cranial parts, but it later became clear that early H. erectus overlapped in time with the more archaic fossils and had comparable leg bones. Unassociated postcranial elements have had to be set aside pending the recovery of more bones clearly linked to identifiable crania.
One Species or Two?
These two sets of early human fossils pose an active problem in paleoanthropology: either H. habilis had greater sexual dimorphism (especially in brain size) than any known primate, or two (rather homogeneous) species were masquerading under a single name, with the smaller set (the supposed females) most similar in facial morphology to both Aus. africanus and later Homo. There are at least two opposing solutions to this problem. One view holds that all the known specimens from Olduvai, the Turkana Basin, and South Africa represent the single species H. habilis, a larger-brained and bigger creature than Aus. africanus, but one with essentially the same dental apparatus—another example of mosaic evolution. It would have had as much sexual dimorphism as did Aus. afarensis, perhaps at least as much as in modern gorillas or orangutans, in both form and size of teeth, face, and brain.
Another suggestion is that almost all of the Olduvai fossils, the smaller Turkana region specimens and some from South Africa, are recognized as H. habilis, while the 1470 specimen and other larger (non-Paranthropus) individuals from Turkana are considered as H. rudolfensis. This two-species view is accepted here. In 1992 a partial mandible was found at Uraha in Malawi that is extremely similar to a Turkana H. rudolfensis specimen. Not only is Malawi geographically intermediate between the known eastern and southern African site regions, but preliminary age estimates based on associated fossil mammals place the find at about 2.5-2.3 Ma, making it one of the oldest representatives of the genus Homo.A temporal bone (the lower middle part of the cranium, where the mandible hinges and the ear is housed) of similar age from central Kenya might also represent this species, and some authorities have even suggested that a few Olduvai specimens belong here as well. One new fossil (the lower part of a face with teeth) described in 2003 from the western part of Olduvai shares certain similarities with both the 1470 fossil and the Olduvai lower jaw first included in H. habilis. The describers of this new specimen suggested that the name H. habilis would thus best be applied to the larger-sized group (including what had previously been called H. rudolfensis), requiring a new name for the remaining smaller fossils; the jury is still out. To further complicate matters, some researchers have suggested that 1470 and its closest relatives might be linked to another Kenyan cranium found in 2001 and dated close to 3.5 Ma. Termed Kenyanthropus platyops (“flat-faced Kenya man”), this fossil is somewhat “squashed” and its surface bone broken into many small, slightly separated pieces. It might represent a new lineage of Pliocene hominins; that lineage might include 1470, renamed as K. rudolfensis; or the new fossil might just be a badly damaged representative of a known species, such as A. afarensis. For the moment, it seems wisest to accept just two named early species of Homo, with other options awaiting stronger supporting evidence.
Although australopiths were habitually bipedal, limb bones questionably attributed to H. rudolfensis suggest a wider pelvis, perhaps to permit the birth of larger-headed (because larger-brained) infants, which resulted also in the leg bones themselves appearing more modern. It is this combination of apparently significant change in the two major human adaptations of locomotion and intelligence that leads most authors to classify these species as Homo, but some have placed both of them in Australopithecus. Those researchers take the position that the anatomy of H. habilis and H. rudolfensis suggests their “adaptive strategies” were more like that of Australopithecus than other Homo species. The problem with this argument is that most other studies support a stronger evolutionary relationship between H. habilis, H. rudolfensis, and the other members of the genus Homo, including H. erectus, Neanderthals and modern humans, than with Australopithecus. While the former idea has not taken hold widely in the paleoanthropological community, it does suggest that a fundamental shift in adaptive strategies took place after these early Homo species.
It is not clear which species of early Homo might have been ancestral in turn to H. erectus. Both early forms have been found in the 2.0-1.6 Ma time range in the Turkana Basin and probably Olduvai, where they overlapped with P. boisei and H. erectus. But neither H. habilis nor H. rudolfensis as known are morphologically very close to early H. erectus. The former species differs not only in small brain and body size but also in limb proportions, although its facial morphology is potentially acceptable in an ancestor of H. erectus. The brain of H. rudolfensis is large, as might be expected in an ancestor of H. erectus, but its teeth are large and distinctive, its face is unique, and its limb bones have not been satisfactorily identified. One wonders if there might not be an as yet undiscovered early species of Homo which combined smaller teeth and face with a larger brain. Perhaps the upper jaw from Makaamitalu (found in 1994 high in the Hadar sequence) dated about 2.35 Ma might be a step toward fulfilling this prediction.
Early Homo Technology
Evidence of archeological activity has been found with or near some of these early Homo specimens, mostly flakes and choppers or “pebble tools” of the Oldowan stone-tool industry (see table), and also the remains of small animal prey. It is not possible to tell which types of humans made the tools, but at present the oldest evidence is from Ethiopia, in the Gona region (near Hadar and Aramis) and the Omo valley, about 2.5 Ma. The close time and space concordance of Oldowan tools and early Homo suggests that at least one of these species was the actual toolmaker; whichever it was may have been an active hunter of small game, collector of plant and insect food items, and scavenger of larger mammals.
Homo Erectus
While H. habilis and H. rudolfensis apparently were short-lived and relatively rare African species, their likely successor, H. erectus, was common, widespread, and long-surviving. The first fossils were found in Java in 1893 and termed Pithecanthropus erectus. Most of the later finds in China and across Africa were given distinctive generic and specific names, but all are now usually considered local variants or subspecies of the single species H. erectus. Some scholars still recognize additional species, especially H. ergaster for the earlier East African populations, but here only one species is accepted. The major anatomical characteristics of this form are the following: a body of nearly modern form and proportions below the neck, topped by a low and slightly elongated skull with cranial capacity averaging 1100 ml (with a range of about 800-1300 ml), smaller teeth in a less projecting face than Aus. africanus or H. rudolfensis, large solid brow ridges, thick cranial bones, and no chin.
Old World archeological variants before 10,000 years ago
| Technological mode | Description | Paleolithic subdivisions | Selected regional industries* |
| *Ka = thousand years ago; Ma = million years ago. | |||
| 4 | Blade cores, to produce scrapers, points, burins; bone harpoons, art | Late (Upper) Paleolithic | Later Stone Age [sub-Saharan Africa, 40 to < 5Ka (mode 5, microlithic)] Upper Paleolithic of Europe, including Perigordian Solutrean, Magdalenian Aurignacian (Europe, SW Asia?, > 40 to 27 Ka) |
| 3 | Widespread use of prepared cores to obtain variety of flake forms, used as scrapers and projectile points | Middle Paleolithic | Chatelperronian (western Europe, 36-30 Ka) Aterian (northern Africa, 100 to 30 Ka) Mousterian (many variants; western Eurasia and northwestern Africa, 200 to 35 Ka) Middle Stone Age (many variants; sub-Saharan Africa and southern Asia, 250 to 40 Ka) |
| 2 | Large bifaces (handaxes and cleavers), also simple cores and retouched flakes | Early (Lower) Paleolithic | In late stages, use of prepared cores and wooden throwing spears Acheulean (Africa and southwestern Asia, 1.5 to < 0.2 Ma; eastern Asia, 0.8 to < 0.2 Ma; Europe and southern Asia, 0.5? to < 0.2 Ma) |
| 1 | Simple cores and “casual” flakes | Early (Lower) Paleolithic | Zhoukoudian (China, 1 to 0.25 Ma) Various (Europe, > 1to < 0.4 Ma) Oldowan (Africa, 2.5 to < 1.5 Ma) |
African Populations
The earliest specimens are probably from East Africa, dating to as much as 1.9 Ma. These populations might have continued to make simple Oldowan artifacts, as no other form of stone tools is known at this time. There is evidence of the use of fire as early as 1.8 Ma and in scattered sites thereafter. Acheulean bifaces (handaxes and cleavers) appear by 1.5 Ma, though only rarely in direct association with H. erectus fossils; evidence for group hunting of large animals is rarer still and has been questioned by some scholars. In addition, the eventual extinction of P. boisei in this time interval may have been caused by direct or indirect competition with the more advanced H. erectus.
The most important African H. erectus find of recent decades is a mostly complete skeleton of an adolescent male (its sex judged by pelvic bones as well as by relative cranial robusticity), recovered on the west side of Lake Turkana in 1984. Judging by the pattern of dental eruption, he was probably 9-10 years old at death, although an age of 11-12 has also been suggested. One still ambiguously answered question is whether this species underwent the adolescent growth spurt which occurs in modern children of ages 12-14. Using formulas based on limb bone lengths of modern human adults and children, it was estimated that the “Turkana boy” might have reached an adult height of nearly 6 ft (183 cm) and a body weight of 150 lb (68 kg). Moreover, his body shape would have been slender or elongate, much as seen in living people of the same region, as a result of similar adaptation to a warm and dry climate. The development of brow ridges and other cranial structures in the adolescent male and a larger skull from Olduvai contrasts markedly with other Turkana skulls, indicating the persistence of strong sexual dimorphism.
A number of interesting new fossils thought to be H. erectus were found in East Africa in 1995-2002. A relatively complete cranium without a face from Lake Turkana and cranial fragments from another Kenyan site represent quite small individuals. The latter was found in the same stratigraphic layer as many bifacial Acheulean tools, good evidence that they were indeed the toolmakers. The first potential H. erectus fossils from Ethiopia and Eritrea have also been recently described, both from around 1 Ma. Measurements of the Ethiopian fossil indicate its similarity to both African and Asian H. erectus fossils. The Eritrean fossil, on the other hand, shows an increase in width of the back of the skull that is also seen in modern humans. Together, these new fossils extend the known variation in African H. erectus considerably.
Eurasian Representatives
Homo erectus (perhaps as a result of increasing population size) spread from Africa into Eurasia through the Middle East, perhaps earlier than has previously been thought. New dates for some long-known Javanese fossils suggested ages around 1.7 Ma, but these have been questioned. The most exciting new finds have come from the site of Dmanisi (Georgia), mainly from 1999 to 2005. There are currently five crania, four mandibles and a large number of postcranial elements along with Mode 1 (Oldowan-like) tools and animal bones dated to 1.8-1.6 Ma. Mode 1 and early Mode 2 artifacts from Israel (as yet not definitively found with human fossils) are slightly younger. Thus, H. erectus in the broad sense accepted here must have been the first human species to leave Africa in large numbers, probably about 1.8 Ma, soon after it first appeared in the fossil record.
Fossils of this species may extend in mainland Asia to nearly 200,000 years ago, mostly associated with fauna from the warmer intervals in this time of alternating glacial climate. From central Java, Indonesia, come a series of cranial and dental remains spanning from perhaps 1.7 Ma to possibly 30 Ka. The earlier range of time yielded several groups of skulls known popularly as Java man, including the first found members of this species; these specimens (and also those from China) tend to be more derived or extreme in their morphology than most African specimens, including the common presence of a thickened “keel” of bone down the midline of the braincase and a more projecting browridge. A dozen younger partial skulls from Ngandong, Java were long thought to be quite recent in age and once were placed in H. sapiens, but later study indicated close similarity to earlier Indonesian H. erectus, as they are now classified. Their age remains uncertain, with some evidence suggesting a range between 400 and 100 Ka, while a dating attempt in 1996 arrived at the surprisingly young age of 50-30 Ka, which has been questioned on several grounds. If this is indeed a correct date for these fossils, this would be the first demonstration of the coexistence of H. erectus with anatomically modern humans, known elsewhere in Indonesia and Australia at that date. Homo erectus specimens continue to be recovered from Java, and one partial cranium was even recognized in 1999 at a New York City shop selling natural history specimens. It was determined that it had been found in Indonesia some years earlier and illegally removed from that country; the shop’s owner returned it to Indonesian authorities.
A fascinating find was made in 2003 on the Indonesian island of Flores in a cave called Liang Bua, which has an archaeological record preserved from 200 Ka to the present. The fossils found there, including a well-preserved skull, additional lower jaws, and postcranial bones, have been dated to about 18 Ka. Dubbed “Hobbits” in the popular press, the remains are those of a small-brained (380 ml) and small-bodied (3 ft 3 in. or 1 m) hominin with a skull that resembles that of a small H. erectus. This led the discovers to conclude that a population of H. erectus somehow reached the island over water and became “dwarfed” after isolation, a phenomenon observed in other large mammals on islands; they named the fossils H. floresiensis. Other scientists have suggested that the remains belonged to pathological modern humans, which seems unlikely given the number of individuals of differing ages recovered. Further finds published in late 2005 highlighted the long arms and other anomalous postcranial features of this species, which are difficult to explain.
The first Chinese H. erectus, once called Peking man, was found at Zhoukoudian, near Beijing, where they occupied a large cave during most of the period between 500 and 250 Ka. Additional fossils of this form are known from Yuanmou in the southwest, Lantian and Yunxian in the center, and Hexian and Tongshan (Nanjing) in eastern China; some may be as old as 1-1.2 Ma. Archeological remains found with these fossils are mainly of the simple Mode 1 variety (no stone tools are unambiguously known from layers with human fossils in Java). However, more complex bifacial tools have been found in the Bose Basin of southern China, from about 0.8 Ma. These tools are comparable in overall form and complexity to Acheulian (Mode 2) tools found in Africa at this time and predate Mode 2 tools in Europe. This contradicts the long held hypothesis that only simple Mode 1 tools were made in East Asia while more complex Acheulean tools were restricted to Africa and Europe at this time. Although there have been claims, no definite H. erectus fossils are yet known from Europe, nor are archeological remains or more modern humans unambiguously documented there older than about 800,000 years. The Dmanisi specimens from Georgia are the most ancient human representatives in the region, but the lack of further remains over the succeeding million years may indicate that Homo required further physical or cultural adaptations in order to survive in Europe proper.
Middle Pleistocene Homo
It has been suggested that the increased rigor of the glacial climate in Europe at this time was the impetus leading to the evolution of humans who seem to be physically more modern in several ways than Afro-Asian H. erectus. These people were often termed early or archaic H. sapiens, or sometimes placed in their own species, H. heidelbergensis. This view was acceptable so long as the most ancient African representatives of this group were poorly dated or younger, but studies in the later 1990s suggested some modification. The earliest human fossils in Europe were long thought to date to about 500 Ka in England (a tibia from Boxgrove) and Germany (the mandible from Mauer near Heidelberg found in 1908). The long-known human fossils from Tighenif (previously called Ternifine, in Algeria), dated to 800-700 Ka, were transferred out of H. erectus by some workers because they present at least one derived feature of the lower jaw. Moreover, the partial skull from Bodo (Ethiopia) was also dated older than 600 Ka, and similar South African fossils were estimated to be of comparable age. These dates suggested that the earliest representatives of these more derived humans may have lived in the northern half of Africa and perhaps evolved there from local H. erectus populations early in the Middle Pleistocene (780-125 Ka).
Then, in 1995, fragmentary human fossils and associated stone tools from the older levels at Atapuerca (Spain) were dated to about 800 Ka, implying that archaic H. sapiens appeared at about this date all over the western Old World. These specimens were named H. antecessor in 1997, and it was suggested that they represented the common ancestor of all later human varieties. Meanwhile, in 1996, a cranium lacking the face was described from Ceprano, Italy, in a context suggestive of a date of about 700 Ka or more. However, this date is not particularly secure. A revised reconstruction claims strong similarity to H. erectus, otherwise unknown in Europe. In late 2005, stone tools (of both Mode 1 and Mode 2 types) and animal fossils were described from Pakefield in eastern Britain, a site estimated to date to 700 Ka; no human fossils are yet known, but they are eagerly awaited.
All of these finds, combined with theoretical arguments about the best way to recognize and delimit species in the fossil record, have led to competing interpretations of the number of species of Homo known in the past million years. Some workers continue to place all post-erectus fossils in “archaic Homo sapiens,” sometimes recognizing a variety of temporal and geographic subspecies (such as the Neanderthals and anatomically modern humans). Afew have gone so far as to include H. erectus within an over-enlarged H. sapiens. At the other extreme, some researchers accept between five and seven species in the same time period: H. antecessor, H. cepranensis (for Ceprano) H. heidelbergensis (either restricted to Europe or extended to Africa and even East Asia), H. rhodesiensis (for African Middle Pleistocene populations), H. neanderthalensis, H. sapiens (restricted to anatomically modern humans), and perhaps others. A possible middle ground would be to (1) include the earliest of these African and European populations in one named group [for example, antecessor, or perhaps mauritanicus (the name originally given to the Tighenif fossils), as suggested by J.-J. Hublin]; (2) combine all post-500 Ka nonmodern European fossils in neanderthalensis (including heidelbergensis here); (3) group nonmodern African fossils younger than Tighenif in rhodesiensis; and (4) restrict sapiens to anatomically modern humans worldwide. These “groups” could be given specific or subspecific status, depending on the theoretical model used. Recent work discussed below supports full species status for Neanderthals based on several lines of evidence, and all four groups are here interpreted as full species. While both fossil and genetic evidence supports an African origin for modern humans, the evolutionary relationships among all of these groups remains unclear.
Homo rhodensis and early representatives of H. neanderthalensis occur in Africa and Europe, respectively, between 650 and 250 Ka, thus contemporaneous with H. erectus populations in eastern Asia. They share somewhat larger brains (for body size), smaller teeth, more expanded facial sinuses and occiput (rear of the skull), but less robustness than in H. erectus. All these features are found in more extreme form in modern humans and in the late “classic” Neanderthals (see below). In most areas, these people still used Acheulean tools, but perhaps with greater efficiency. It is likely that these “intermediate-grade” humans spread gradually eastward across the Old World, replacing late-surviving populations of H. erectus everywhere by 200 Ka. These geographic variants were not only distinct from H. erectus but also from each other to a greater degree than is true among living varieties or “races” of anatomically modern humans. In southern Africa, one cranium was found at Broken Hill, now Kabwe, Zambia (formerly Northern Rhodesia, hence the name Rhodesian man), and broadly similar specimens are known in South Africa (Saldanha and Florisbad), Tanzania (Ndutu), Ethiopia (Bodo), and Morocco (Salé and Thomas quarries). These people made Acheulean or equivalent Mode 2 tools and apparently hunted big game across most of Africa. In early 2006, a new partial cranium probably belonging to this species from Gawis (Ethiopia, near the Gona Pliocene tool site) was reported by Ethiopian and U.S. researchers led by Sileshi Semaw. Its age is as yet uncertain but probably in the 500-250 Ka interval.
Rare specimens from China appear to be younger, mainly dating to about 250-150 Ka. These include a nearly complete cranium from Dali, in central China, and a partial skull and skeleton from Jinniushan (or Yingkou), in the northeast, as well as scattered, less complete remains. All of these fossils, especially Dali, are broadly similar to the African specimens just mentioned, as well as to some of the earliest European H. neanderthalensis. Other Chinese specimens, such as Maba (from the southeast), and the central Indian Hathnora (or Narmada) fossil, are partial crania which are both younger (perhaps about 150-75 Ka) and more derived morphologically, although not in the direction of either early anatomically modern people or the contemporaneous Neanderthals.
Neanderthals
The best known of the archaic varieties are the Neanderthals, from Europe and western Asia. It now seems likely that this group evolved locally in Europe from early Middle Pleistocene Homo via intermediate populations previously called Homo heidelbergensis such as those known from England (Swanscombe, Boxgrove), Spain (Atapuerca), France (Arago, Montmaurin), Germany (Mauer, Steinheim), and Greece (Petralona). They became adapted to the cold climates of glaciated Europe, with prototypical Neanderthal anatomy well established by about 200 Ka. During the warm interval about 120 Ka, they may have spread into the Near East and central Asia. In the cold glacial phase between 110 and 35 Ka, “classic” (or extreme), cold-adapted Neanderthals were abundant in cold northern parts of western and central Europe, while less extreme forms (perhaps more like their immediate predecessors) inhabited areas to the south and east. They were essentially stocky humans, but had long, low skulls with a projecting occipital region, large faces, teeth, and brow ridges; and brains averaging 1500 ml in volume. Their limbs and trunks were heavily muscled, indicating great strength, but many bones were broken and healed during life. They made Mousterian tools (a variant of Middle Paleolithic or Mode 3 flake-based tool kits), often lived in caves or wooden shelters where they controlled fire, hunted big game, and had primitive religious beliefs, including burial of the dead with grave goods. There is intense argument among paleoanthropologists as to how modern the Neanderthals were behaviorally, in terms of their stoneworking and hunting techniques and modes of foraging, whether planned or merely ad hoc. For example, the frequent broken bones may have resulted from hunting large game at close proximity, rather than using projectile technology from a distance. Such controversies feed back into the question of whether the Neanderthals are a distinct species or a distinctive subspecies of H. sapiens. A related question is whether the Neanderthals were in any way ancestral to anatomically modern humans, especially of Europe.
Recognition of a separate Neanderthal species implies an almost absolute reproductive isolation and lack of genealogical continuity, while the opposite is true for most interpretations of Neanderthals as members of H. sapiens. The interpretation accepted here is that H. neanderthalensis is considered to have been geographically and culturally isolated from early anatomically modern humans and their ancestors, two independent but closely related lineages (species) evolving in parallel until they finally met, after which the former group soon became extinct. Indeed, current research suggests that differences in the skulls between Neanderthals and modern humans appeared early in individual growth, meaning that Neanderthal children looked quite different from modern human children. Neanderthals also differ in developing more rapidly than modern humans. In other words, they achieved their adult anatomy faster than modern humans do. In addition, work by K. Harvati and colleagues published in 2004 shows that the differences in skull anatomy between Neanderthals fossils and modern humans are at the level of separate species when compared to other primates. Furthermore, when scientists extracted mitochondrial DNA from the bones of several Neanderthal fossils they found that Neanderthals have some unique sequences of DNA when compared to modern humans. Neanderthals, possibly due to local climatic differences, were probably isolated from other hominins and as a result became quite distinct in their development as well as their skull shape compared to modern humans.
Spread of Modern Humans
One of the major foci of recent paleoanthropological research is the clarification of the area of origin and the early history of modern humans, H. sapiens. The skull of this form is characterized by a small, nonprotruding face; small teeth and brow ridges; a chin; and a high, rounded braincase. There are no specimens of this type known (or even hinted at) anywhere in the world earlier than about 200-150 Ka. New fossils representing early modern humans were recovered from Herto, Ethiopia, in 1997 (published 2003) and dated 164-150 Ka. Aside from having larger faces than recent humans, these are quite modern looking. Moreover, in 2005, the long-known fossils from Kibish (Omo Valley, Ethiopia) were dated close to 200 Ka. This confirms previous suggestions that about 200-100 Ka some fossils from eastern and southern Africa (for example, one from Kibish and one from a late horizon at Laetoli) suggest the persistence of a “Rhodesian-like” morphology, while others (for example, another from Kibish and several from Djebel Irhoud, Morocco) appear to be nearly modern. Two somewhat younger sites in South Africa have produced important additional modern human fossils. At Border Cave, a partial cranium and other fragments may date to nearly 90 Ka; they are clearly modern in form, but their date is questionable. The Klasies River Mouth caves, on the southern coast, have yielded a sequence of layers with good dates and archeological context; the human remains dated about 100 Ka are scrappy but appear modern, with a chin, small brow ridges, and overall gracility. Such gracilization, especially of the face, is a major feature of the evolution of (and within) Homo sapiens. In combination, these remains and other, less complete fossils indicate that early moderns were living in sub-Saharan Africa by 150 Ka. Archeological remains dated 100-75 Ka in South Africa and D. R. Congo (ex-Zaire) indicate that at least some of these people were making Mode 3 (MSA) tool kits with elements (such as bone harpoons, engraved bone, and minerals and shell beadwork) that do not appear in Europe until after 30 Ka.
From such a possible sub-Saharan origin, anatomically modern H. sapiens appear to have spread across the Old World, differentiating into local races by 80-50 Ka. This view of human dispersal has received support from studies of the distribution pattern of human mitochondrial DNA haplotypes (variants) and other genetic evidence. The majority of these studies suggest that the major dichotomy in modern human population genetics is between Eurasians and Africans. Such results fit well with the fossil evidence for African versus Eurasian divergence about 100 Ka. Moreover, dates on early anatomically modern remains from Israel (Djebel Qafzeh and Skhul) documented the presence of the ancestors of Eurasians outside Africa by about 110-90 Ka. This is especially intriguing because most Israeli Neanderthals have been dated to about 65-45 Ka, which is significantly younger than the early moderns and implies that they may have overlapped in this region of the world. Even more complexity is implied by the strong similarity of Mousterian tool kits associated with such diverse human forms as the Neanderthals of Europe and Israel (and farther east), the early moderns from Israel, and the “premoderns” from Jebel Irhoud (Morocco), but the implications of this cultural similarity are as yet unclear.
Found in the late 1970s, the youngest known Neanderthal skull comes from southern France and is associated with tools of the Châtelperronian industry, a Mode 3 or 4 variety previously thought to have been made by H. sapiens. This specimen dated to 34 Ka has been alternatively interpreted as the maker of these tools (possibly after contact with Late Paleolithic moderns) or as evidence for direct Neanderthal ancestry of moderns. In some cases, more fossils do not answer questions but create new ones. Sites in southern Spain and Portugal have yielded less complete Neanderthal fossils and Mousterian tools dated about 30 Ka, after which modern H. sapiens was the sole form of human to be found anywhere. One reason for the success of H. sapiens may have been their greater tool-making efficiency, as documented by the Late (or Upper) Paleolithic Mode 4 blade-and-burin industries. These people included large quantities of worked bone in their tool kits (using chisel-like burins to carve and engrave the bone), constructed dwellings of wood or of already fossilized animal bone, hunted large game, fished with harpoons, and in general behaved much like their living descendants. In many parts of the world, they also wore personal ornamentation (jewelry) and engaged in artistic pursuits, including carving small animal statues and perhaps calendars, as well as painting on the walls of rock overhangs and deep caves.
In 1999, Portuguese and American paleoanthropologists described the remains of a 4-year-old child, buried near Lagar Velho, Portugal, about 25 Ka. The burial pattern and most of the child’s morphology indicated links to the Gravettian culture, made by early anatomically modern people elsewhere in Europe at this time. However, features of the lower leg bone (tibia) and lower jaw (the cranium was crushed and later reconstructed) suggested similarity to Neanderthals. The describers hypothesized originally and in a major publication in 2005 that this individual might have been the result of hybridization between Neanderthals and moderns, but other researchers argued that the morphology was not that different from what could be expected in a robust anatomically modern child and that hybridization would have resulted in features intermediate between Neanderthals and moderns, not clear features of each.
Many names have been given to early modern humans, especially in Europe, but these indicate only minor differences. The term Cro-Magnon derives from several skeletons found in 1868 in Les Eyzies, France. They gave their name to a “race” said to occur either just in France or across most of Europe. In fact, Cro-Magnon people were already essentially Europeans, while early Africans are known from sites in eastern and southern Africa. Australasia was colonized over water after about 70 Ka, with important finds at Keilor and Lake Mungo. New World Indians likely originated from Siberia, by means of crossing a land bridge over what is now the Bering Strait. Many human fossil remains are known in the Americas as far back as 12 Ka, but some dates as old as 35 Ka have been obtained on archeological sites, indicating that perhaps several crossings of the land bridge occurred.
In contrast to the above “Out of Africa” view of human dispersal (based on the idea that modern humans evolved in sub-Saharan Africa more than 100,000 years ago from populations of archaic Homo) accepted here, a minority view (the “Multi-regional” hypothesis) interprets the fossil record to document the nearly parallel origin of modern humans in different regions of the Old World from a H. erectus ancestry. Each regional variety is said to present morphological characteristics linking archaic to modern populations, while gene flow (migration and interbreeding) between regions kept the geographical varieties united in a single species at any one time. While most researchers reject a significant contribution from Neanderthals or H. erectus to the modern human gene pool, there may still have been some interbreeding among the species.