All prehistoric skeletal remains of humans which are archeologically earlier than 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.
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.
The oldest certain representatives of the Catarrhini are fossils from the Fayum beds of northern Egypt dated around 34 million years ago (Ma). The best known is Propliopithecus (=Aegyptopithecus) zeuxis, a species near the common ancestor of apes, humans, and Old World monkeys (see illustration). Hominoid evolution took place only in Africa in the late Oligocene and early Miocene (26–17 Ma). See also Fossil apes; Fossil primates; Molecular anthropology; Monkey; Primates.
Human phylogeny from the Oligocene to the present time, showing the skulls of the major known fossil relatives and possible ancestors of modern humans.
Pliocene Homininae
Australopithecus, the first truly humanlike beings, appear in the fossil record in quantity some 4.5 Ma, during the early Pliocene. Pliocene humans have been grouped in various ways, but it now seems that four main types can be distinguished. Three of these, dating from 4.5–1 Ma, have often been assigned to the genus Australopithecus and can informally be termed australopiths; the fourth group includes early species of Homo, beginning about 2.5 Ma. All australopith species appear to share a number of basic characteristics distinguishing them from living and fossil apes and also from later humans, although clearly linking them to the latter. Many researchers are coming to accept a division into at least the two genera Australopithecus and Paranthropus, and one species has been placed in its own genus, Ardipithecus, but others continue to recognize only a single genus, Australopithecus. See also Australopithecine.
The oldest known probable human species, Ardipithecus ramidus, is based upon a small group of fossils found at the Aramis locality in the Middle Awash Valley, Ethiopia. The remains of Ard. ramidus described so far document a mosaic pattern combining features similar to those of younger humans with others reflecting retention of apelike conditions. The remaining components of the Ard. ramidus mosaic are all reasonably interpreted as ancestral conditions, to be expected in an ancient human ancestor. Slightly younger hominin fossils named Aus. anamensis have been found since 1994 at sites in the southern Lake Turkana region of Kenya.
Fossils from sites in Ethiopia and Tanzania reveal far more details about a still younger species, Aus. afarensis. 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. 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. Arguments as to the priority of brain expansion or walking ability in human evolution thus have yet to be resolved. Australopithecus afarensis combines both of these advanced, human characteristics with numerous other features reminiscent of later Miocene hominids and modern apes.
Early Homo
The only other genus of the Hominini is Homo, true humans, into which all later forms are placed. 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. It was made clear from additional finds at Olduvai, Lake Turkana, and probably an upper level at Sterkfontein that a relatively small-brained (510–700 ml) and small-toothed Homo was present in the 2.0–1.5 Ma time period.
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 brain size of about 750 ml, a high rounded vault and probably large teeth but a relatively protruding face. It has been suggested that the known variation in brain size and other aspects of craniofacial morphology is too great to represent merely the sexes of even a strongly dimorphic species. 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 gaining adherents and is accepted here.
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. Each 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.
The earliest specimens are probably from East Africa, dating to as much as 1.9 Ma. Homo erectus (presumably as a result of increasing population size) spread into Eurasia through the Middle East, perhaps earlier than has previously been thought. Nonetheless, H. erectus must have been the first human species to leave Africa in large numbers. Fossils of this species may extend in Asia to nearly 200,000 years ago, mostly associated with fauna from the warmer intervals in this time of alternating glacial climate. The first Chinese H. erectus, called Beijing (Peking) man, was found at Zhoukoudian, near Beijing, where they occupied a large cave during most of the period between 500 and 250 Ka. 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 as older than about 800,000 years.
Premodern Homo sapiens
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 are often termed early or archaic H. sapiens, or sometimes placed in their own species, H. heidelbergensis. There are 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). However, some researchers accept between three and six species in the same time period: H. antecessor, H. heidelbergensis (either restricted to Europe or extended to Africa and even East Asia), H. rhodesiensis (for early African “archaics”), H. neanderthalensis, H. sapiens (restricted to anatomically modern humans), and perhaps others.
Early representatives of H. s. neanderthalensis and H. s. rhodesiensis occur in Europe and Africa between 500 (or even 600) and 250 Ka, thus contemporaneous with H. erectus populations in eastern Asia. It is likely that these archaic H. sapiens spread gradually eastward across the Old World, replacing late-surviving populations of the broadly ancestral H. erectus everywhere by 200 Ka, when a poorly known (and here unnamed) variant occurs in northern China. These three 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.
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 earliest H. sapiens via intermediate forms (“pre-Neanderthals” or “ante-Neanderthals”). 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. 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. Such controversies feed back into the question of whether the Neanderthals are a distinct species or, as accepted here, 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.
Spread of modern humans
One of the major foci of recent paleoanthropological research is the clarification of the area of origin and early history of anatomically modern humans, H. s. sapiens. The skull of this form is characterized by a small, upright face; small teeth and brow ridges; chin; and high, rounded braincase. There are no specimens of this type known (or even hinted at) anywhere in the world earlier than about 150 thousand years ago (Ka). But from about 150–100 Ka, in eastern and southern Africa, some fossils suggest the persistence of a “Rhodesian-like” morphology, while others are often considered to be nearly modern. Two somewhat younger sites in South Africa have produced the most important evidence. In combination, these remains and other, less complete fossils indicate that early moderns were living in sub-Saharan African by about 100 Ka. From such a possible sub-Saharan origin, anatomically modern H. s. sapiens may 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 deoxyribonucleic acid (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.
In contrast to the “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 Neanderthal populations), a minority view (the “Multiregional” 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 between regions kept the geographical varieties united in a single species at any one time. Most scholars reject the implication that Neanderthals, for example, were ancestral to modern Europeans, or Chinese H. erectus to modern north Asians. See also Early modern humans.
