Hominins

Hominin Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Chordata

Subphylum: Vertebrata

Superclass: Tetrapoda

Class: Mammalia

Subclass: Theria

Infraclass: Eutheria

Order: Primates

Suborder: Haplorrhini

Superfamily: Hominoidea

Family: Hominidae

Subfamily: Homininae

Tribe: Hominini

Geographical location: Originally Africa, now worldwide

Habitat: Originally savannas, now spread to all habitats

Gestational period: Nine months

Life span: Now over one hundred years, with the average life expectancy at birth ranging from fifty to ninety years, depending on country

Special anatomy: Large, well-developed brain; upright, bipedal posture; opposable thumbs; larynx, vocal chords, and tongue adapted to produce a wide variety of sounds for language

The idea that humankind might be significantly older than the six thousand years previously allotted by biblical scholars, who tried to calculate the generations since Adam and Eve, was not widely maintained until 1859, when human stone tools and the bones of extinct animals were found lying close to each other in the Somme valley in France. Charles Darwin’s On the Origin of Species appeared on November 1 of the same year, but it suggested only that “light will be thrown on the origin of man and his history” by the theory of evolution he had just proposed.

A series of important and widely influential books then followed, including J. Boucher de Perthes’ De l’homme antédiluvien et de ses œuvres (1860; of antediluvian man and his works); Thomas Henry Huxley’s Evidence as to Man’s Place in Nature (1863); and Charles Lyell’s Antiquity of Man (1863), with others later by John Lubbock, James Geikie, and W. Boyd Dawkins. In The Descent of Man (1871), Darwin sagely hypothesized that the human line evolved in Africa (not Asia, as had previously been assumed) from a long-tailed, probably arboreal, ancestor. Yet, in Darwin’s time only two fossil apes were known at all, together with some controversial bones of a creature known as Neanderthal man found in 1848. Extinct species such as the australopithecines and Homo erectus had not been discovered.

Humans and Other Primates

The scientific name for modern humans is Homo sapiens sapiens; the taxonomic family is the Hominidae, which also includes chimpanzees, bonobos, gorillas, and orangutans. Bone for bone, the skeletons of other great apes are almost identical to human skeletons. On other evidence as well, humans and the other great apes appear to be remarkably alike. The protein sequences in chimpanzee and human hemoglobin, for example, are identical; there are only two chemical differences between gorilla and human hemoglobin. Between humans and all other animals, there are more than two. Strands of deoxyribonucleic acid (DNA) from chimpanzees and humans, moreover, are 99 percent identical. Chimpanzees and gorillas, like humans, are also highly intelligent; their brains closely resemble those of humans. These remarkable similarities attest a common ancestry for all the Hominidae, as the classification itself would imply, and a fairly recent differentiation among its members. Some biochemists have argued that humans, chimpanzees, and gorillas shared a common ancestor no more than six or eight million years ago.

Whatever the timing may have been, it is almost universally accepted that the link between humans and their protosimian ancestors was a now extinct genus of ape, the australopithecines (southern apes). The first of these, called, originally, the Taung child, was discovered in South Africa by Robert Dart in 1924. Its identity and significance remained controversial until 1936, when further discoveries by Robert Bloom convinced skeptical professionals and the public.

The Australopithecines

The australopithecines arose at least four million years ago. They lasted until two million years ago, evolving into a series of species. Of these, Australopithecus afarensis (found in the Afar region of Ethiopia) was the oldest and smallest. Males stood no taller than four feet, and the females were smaller. Most significantly, however, afarensis was fully bipedal; unlike the apes, it walked upright, with increasingly specialized hands, and with legs a bit longer than arms. Chimpanzee-like hips, together with curved toe and finger bones, suggest that it was essentially a tree-dweller living on fruits and seeds. A remarkably complete skeleton of afarensis, familiarly called Lucy, was found in 1974; it is about three million years old and is the oldest hominin skeleton yet found. Afarensis died out around 2.5 million years ago; it is thought to be ancestral to the later australopithecines and to modern humans.

Australopithecus africanus, deriving from Africa some three to one million years ago, probably evolved from afarensis. Most specimens come from Sterkfontein in South Africa, though others have been found in Ethiopia, Kenya, and Tanzania. This species was about the same size as afarensis, but it had a less apelike face. The arms were proportionately longer than a modern human’s, yet shorter than those of afarensis; hands and teeth show similar “modernization.” Dart’s Taung child was the first (and is still the most famous) example of this species.

Australopithecus robustus (robust southern ape), found in South African caves, was once thought identical to Australopithecus boisei; in older literature, it was also known as Paranthropus (past man). Larger and more strongly built than africanus, robustus was more than a foot taller and had a larger brain. His teeth indicate that robustus was a plant eater. There is also a remarkable specimen (discovered by C. K. Brain at Swartkrans) of a child’s skullcap in which the imprint of a leopard’s lower canines can be seen. Since an exact-fit leopard’s jaw was found nearby, it is assumed that the leopard killed the child and was then itself killed by an adult robustus armed with some kind of weapon. (A diorama at the Transvaal Museum, Pretoria, reconstructs this hypothetical incident.) Australopithecus boisei—a famous discovery by Mary Leakey, named for the Leakeys’ sponsor, Charles Boise—called Zinjanthropus, was even bigger than robustus and lived at the same time, though in East Africa. A boisei skull discovered in 1985 in Kenya proved not only to be particularly massive but also considerably older (2.5 to 2.6 million years) than any known robustus specimen. Austra lopithecus boisei must therefore have been a separate and earlier species probably not descended from africanus. If so, then there was more than one australopithecine lineage, and the previously held idea that the australopithecines became increasingly robust through time must be reversed. As a result of this one find, there no longer are widely accepted ideas as to who gave rise to whom.

Throughout their history, the australopithecines manifest a regular progression from apelike characteristics to human ones. All the australopithecines walked upright, a fact that evidently encouraged increasing height, hand specialization, and brain development. Gradual changes in australopithecine dentition, moreover, suggest not only changing diet but revised habits as well. With advanced hands and evolving arms and shoulders, Australopithecus probably carried loads and used weapons, regardless of whether they were capable of making them. Though australopithecines may have done some hunting, they probably depended primarily upon foraging and scavenging—filching from leopard kills, for example.

The Evolution into Homo

At what point Australopithecus evolved into Homo is unclear, in part because the distinction between them is rather arbitrary. Though still regarded by some researchers as an advanced form of Australopithecus, Homo habilis (handy man), another Leakey family discovery, is otherwise usually accepted as the earliest member of a distinctly human line. Homo habilis was still only about five feet high (perhaps an optimum height for the environmental conditions), but had a larger brain, a rounder head, a less projecting face, advanced dentition, reduced jaws, and essentially modern feet. Stone artifacts have been found in close association with these remains; habilis, who lived between 2 and 1.5 million years ago, almost certainly made tools, hunted, built shelters, gathered plants, and scavenged. Homo habilis is assumed to be ancestral to Homo erectus and may have exterminated Australopithecus.

In 1859, when the prehistory of humankind was first broadly acknowledged, the only remains then known belonged to Neanderthal man (now called Homo sapiens neanderthalensis). Until 1924, when Australopithecus was discovered, all the intervening finds (excluding “Piltdown man,” a deliberately planted fake) have since been classified as varieties of Homo erectus (erect man—a designation assuming that its predecessors stooped). Of these, the two best-known are Java man, discovered in 1891 by Eugène Dubois in Java, and Peking man, found in China by Davison Black in 1926. Only after a number of specimens had accumulated was it realized that Java man and Peking man were examples of the same species. An exceptionally complete Homo erectus skeleton was discovered in Kenya in 1984 and dated at 1.6 million years old. Overall, Homo erectus lived from some time before 1.6 million years ago to as recently as two hundred thousand years ago. Homo erectus probably evolved in Africa but migrated from there to Europe and the Pacific shores of Asia. This species was as tall as modern humans, but more robust overall, with a noticeably thicker, somewhat “old-fashioned” skull that still included prominent brow ridges and a sloping forehead. Homo erectus had large, projecting jaws, no chin, and a brain and teeth that were larger than that of modern humans. Homo erectus was not only widespread in distribution, but also showed considerable regional variation. The success of Homo erectus as a colonizer was attributable in large part to their intelligence, which was manifested in standardized but increasingly sophisticated toolkits, big-game hunting (almost certainly cooperative), the use of fire, and advanced housing. Homo erectus lived during the Pleistocene, or glacial, epoch, and was probably stimulated to use creative abilities by the deteriorating environments sometimes encountered. Very late examples of Homo erectus are sometimes alternatively classified as Homo sapiens. Despite some continuing opposition, it is now usually accepted that Homo erectus gave rise to Homo sapiens.

While for some time it was believed that Homo erectus was the first hominin species to migrate outside of Africa, archaeological evidence gathered painstakingly since the beginning of the 2000s led to an announcement in 2018 that hominins traveled the long distance to Asia earlier than once thought—and they may not have been Homo erectus. Over the span of more than a decade, researchers excavated at the Shangchen site of China's Loess Plateau in Lantian county and unearthed close to one hundred stone tools that they dated to approximately 2.1 million years ago. These findings served as further proof that work still needs to be done to better understand the timeline of human evolution.

Searching for the Fossil Remains

Considering the efforts that have been made to find them, hominin fossils are remarkably few. This is the case for three main reasons: First, the early hominins (unlike modern humans) did not exist in huge numbers; second, the majority of their bones were not preserved as fossils; and third, it is certain that only a small proportion of the hominin fossils that do exist have been found.

The hominin line evolved in Africa, and its earlier members (including the australopithecines and Homo habilis) have been found only there. Homo erectus was both more widespread and more numerous, but none of these types practiced ritual burials (Neanderthals were the first to do that), so the most usual agent of presentation was some sort of nonhuman carnivore. Predators, such as the large cats, might actually have hunted the early hominins; in any case, they certainly scavenged hominin carcasses. Hyenas and other cleanup animals then grabbed what they could, taking the leftover pieces to their dens in limestone caves. (There must be some truth to this scenario, because leopards and hyenas have left their toothmarks on australopithecine bones.) The gnawed bones, now thoroughly disarticulated, were scattered about the cave—and eventually solidified by limy deposits into a bone breccia.

When investigators find such embedded hominin bones, or suspect their presence, they collect chunks of the breccia and dissolve the limy matrix with acetic acid, a procedure that does no harm to fossil bones. It is unlikely that any of the latter will be whole. Once the fragmented bones have been freed from the matrix, they are cleaned, preserved, sorted by type, and tallied. Whether big or small, routine or not, each must be identified. By far the great majority of the bones will belong to antelope of various kinds; less than one in a thousand, normally, proves to be hominin.

Such procedures are standard when dealing with cave deposits at the famous South African australopithecine sites of Sterkfontein, Krom draai, and Swartkrans, all of which are adjacent to each other and to Pretoria. At Olduvai Gorge in Tanzania, where the Leakeys and others have found both australopithecine and Homo habilis remains, the geology is entirely different. Here, the erosive power of a now-defunct river has exposed primarily volcanic sediments that once bordered a shifting saline lake. The disadvantage of this site is that it lacked any obvious place for the location of bones; years of determined effort were required to locate productive sites. The advantages of the site were that the presence of early humans was virtually assured because stone tools were scattered about plentifully (whole campsites were eventually found); the bones involved had not been dragged about and disarticulated by animals; and the involved stratigraphy made fairly precise dating at least theoretically possible. Still other sites have provided additional unique information.

The collection, preservation, and interpretation of hominin fossil bones is very much a multidisciplinary effort. In particular, detailed geological understanding of the site is essential. Only through stratigraphical analysis, usually, can the age and situation of the discovered fossil be understood. Stratigraphy aside, certain rocks can also be dated according to the radioactive elements they contain. More often, bones can be dated approximately because they occur in association with a particular assemblage of animal bones, the animal species themselves being of reliable short-term ages. In some cases, pollen samples have been of use. All this additional information, together with comparative anatomical analysis, helps to give hominin fossils a defensible identity.

The Search for Ancestors and Origins

Throughout human history, various cultures have asserted deeply meaningful identities by attributing their present being to a particular origin. Most of these genealogies and origin myths relied upon some divine agency to explain human existence. From a surprisingly early time, however, civilized humans (such as the Greeks) recognized that there had been a time when humankind did not know the use of metals. Subsequently, many thinkers took the concept of cultural evolution for granted.

By the seventeenth century CE, anatomy had become a popular field of study. Comparisons soon established how like human anatomy that of the higher primates was. By the mid-eighteenth century, Carolus Linnaeus, the originator of modern biological classification, even ventured to place man and the apes within the same family. Yet this classification did not imply any necessary common ancestry. Linneaus and others of his time created the notion that individual species arose through a special, divine plan. The idea of special creation lost credibility when the fact of extinction became established at the end of the eighteenth century and as the diversity of species and varieties came increasingly to be appreciated. Nature, moreover, was no longer seen to be a benign reflection of its creator. As opinions of a distinctly human nature likewise declined, the realization that humans are animals encountered lessening resistance.

Before human evolution was generally accepted during the twentieth century, the evolution of cultures, language, law, institutions, and at least some animals had already been established. Though the idea was there, reliable evidence for the biological evolution of humans remained elusive. Before 1891, the only known prehistoric human bones belonged to Neanderthals; they were quickly and effectively dismissed as pathological freaks. Since Raymond Dart’s Taung child of 1924 was likewise dismissed with ridicule, only a few specimens of what would later be recognized as Homo erectus (Java man and Peking man) survived to satisfy the now fashionable quest for a “missing link.” (There are still innumerable missing links, but the essential connection between ancestral apes and man was confirmed by the discovery of australopithecines.) It was Robert Broom who, during the 1930s, established the reality of the australopithecines and, by implication, of human evolution.

Though the evolution of the hominin line is certainly a worthwhile scientific topic, it has always been regarded as much more than that, because the claims to ancestry define humanity. Yet the formal constraints of science are limited. In a remarkable series of thirty-nine papers (published between 1949 and 1965), for example, Raymond Dart promulgated an interpretation of the australopithecines as aggressive, predatory, and cannibalistic hunters. Because of the recurrent wars in which civilization had engaged during this period and shortly before, this image of human (or almost human) nature appealed to the popular imagination—so much so that less technical restatements of the same views by Robert Ardrey were not only commercially successful but also politically influential. Interpretations of australopithecine and early hominin behavior have subsequently changed, however—many now see these species as abject scavengers disputing the possession of already picked-over animal corpses with hyenas. It is arguable whether such changing interpretations are attributable to scientific advances or are the result of changing philosophical views of humanity.

Principal Terms

apes: large, tailless, semierect anthropoid primates, including chimpanzees, gorillas, gibbons, and orangutans, and their direct ancestors—but excluding man and his ancestors

australopithecines: nonhuman hominins, commonly regarded as ancestral to present-day humans

dryopithecines: extinct Miocene-Pliocene apes (sometimes including Proconsul, from Africa) found in Europe and Asia; their evolutionary significance is unclear

humans: hominins of the genus Homo, whether Homo sapiens sapiens (to which all varieties of modern man belong), earlier forms of Homo sapiens, or such presumably related types as Homo erectus and the still earlier (and more problematic) Homo habilis

primates: placental mammals, primarily arboreal, of the suborders Strepsirrhini (lemurs, galagos, and lorisids) and Haplorhini (tarsiers, monkeys, and apes, including humans)

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