Human evolution

Human evolution is the part of the theory of evolution by which human beings emerged as a distinct species. It is the subject of a broad scientific inquiry that seeks to understand and describe how this change and development occurred. The study of human evolution encompasses many scientific disciplines, most notably physical anthropology and genetics. The term "human", in the context of human evolution, refers to the genus Homo, but studies of human evolution usually include other hominins, such as the australopithecines.

History of paleoanthropology
The modern field of paleoanthropology began in the 19th century with the discovery of "Neanderthal man" (the eponymous skeleton was found in 1856, but there had been finds elsewhere since 1830), and with evidence of so-called cave men. The idea that humans are similar to certain great apes had been obvious to people for some time, but the idea of the biological evolution of species in general was not legitimized until after Charles Darwin published On the Origin of Species in 1859. Though Darwin's first book on evolution did not address the specific question of human evolution&mdash; "light will be thrown on the origin of man and his history," was all Darwin wrote on the subject&mdash; the implications of evolutionary theory were clear to contemporary readers. Debates between Thomas Huxley and Richard Owen focused on the idea of human evolution. Huxley convincingly illustrated many of the similarities and differences between humans and apes in his 1863 book Evidence as to Man's Place in Nature. By the time Darwin published his own book on the subject, Descent of Man, it was already a well-known interpretation of his theory&mdash; and the interpretation which made the theory highly controversial. Even many of Darwin's original supporters (such as Alfred Russel Wallace and Charles Lyell) balked at the idea that human beings could have evolved their apparently boundless mental capacities and moral sensibilities through natural selection.

Since the time of Carolus Linnaeus, the great apes were considered the closest relatives of human beings, based on morphological similarity. In the 19th century, it was speculated that our closest living relatives were chimpanzees and gorillas, and based on the natural range of these creatures, it was surmised humans share a common ancestor with African apes and that fossils of these ancestors would ultimately be found in Africa.

It was not until the 1920s that hominin fossils were discovered in Africa. In 1924, Raymond Dart described Australopithecus africanus. The type specimen was the Taung Child, an australopithecine infant discovered in a cave deposit being mined for concrete at Taung, South Africa. The remains were a remarkably well-preserved tiny skull and an endocranial cast of the individual's brain. Although the brain was small (410 cm³), its shape was rounded, unlike that of chimpanzees and gorillas, and more like a modern human brain. Also, the specimen exhibited short canine teeth, and the position of the foramen magnum was evidence of bipedal locomotion. All of these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between apes and humans. Another 20 years would pass before Dart's claims were taken seriously, following the discovery of more fossils that resembled his find. The prevailing view of the time was that a large brain evolved before bipedality. It was thought that intelligence on par with modern humans was a prerequisite to bipedalism.

The australopithecines are now thought to be the immediate ancestors of the genus Homo, the group to which modern humans belong. Both australopithecines and Homo sapiens are part of the tribe Hominini, but recent data has brought into doubt the position of A. africanus as a direct ancestor of modern humans; it may well have been a dead-end cousin. The australopithecines were originally classified as either gracile or robust. The robust variety of Australopithecus has since been reclassified as Paranthropus. In the 1930s, when the robust specimens were first described, the Paranthropus genus was used. During the 1960s, the robust variety was moved into Australopithecus. The recent trend has been back to the original classification as a separate genus.

Before Homo
The evolutionary history of the primates can be traced back for some 60 million years, as one of the oldest of all surviving placental mammal groups. Most paleontologists consider that primates share a common ancestor with the bats, another extremely ancient lineage, and that this ancestor probably lived during the late Cretaceous together with the last dinosaurs. The oldest known primates come from North America, but they were widespread in Eurasia and Africa as well, during the tropical conditions of the Paleocene and Eocene. With the beginning of modern climates, marked by the formation of the first Antarctic ice in the early Oligocene around 40 million years ago, primates went extinct everywhere but Africa and southern Asia. The surviving tropical population, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Fayum depression southwest of Cairo, gave rise to all living primates - lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and the anthropoids, i.e. platyrrhines or New World monkeys, and catarrhines or Old World monkeys and the great apes and humans.

The earliest know catarrhine is Kamoyapithecus from uppermost Oligocene at Eragaleit in the northern Kenya rift valley, dated to 24 Ma (millions of years before present). Its ancestry is generally thought to be close to such genera as Aegyptopithecus, Propliopithecus, and Parapithecus from the Fayum, at around 35 Ma. There are no fossils from the intervening 11 million years. No near ancestor to South American platyrrhines, whose fossil record begins at around 30 Ma, can be identified among the North African fossil species, and possibly lies in other forms that lived in West Africa that were caught up in the still-mysterious transatlantic sweepstakes that sent primates, rodents, boa constrictors, and cichlid fishes from Africa to South America sometime in the Oligocene.

In the early Miocene, after 22 Ma, many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Because the fossils at 20 Ma include fragments attributed to Victoriapithecus, the earliest cercopithecoid, the other forms are (by default) grouped as hominoids, without clear evidence as to which are closest to living apes and humans. Among the presently recognized genera in this group, which ranges up to 13 Ma, we find Proconsul, Rangwapithecus, Dendropithecus, Limnopithecus, Nacholapithecus, Equatorius, Nyanzapithecus, Afropithecus, Heliopithecus, and Kenyapithecus, all from East Africa. The presence of other generalized non-cercopithecids of middle Miocene age from sites far distant -- Otavipithecus from cave deposits in Namibia, and Pieroloapithecus and Dryopithecus from France, Spain and Austria -- is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the early and middle Miocene. The youngest of the Miocene hominoids, Oreopithecus, is from 9-Ma coal beds in Italy.

Molecular evidence indicates that the lineage of gibbons (family Hylobatidae) became distinct between 18 and 12 Ma, and that of orangutans (subfamily Ponginae) at about 12 Ma; we have no fossils that clearly document the ancestry of gibbons, which may have originated in a so far unknown SE-asian hominoid population, but fossil proto-orangutans may be represented by Ramapithecus from India and Griphopithecus from Turkey, dated to around 10 Ma.

Molecular evidence further suggests that between 8 and 4 MYA, first the gorillas, and then the chimpanzee (genus Pan) split off from the line leading to the humans; we have no fossil record, however, of either group of African great apes, possibly because bones do not fossilize in rain forest environments. Hominines, however, seem to have been one of the mammal groups (as well as antelopes, hyaenas, dogs, pigs, elephants, and horses) that adapted to the open grasslands as soon as this biome appeared, due to increasingly seasonal climates, about 8 Ma, and their fossils are relatively well known. The earliest are Sahelanthropus tchadensis (7-6 MYA) and Orrorin tugenensis (6 MYA), followed by:
 * Ardipithecus (5.5-4.4 MYA), with species Ar. kadabba and Ar. ramidus;
 * Australopithecus (4-2 MYA), with species Au. anamensis, Au. afarensis, Au. africanus, Au. bahrelghazali, and Au. garhi;
 * Paranthropus (3-1.2 MYA), with species P. aethiopicus, P. boisei, and P. robustus;
 * Homo (2 MYA-present).

The Homo genus
In modern taxonomy, Homo sapiens is the only extant species of its genus, Homo. Likewise, the ongoing study of the origins of Homo sapiens often demonstrates that there were other Homo species, all of which are now extinct. While some of these other species might have been ancestors of H. sapiens, many were likely our "cousins", having speciated away from our ancestral line. There is not yet a consensus as to which of these groups should count as separate species and which as subspecies of another species. In some cases this is due to the paucity of fossils, in other cases it is due to the slight differences used to classify species in the Homo genus.

The word homo is Latin for "person", chosen originally by Carolus Linnaeus in his classification system. It is often translated as "man", although this can lead to confusion, given that the English word "man" can be generic like homo, but can also specifically refer to males. Latin for "man" in the gender-specific sense is vir, cognate with "virile" and "werewolf". The word "human" is from humanus, the adjectival form of homo.

Homo habilis
H. habilis lived from about 2.4 to 1.5 million years ago (MYA). H. habilis, the first species of the genus Homo, evolved in South and East Africa in the late Pliocene or early Pleistocene, 2.5–2 MYA, when it diverged from the Australopithecines. H. habilis had smaller molars and larger brains than the Australopithecines, and made tools from stone and perhaps animal bones. One of the first known hominids, it was nicknamed 'handy man' by its discoverer, Louis Leakey. Some scientists have proposed moving this species out of Homo and into Australopithecus.

Homo rudolfensis and Homo georgicus
These are proposed species names for fossils from about 1.9-1.6 MYA, the relation of which with H. habilis is not yet clear.
 * H. rudolfensis refers to a single, incomplete skull from Kenya.
 * H.georgicus, from Georgia, may be an intermediate form between H. habilis and H. erectus.

Homo ergaster and Homo erectus
The first fossils of Homo erectus were discovered by Dutch physician Eugene Dubois in 1891 on the Indonesian island of Java. He originally gave the material the name Pithecanthropus erectus based on its morphology that he considered to be intermediate between that of humans and apes.

H. erectus lived from about 1.8 MYA to 70,000 years ago. Often the early phase, from 1.8 to 1.25 MYA, is considered to be a separate species, H. ergaster, or it is seen as a subspecies of erectus, Homo erectus ergaster.

In the Early Pleistocene, 1.5–1 MYA, in Africa, Asia, and Europe, presumably, Homo habilis evolved larger brains and made more elaborate stone tools; these differences and others are sufficient for anthropologists to classify them as a new species, H. erectus. In addition H. erectus was the first human ancestor to walk truly upright. This was made possible by the evolution of locking knees and a different location of the foramen magnum (the hole in the skull where the spine enters). They may have used fire to cook their meat.

A famous example of Homo erectus is Peking Man; others were found in Asia (notably in Indonesia), Africa, and Europe. Many paleoanthropologists are now using the term Homo ergaster for the non-Asian forms of this group, and reserving H. erectus only for those fossils found in the Asian region and meeting certain skeletal and dental requirements which differ slightly from ergaster.

Homo cepranensis and Homo antecessor
These are proposed as species that may be intermediate between H. erectus and H. heidelbergensis.
 * H. cepranensis refers to a single skull cap from Italy, estimated to be about 800,000 years old.
 * H. antecessor is known from fossils from Spain and England that are 800,000-500,000 years old.

Homo heidelbergensis
H. heidelbergensis (Heidelberg Man) lived from about 800,000 to about 300,000 years ago. Also proposed as Homo sapiens heidelbergensis or Homo sapiens paleohungaricus.

Homo neanderthalensis
H. neanderthalensis lived from about 250,000 to as recent as 30,000 years ago. Also proposed as Homo sapiens neanderthalensis: there is ongoing debate over whether the 'Neanderthal Man' was a separate species, Homo neanderthalensis, or a subspecies of H. sapiens. While the debate remains unsettled, the prevailing view of evidence, collected by examining mitochondrial DNA and Y-chromosomal DNA, currently indicates that little or no gene flow occurred between H. neanderthalensis and H. sapiens, and, therefore, the two were separate species. In 1997, Dr. Mark Stoneking, then an associate professor of anthropology at Pennsylvania State University, stated: "These results [based on mitochondrial DNA extracted from Neanderthal bone] indicate that Neanderthals did not contribute mitochondrial DNA to modern humans… Neanderthals are not our ancestors." Subsequent investigation of a second source of Neanderthal DNA confirmed these findings. However, supporters of the multiregional hypothesis point to recent studies indicating non-African nuclear DNA heritage dating to one MYA, as well as apparent hybrid fossils found in Portugal and elsewhere, in rebuttal to the prevailing view.

Homo rhodesiensis, and the Gawis cranium

 * H. rhodesiensis, estimated to be 300,000-125,000 years old, most current experts believe Rhodesian Man to be within the group of Homo heidelbergensis though other designations such as Archaic Homo sapiens and Homo sapiens rhodesiensis have also been proposed.
 * In February 2006 a fossil, the Gawis cranium, was found which is suspected to be a species intermediate between H. erectus and H. sapiens. The skull from Gawis, Ethiopia, is believed to be 500,000-250,000 years old. Only summary details are known. Gawis man used tools and fire, and its facial features suggest its being an intermediate species. So far, it has not yet been named.

Homo sapiens
H. sapiens ("sapiens" means wise or intelligent) has lived from about 200,000 years ago to the present. Between 400,000 years ago and the second interglacial period in the Middle Pleistocene, around 250,000 years ago, the trend in cranial expansion and the elaboration of stone tool technologies developed, providing evidence for a transition from H. erectus to H. sapiens. The direct evidence suggests there was a migration of H. erectus out of Africa, then a further speciation of H. sapiens from H. erectus in Africa (there is little evidence that this speciation occurred elsewhere). Then a subsequent migration within and out of Africa eventually replaced the earlier dispersed H. erectus. However, the current evidence does not preclude multiregional speciation, either. This is a hotly debated area in paleoanthropology.

Current research establishes that human beings are highly genetically homogenous, meaning that the DNA of individual Homo sapiens is more alike than usual for most species, a result of our relatively recent evolution. Distinctive genetic characteristics have arisen however, primarily as the result of small groups of people moving into new environmental circumstances. Such small groups are initially highly inbred, allowing the relatively rapid transmission of traits favorable to the new environment. These adapted traits are a very small component of the Homo sapiens genome and include such outward "racial" characteristics as skin color and nose form in addition to internal characteristics such as the ability to breathe more efficiently in high altitudes.

 H. sapiens idaltu , from Ethiopia, lived from about 160,000 years ago (proposed subspecies). It is the oldest known anatomically modern human.

Homo floresiensis
H. floresiensis, which lived about 100,000 - 12,000 years ago (announced 28 October 2004 in the science journal Nature), has been nicknamed hobbit for its small size, probably a result of insular dwarfism. H. floresiensis is intriguing both for its size and its age; being a concrete example of a recent species of Homo that exhibits derived traits not shared with modern humans. In other words, 'floresiensis' share a common ancestor with modern humans, but split from the modern human lineage and followed a distinct evolutionary path. The main find was a fossil believed to be a woman of about 30 years of age. Found in 2003 it has been dated to approximatly 18,000 years old. Her brain size was only 380 cm³ (which can be considered small even for a chimp). She was only 1 meter in height.

Use of tools
Using tools is not only a sign of intelligence, it also may have acted as a stimulant for human evolution. Over the past 3 or 2 million years, human brain size has increased threefold. A brain needs a lot of energy: the brain of modern man uses about 20 Watts (about 400 calories per day), one fifth of total human energy consumption. Early hominoids, like apes, were essentially plant eaters (fruit, leaves, roots), their diet only occasionally supplemented by meat (often from scavenging). However, plant food in general yields considerably less energy and nutritive value than meat. Therefore, being able to hunt for large animals, which was only possible by using tools such as spears, made it possible for humans to sustain larger and more complex brains, which in turn allowed them to develop yet more intelligent and efficient tools.

Precisely when early man started to use tools is difficult to determine, because the more primitive these tools are (for example, sharp-edged stones) the more difficult it is to decide whether they are natural objects or human artifacts. There is some evidence that the australopithecines (4 MYA) may have used broken bones as tools, but this is debated.

Stone tools
Stone tools are first attested around 2.6 million years ago, when H. habilis in Eastern Africa used so-called pebble tools, choppers made out of round pebbles that had been split by simple strikes. This marks the beginning of the Paleolithic, or Old Stone Age; its end is taken to be the end of the last Ice Age, around 10,000 years ago. The Paleolithic is subdivided into the Lower Paleolithic (Early Stone Age, ending around 350,000 – 300,000 years ago), the Middle Paleolithic (Middle Stone Age, until 50,000 – 30,000 years ago), and the Upper Paleolithic.

The period from 700,000 – 300,000 years ago is also known as the Acheulean, when H. ergaster (or erectus) made large stone hand-axes out of flint and quartzite, at first quite rough (Early Acheulian), later "retouched" by additional, more subtle strikes at the sides of the flakes. After 350,000 BP (Before Present) the more refined so-called Levallois technique was developed. It consisted of series of consecutive strikes, by which scrapers, slicers ("racloirs"), needles, and flattened needles were made. Finally, after about 50,000 BP, ever more refined and specialized flint tools were made by the Neanderthals and the immigrant Cro-Magnons (knives, blades, skimmers). In this period they also started to make tools out of bone.

The "modern man" debate and the Great Leap Forward
Until about 50,000–40,000 years ago the use of stone tools seems to have progressed stepwise: each phase (habilis, ergaster, neanderthal) started at a higher level than the previous one, but once that phase had started further development was slow. In other words, one might call these Homo species culturally conservative. After 50,000 BP, what Jared Diamond and other anthropologists characterize as a Great Leap Forward, human culture apparently started to change at much greater speed: "modern" humans started to bury their dead carefully, made clothing out of hides, developed sophisticated hunting techniques (such as pitfall traps, or driving animals to fall off cliffs), and made cave paintings. This speed-up of cultural change seems connected with the arrival of modern humans, homo sapiens sapiens. Additionally, human culture began to become more advanced, in that, different populations of humans begin to create novelty in existing technologies. Artifacts such as fish hooks, buttons and bone needles begin to show signs of variation among different population of humans, something that has not been seen in human cultures prior to 50,000 BP.

Whether this was an acceleration from standstill, or a trend that had in fact already started earlier, is debated: many innovations of the past 50,000 years are known to have occasional precursors in the Middle Stone Age. Among innovations of the Acheulian and the Middle Stone Age are the use of fire (500,000 BP), building shelters (400,000 BP), production of stone blades or knives (280,000 BP), grinding stones (280,000 BP), long distance barter trade (140,000 BP), fishing gear (110,000 BP), mining (100,000 BP), and beads (75,000 BP). However, these mostly remain incidents and do not become common until after 50,000 BP.

Theoretically, modern human behaviour is taken to include four ingredient capabilities: abstract thinking (concepts free from specific examples), planning (taking steps to achieve a farther goal), innovation (finding new solutions), and symbolic behaviour (such as images, or rituals). Among concrete examples of modern human behaviour, anthropologists include specialization of tools, use of jewelry and images (such as cave drawings), organization of living space, rituals (for example, burials with grave gifts), specialized hunting techniques, exploration of less hospitable geographical areas, and barter trade networks. Debate continues whether there was indeed a "Revolution" leading to modern man ("the big bang of human consciousness"), or a more gradual evolution.

Notable human evolution researchers

 * Henry McHenry
 * Svante Paabo
 * David Pilbeam
 * Jeffrey H. Schwartz
 * Erik Trinkaus
 * Milford H. Wolpoff

Additional notes
The validity of evolution and the origins of humanity have often been a subject of great political and religious controversy (see Creation-evolution controversy and Hybrid-origin).

The classification of humans and their relatives has changed considerably over time (see History of hominoid taxonomy).

Speculation about the future evolution of humans is often explored in science fiction as continued speciation of humans as they fill various ecological niches (see adaptive radiation and Co-evolution).

Currently, scientists estimate that humans branched off from their common ancestor with chimpanzees about 5-7 MYA.