Human evolution is the part of biological evolution concerning the emergence of humans 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, linguistics 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 evolutionary history of the primates can be traced back for some 85 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. Fossil evidence found in Germany 20 years ago was determined to be about 16.5 million years old, some 1.5 million years older than similar species from East Africa. It suggests that the great ape and human lineage first appeared in Eurasia and not Africa.
The discoveries suggest that the early ancestors of the hominids (the family of great apes and humans) migrated to Eurasia from Africa about 17 million years ago, just before these two continents were cut off from each other by an expansion of the Mediterranean Sea. Begun says that the great apes flourished in Eurasia and that their lineage leading to the African apes and humans—Dryopithecus—migrated south from Europe or Western Asia into Africa. 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; platyrrhines or New World monkeys, and catarrhines or Old World monkeys and the great apes and humans.
The earliest known 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 mya. There are no fossils from the intervening 11 million years. No near ancestor to South American platyrrhines, whose fossil record begins at around 30 mya, 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 mya, many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Because the fossils at 20 mya 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 mya, 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 Pierolapithecus 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 mya 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 South East Asian hominid 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; human DNA is 98.4 percent identical to the DNA of chimpanzees. 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, hyenas, dogs, pigs, elephants, and horses) that adapted to the open grasslands as soon as this biome appeared, due to increasingly seasonal climates, about 8 mya, 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). Before Homo
The word homo is Latin for "human", 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.
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 Sahara pump theory provides an explanation of the early variation in the genus Homo.
The genus Homo
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 habilis
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. Homo ergaster and Homo erectus
These are proposed as species that may be intermediate between H. erectus and H. heidelbergensis. Homo cepranensis and Homo antecessor
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 heidelbergensis
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.
Homo neanderthalensisH. 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 might possibly be a species intermediate between H. erectus and H. sapiens or one of many evolutionary dead ends. The skull from Gawis, Ethiopia, is believed to be 500,000–250,000 years old. Only summary details are known, and no peer reviewed studies have been released by the finding team. Gawis man's facial features suggest its being either an intermediate species and an example of a "Bodo man" female. Homo rhodesiensis, and the Gawis cranium
H. sapiens ("sapiens" means wise or intelligent) has lived from about 250,000 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. This migration and origin theory is usually referred to as the single-origin theory. However, the current evidence does not preclude multiregional speciation, either. This is a hotly debated area in paleoanthropology.
Current research has established that human beings are genetically highly homogenous, that is the DNA of individuals is more alike than usual for most species, which may have resulted from their relatively recent evolution or the Toba catastrophe. 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 sapiens
H. floresiensis, which lived about 100,000–12,000 years ago has been nicknamed hobbit for its small size, probably a result of insular (island) dwarfism.
Homo floresiensis
Bolded species names indicate the existence of numerous fossil records.
Comparative table of Homo species
Using tools is not only a sign of intelligence, it also may have acted as a stimulus 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 5 Watts (about 400 kilo 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.
Use of 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.
Stone tools
James Burnett, Lord Monboddo, most famous today as a founder of modern comparative historical linguistics
Henry McHenry, specializes in studies of human evolution, the origins of bipedality, and paleoanthropology
Svante Pääbo, a biologist specializing in evolutionary genetics
Jeffrey H. Schwartz, an American physical anthropologist and professor of biological anthropology
Erik Trinkaus, a prominent paleoanthropologist and expert on Neanderthal biology and human evolution
Milford H. Wolpoff, a paleoanthropologist
Charles Darwin, an English naturalist who documented considerable evidence that species originate through evolutionary change
J. B. S. Haldane, a British geneticist and evolutionary biologist
Leonard Shlain, a surgeon and author of three books
Richard Dawkins, a British ethologist, evolutionary biologist who has promoted a gene-centered view of evolution.
Sir Alister Hardy, a British zoologist, who first hypothesised the aquatic ape theory of human evolution.
Louis Leakey
Richard Leakey Notable human evolution researchers
This list will conduct in chronological order, following genus.
Sahelanthropus
- Sahelanthropus tchadensis
Orrorin
- Orrorin tugenensis
Ardipithecus
- Ardipithecus kadabba
Ardipithecus ramidus
Australopithecus
- Australopithecus anamensis
Australopithecus afarensis
Australopithecus bahrelghazali
Australopithecus africanus
Australopithecus garhi
Paranthropus
- Paranthropus aethiopicus
Paranthropus boisei
Paranthropus robustus
Kenyanthropus
- Kenyanthropus platyops
Homo
- Homo habilis
Homo rudolfensis
Homo ergaster
Homo georgicus
Homo erectus
Homo cepranensis
Homo antecessor
Homo heidelbergensis
Homo rhodesiensis
Homo neanderthalensis
Homo sapiens idaltu
Homo sapiens (Cro-magnon)
Homo sapiens sapiens
Homo floresiensisSpecies list
The validity of evolution and the origins of humanity have often been a subject of great political and religious controversy within the non-scientific community (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), as well as deliberate self-modification (see Participant evolution).
Currently, scientists have estimated that humans branched off from their common ancestor with chimpanzees about 5–7 mya. See also
Wolfgang Enard et al. (2002-08-22). "Molecular evolution of FOXP2, a gene involved in speech and language". Nature 418: 870.
DNA Shows Neandertals Were Not Our Ancestors
J. W. IJdo, A. Baldini, D. C. Ward, S. T. Reeders, R. A. Wells (October 1991). "Origin of human chromosome 2: An ancestral telomere-telomere fusion". Genetics 88: 9051–9055. —two ancestral ape chromosomes fused to give rise to human chromosome 2.
Ovchinnikov, et al. (2000). "Molecular analysis of Neanderthal DNA from the Northern Caucasus". Nature 404: 490.
Heizmann, Elmar P J, Begun, David R (2001). "The oldest Eurasian hominoid". Journal of Human Evolution 41 (5).
JBS Haldane (1955). "Origin of Man". Nature 176 (169).
- Homo habilis
- Kenyanthropus platyops
- Paranthropus aethiopicus
- Australopithecus anamensis
- Ardipithecus kadabba
- Orrorin tugenensis
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