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Replica of fossil skull ofHomo habilis. Fossil number KNM ER 1813, found atKoobi Fora, KenyaReplica of fossil skull ofHomo ergaster (AfricanHomo erectus). Fossil number Khm-Heu 3733 discovered in 1975 in Kenya
Human evolution is about the origin ofhuman beings. All humans belong to the samespecies,Homo sapiens. The species appeared first inAfrica and later spread to almost all parts of the world.Fossils found in Africa prove that humans first appeared there.[1][2][3]
The word 'human' in this context means thegenusHomo. However, studies of humanevolution usually include otherhominids, such as theAustralopithecines. This group diverged (split) from the genusHomo in Africa by about 2.3 to 2.4 million years ago.[4][5] The firstHomo sapiens, theancestors of today's humans,evolved around 200,000 years ago.[6]
The great apes live intropicalrainforests. It is thought that human evolution started when a group of apes (now called theaustralopithecines) began to live more in thesavannah. A savannah is more open, with trees,shrubs and grass. This group started walking on two legs. They began to use their hands to carry things. Life in the open was different, and there was a big advantage in having better brains. Their brains grew larger, and they began to make simpletools. This process began at least 5 million years ago. We have fossils of two or three different groups of walking apes, and one was theancestor of humans.
Thebiological name for "human" or "man" isHomo. The modern human species is calledHomo sapiens. "Sapiens" means "thought".Homo sapiens means "the thinking man".
Paleoanthropology looks at ancient human fossils, tools, and other signs of early human life. It began in the 19th century with the discovery of askull of "Neanderthal man" in 1856.
Showssimilarities ofapes and man. The human skeleton is on the right. Figures are drawn to scale, but thegibbon, on the left, is drawn at double size.
By 1859,zoologists had known for a long time that humans are, in theiranatomy, similar to thegreat apes. There are also differences: humans can speak, for example. But the similarities are more basic than the differences. Humans also have features with a much older history, from early in the life of vertebrates.[7]
The idea that species are caused by evolution had been proposed before Darwin, but his book gave much evidence, and many were persuaded by it. The book wasOn the Origin of Species by means of Natural Selection, published in November 1859. In this book, Darwin wrote about the idea of evolution in general, rather than the evolution of humans.Light will be thrown on the origin of man and his history, was all Darwin wrote on the subject. Nevertheless, theimplication of the theory was clear to readers at the time.[8]
Several people discussed the evolution of humans. Among them wereThomas Huxley andCharles Lyell. Huxley convincingly showed many of the similarities and differences between humans and apes in his 1863 bookEvidence as toMan's Place in Nature. When Darwin published his own book on the subject,The Descent of Man, and selection in relation to sex, the idea of human evolution was already well-known. The theory wascontroversial. Even some of Darwin's supporters (such asAlfred Russel Wallace andCharles Lyell) did not like the idea that human beings have evolved their impressive mental capacities and moral sensibilities throughnatural selection.
Since the 18th century, scientists thought the great apes to be closely related to human beings. In the 19th century, they speculated that the closest living relatives of humans were eitherchimpanzees orgorillas. Both live in central Africa intropical rainforests. In fact, chimpanzees are closest to us.[9] Biologists believed that humans share acommon ancestor with other African great apes and thatfossils of these ancestors would be found in Africa, which they have been. It is now accepted by virtually all biologists that humans are not only similar to the great apes, but actually are great apes.
The issue was confirmed by late 20th century studies on the sequences ofproteins andgenes in apes and man. These studies showed that man shares about 95 to 98% of these structures with chimpanzees.[10][11][12] This is a much closer relationship than with any other type of animal, and fully supports the ideas put forward in the 19th century by Darwin and Huxley.
"Currently available genetic and archaeological evidence is generally interpreted as supportive of a recent single origin of modern humans in East Africa. However, this is where the near consensus on human settlement history ends, and considerable uncertainty clouds any more detailed aspect of human colonization history".[13]
Primates have diversified inhabitats such as trees and bushes. They have many features which areadaptations to their environment.[14] Here are some of those traits:
Shoulder joints which allow high degrees of movement in all directions.[14]
Five digits on the fore and hind limbs withopposable thumbs and big toes; hands can grasp, and usually big toes as well.[14]
Nails on the fingers and toes (in most species).[15]
Sensitive tactile pads on the ends of the digits.[14]
A trend towards holding the torso upright leading tobipedalism.[14]
Not all primates have these anatomical traits, nor is every trait unique to primates. Primates are frequently highlysocial, live in groups with 'flexible dominance hierarchies'.[17][18]
Closely related animals almost always have closely relatedparasites. This usually comes about because parasites evolve with their hosts, and when host populations split, their parasites split also.[19] It is also possible for parasites to get from one species to another. Two of the most serious parasitic infections of humans in Africa come from apes. Each may have been transferred to humans by a single cross-species event.
There are several species ofmosquito, and several species of themalarial parasitePlasmodium. The most serious type,P. falciparum, which kills many millions of people each year, originated ingorillas.[20] It is now virtually certain thatchimpanzees are the source ofHIV-1, the major cause ofAIDS.[21] This information is got by thesequence analysis of ape and humanviruses.
The relevance of this to evolution is that ourphysiology is so close to the apes that their parasites were able to transfer to humans with great success. Humans have much less resistance to these parasites, which are ancient in origin, but comparatively new to our species.
A cast of the cranium of "Tournai",Sahelanthropus tchadensis, a member of an extinct hominid species who lived about 7 million years ago
It was not until the 1920s thathominid fossils were discovered inAfrica. In 1924,Raymond Dart describedAustralopithecus africanus.[22] The specimen was called theTaung Child, anaustralopithecine infant discovered in a cave deposit being mined for concrete at Taung,South Africa. The remains were a remarkably well-preserved tiny skull and a cast of the inside of the individual's skull. 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 had shortcanine teeth, and the position of theforamen magnum was evidence ofbipedal locomotion.[23] All of these traits convinced Dart that the Taung baby was a bipedal human ancestor, a transitional form between apes and humans.
It took another 20 years before Dart's claims were taken seriously. This was after other similar skeletons had been found. The most common view of the time was that a large brain evolvedbefore bipedality, the ability to walk on two feet more or less upright. It was thought thatintelligence similar to that of modern humans was necessary for bipedalism. This turned out to be wrong: bipedality came first.
The australopithecines are now thought to be immediate ancestors of the genusHomo, the group to which modern humans belong.[24] Both australopithecines andHomo sapiens are part of the tribeHominini, but recent data has brought into doubt the position ofA. africanus as a direct ancestor of modern humans; it may well have been a cousin.[25] The australopithecines were originally classified as eithergracile orrobust. The robust variety ofAustralopithecus has since been reclassified asParanthropus, although it is still regarded as a subgenus ofAustralopithecus by some authors.[26]
In the 1930s, when the robust specimens were first described, theParanthropus genus was used. During the 1960s, the robust variety was moved intoAustralopithecus. The recent trend has been back to the original classification as a separate genus.
A chart showing different types of the genus homo, classified by where they lived, and when they lived
It wasCarolus Linnaeus who chose the nameHomo. Today, there is only one species in the genus:Homo sapiens. There were other species, but they became extinct.
The figure shows where some of them lived and at what time. Some of the other species might have been ancestors ofH. sapiens. Many were likely our "cousins", they developed away from our ancestral line.[27]
Anthropologists are still investigating the exact line of descent. A consensus on which should count as separate species and which as subspecies has not been reached yet. In some cases this is because there are very few fossils, in other cases it is due to the slight differences used to classify species in theHomo genus.
Homo habilils was likely the first species of Homo. It developed from theAustralopithecus, about 2.5 million years ago. It lived until about 1.4 million years ago. It had smallermolars (back teeth) and larger brains than the Australopithecines.
There are two proposed species that lived from 1.9 to 1.6 million years ago. Their relation has not been clarified. One of them is calledHomo rudolfensis. It is known from a single incomplete skull fromKenya. Scientists have suggested that this was just another habilis, but this has not been confirmed.[30] The other is currently calledHomo georgicus. It is fromGeorgia and may be an intermediate form betweenH. habilis andH. erectus,[31] or a sub-species ofH. erectus.[32]
Homo erectus was first discovered on the island ofJava inIndonesia, in 1891. The discoverer,Eugene Dubois originally called itPithecanthropus erectus based on its morphology that he considered to be intermediate between that of humans and apes.[33] Homo erectus lived from about 1.8 million to 70,000 years ago. The earlier specimens (from 1.8 to 1.2 million years ago) are sometimes seen as a different species, or a subspecies. calledHomo ergaster, orHomo erectus ergaster.
In the earlyPleistocene, 1.5–1 mya, in Africa,Asia, andEurope, presumably, some populations ofHomo 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 additionH. erectus was the first human ancestor to walk truly upright.[34] This was made possible by the evolution of locking knees and a different location of theforamen magnum (the hole in the skull where the spine enters). They may have usedfire tocook theirmeat.
A famous example ofHomo erectus isPeking Man; others were found in Asia (notably in Indonesia), Africa, and Europe. Many paleoanthropologists are now using the termHomo ergaster for the non-Asian forms of this group. They reserveH. erectus only for those fossils found in the Asian region that meet certain requirements (as to skeleton and skull) which differ slightly from ergaster.
Homo neaderthalensis (usually calledNeanderthal man) lived from about 250,000 to about 30,000 years ago. Also, less usual, asHomo sapiens neanderthalensis: there is still some discussion if it was a separate speciesHomo neanderthalensis, or a subspecies ofH. sapiens.[35] While the debate remains unsettled, evidence frommitochondrial DNA andY-chromosomalDNA sequencing indicates that little or no gene flow occurred betweenH. neanderthalensis andH. sapiens, and, therefore, the two were separate species.[36] In 1997, Dr. Mark Stoneking, then an associate professor of anthropology atPennsylvania State University, stated:
"These results based on mitochondrialDNA extracted from Neanderthal bone] indicate that Neanderthals did not contribute mitochondrial DNA to modern humans… Neanderthals are not our ancestors".
More investigation of a second source of Neanderthal DNA supported these findings.[37]
A genetic analysis of a piece of finger bone found inSiberia has produced a surprise result. It dates to about 40,000 years ago, at a time when Neanderthals and modern man were living in the area.German researchers found itsmitochondrialDNA did not match either that of our species or that of Neanderthals. If this result is correct, the bone belongs to a previously unknown species. The degree of difference in the DNA suggests this species split off from our family tree about a million years ago, well before the split between our species and Neanderthals.[38]
Homo floresiensis, which lived about 100,000–12,000 years ago has been nicknamedhobbit for its small size. Its size may be a result ofisland dwarfism, the tendency for largemammals to evolve smaller forms on islands.[39]H. floresiensis is intriguing both for its size and its age. It is a concrete example of a recent species of the genusHomo that shows derived traits not shared with modern humans. In other words,H. floresiensis share a common ancestor with modern humans, but split from the modern human lineage and followed a different evolutionary path. The main find was a skeleton believed to be a woman of about 30 years of age. Found in 2003 it has been dated to approximately 18,000 years old. The living woman was estimated to be one meter in height, with a brain volume of just 380 cm3 This is small for a chimpanzee and less than a third of theH. sapiens average of 1400 cm3.
There is an ongoing debate over whetherH. floresiensis is indeed a separate species.[40] Some scientists believe thatH. floresiensis was a modernH. sapiens suffering from pathological dwarfism.[41] Modern humans who live on Flores, the island where the skeleton was found, arepygmies. This fact is consistent with either theory. One line of attack onH. floresiensis is that it was found with tools only associated withH. sapiens.[41]
Stone artifacts have been found on Flores which can be dated to a million years ago. These artifacts areproxies; which means there were no skeletons of humans, but only a species ofHomo could have made them. The artifacts are flakes and other implements, 48 in all, some of which show signs of being worked to produce a cutting edge. This means that humans were present on Flores by that date, but it does not tell us which species that was.[42]
Homo sapiens has lived from about 300,000 years ago to the present. Between 400,000 years ago and the second warm period in the MiddlePleistocene, about 250,000 years ago, the human skull grew more like its present shape. Our species developed more sophisticated technologies based on stone tools. One possibility is that a transition betweenH. erectus toH. sapiens occurred. The evidence ofJava Man suggests there was a migration ofH. erectus out of Africa. Then, much later, a further development ofH. sapiens fromH. erectus in Africa. Then a subsequent migration within and out of Africa eventually replaced the earlierH. erectus.
H sapiens shows adaptations and skills calledbehavioural modernity by some anthropologists.
Studies of thehuman genome, especially the Y-chromosomeDNA andmitochondrial DNA, support a relatively recent African origin.[43] Evidence from DNA also supports the recent African origin. The details of this great saga are not fully established yet, but by about 90,000 years ago modern humans had moved intoEurasia and theMiddle East. This was the area whereNeanderthals,Homo neanderthalensis, had been living for a long time (at least 500,000 years in western Europe).
By about 42 to 44,000 years agoHomo sapiens had reached westernEurope, includingBritain.[44] In Europe and western Asia,Homo sapiens replaced the Neanderthals by about 35,000 years ago. The details of how this happened are not known.
At roughly the same timeHomo sapiens arrived inAustralia.[45] Their arrival in theAmericas was much later, about 15,000 years ago.[46] All these earlier groups of modern man werehunter-gatherers.
Research has shown that human beings are genetically rather similar. TheDNA of individuals is more alike than usual in most species. This may have resulted from their relatively recent evolution or from theToba catastrophe. Skin colour is anadaptation to differing climates. Thesetraits are a very small component of theHomo sapiens genome and include such outward characteristics asskin color and nose shape, and internal characteristics such as the ability to breathe more efficiently at high altitudes.
H. sapiens idaltu, fromEthiopia, about 160,000 years ago, is a proposed subspecies. It is the oldest known anatomically modern human.[47]
↑Stringer C.B. (1994). "Evolution of early humans". In Steve Jones; Robert Martin; David Pilbeam (eds.).The Cambridge Encyclopedia of Human Evolution. Cambridge: Cambridge University Press. p. 242.ISBN978-0-521-32370-3. AlsoISBN978-0-521-46786-5 (paperback)
↑Soligo C. & Müller A.E. (1999). "Nails and claws in primate evolution".Journal of Human Evolution.36 (1):97–114.doi:10.1006/jhev.1998.0263.PMID9924135.
↑The foramen magnum is where the spine joins the skull. In apes, the skull in life tilts forward, but in humans it is more evenly balanced on top of the spine. During human evolution, the position of the foramen magnum moved towards the middle underside the skull.
↑Gabounia L. & others (2002). "Discovery of a new hominid at Dmanisi (Transcaucasia, Georgia)".Comptes Rendus Palevol.1 (4):243–53.doi:10.1016/S1631-0683(02)00032-5.
↑Lordkipanidze D. & others (2006). "A fourth hominin skull from Dmanisi, Georgia".The Anatomical Record. Part A, Discoveries in Molecular, Cellular, and Evolutionary Biology.288 (11):1146–57.doi:10.1002/ar.a.20379.PMID17031841.S2CID15624621.
↑Harvati K (2003). "The Neanderthal taxonomic position: models of intra- and inter-specific craniofacial variation".J. Hum. Evol.44 (1):107–132.doi:10.1016/s0047-2484(02)00208-7.PMID12604307.
↑41.041.1Martin R.D. & others (2006). "Flores hominid: new species or microcephalic dwarf?".The Anatomical Record. Part A, Discoveries in Molecular, Cellular, and Evolutionary Biology.288 (11):1123–1145.doi:10.1002/ar.a.20389.PMID17031806.
↑Brumm, Adam & others 2010. Hominins on Flores, Indonesia, by one million years ago.Nature464, 748–752.
