Human evolution

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Picture showing similarities of primates and man

Human evolution is the theory by which scientists think that humans became a species (a group of animals that are the same, but different from other groups of animals). A lot of people study it and this study affects and involves several fields, for example the study of genes. Physical anthropology means the study of human evolution. The theory says that some apes started walking on two legs and began to use their hands to carry things about 5 million years ago. Then they got separated from each other and became different. There were two or three different groups of walking apes, and one was the forefather of humans. They learned how to make simple tools.

The scientific name for "human" is Homo. The modern human is called Homo sapiens. Sapiens means "wise" or "knowing". Homo sapiens means "wise human".

According to the theory of evolution, humans developed from other species. The science that is concerned with fining out how this happened is called Paleoanthropology. It wants to explain how the human race developed,by looking at ancient humans based on fossil evidence, tools, and other signs of human habitation. The modern field of paleoanthropology began in the 19th century with the discovery of "Neanderthal man". The skeleton that gave the name was found in 1856, but there had been finds elsewhere since 1830.

[change] The idea of evolution: Humans are similar to great apes

By 1859, people had found out that humans were in some respect very similar to some of the great apes. The idea that species were the result of biological evolution was only discovered, when Charles Darwin published his book On the Origin of Species in November 1859. In his first book, Darwin wrote about the idea of evolution in general, not about the evolution of humans. Light will be thrown on the origin of man and his history, was all Darwin wrote on the subject. Nevertheless, the implications of the theory were clear to the readers of the time.^[1]

Different people discussed the evolution of humans. Among them were Thomas Huxley and Richard Owen. 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. When Darwin published his own book on the subject, The Descent of Man, and Selection in Relation to Sex, it was already a well-known interpretation of his theory. The interpretation he gave made the theory highly controversial. Even many of Darwin's original supporters (such as Alfred Russel Wallace and Charles Lyell) did not like the idea that human beings could have evolved their impressive mental capacities and moral sensibilities through natural selection.

Since the time of Carolus Linnaeus, scientists have considered the great apes to be the closest relatives of human beings because they look very similar. In the 19th century, they speculated that the closest living relatives of humans are chimpanzees. Chimpanzees naturally occur in Africa. Scientists believed that humans share a common ancestor with other African great apes and that fossils of these ancestors would be found in Africa. It is now accepted by virtually all biologists that humans are not only similar to the great apes but, in fact, are great apes.

[change] Common ancestors of humans and great apes

A reconstruction of Australopithecus afarensis, a human ancestor which had developed bipedalism, but as yet lacked the large brains of modern humans.

It was not until the 1920s that hominan fossils were discovered in Africa. In 1924, Raymond Dart described Australopithecus africanus.^[2] 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.

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 evolved before bipedality, the capability to move on two feet only. It was thought that intelligence similar to that of modern humans was a prerequisite to bipedalism.

The australopithecines are now thought to be immediate ancestors of the genus Homo, the group to which modern humans belong.^[3] 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.^[4] The australopithecines were originally classified as either wikt:gracile or wikt:robust. The robust variety of Australopithecus has since been reclassified as Paranthropus, although it is still regarded as a subgenus of Australopithecus by some authors.^[5]

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.

[change] The evolution of primates

The evolution of primates can be traced for a very long time. The oldest specimens found date from some 85 million years ago. This makes primates one of the oldest groups of mammals. Paleontologists believe that primates and bats have a comomn ancestor. This ancestor probably lived at the time of the last [[dinosaur]s.

When modern climates started, about 40 million years ago, primates were extinct, except for Africa and southern Asia. One such primate from this time was Notharctus. A fossil was found in Germany about 20 years ago. Analysis showed it was about 16.5 million years old, some 1.5 million years older than similar species from East Africa.^[6] This suggests that the primate lineage of the great apes first appeared in Eurasia and not Africa.

Primates migrated to Eurasia from Africa, just before the two continents spilt. This was about 17 million years ago. Begun says that these primates flourished in Eurasia and that their lineage leading to the African apes and humans—Dryopithecus—migrated south from Europe or Western Asia into Africa.^[6] Fossils from the Fayum depression southwest of Cairo show that all primates living to day seem to come from these migrants. This includes 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.

[change] The genus Homo

A chart showing different types of the genus homo, clasified by where they lived, and when they lived

It was Carolus Linnaeus who chose the name Homo. Today, there is only one species in the genus Homo, Homo sapiens sapiens. There were other species in there, but they became extinct. The figure shows where some of them lived and at what time. Sone of the other species might have been ancestors of H. sapiens. Many were likely our "cousins", they developed away from our ancestral line.^[7] Scientists are still discussing the exact line of development. A consensus, which of these groups should count as separate species and which as subspecies of another species 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 the Homo genus. The Sahara pump theory provides an explanation of the early variation in the genus Homo.

[change] Homo habilis

Homo Habilils was likely the first species of homo. It developed from the Australopitecus, about 2.5 milion years ago. It lived until about 1.4 million years ago. It had smaller molars and larger brains than the Australopitecus.

[change] Towards homo erectus

There are two proposed specied that lived from 1.9 to 1.6 mililion years ago. Their relation has not been clarified. One of them is called Homo rudolfensis. It is bases on a single incomplete skull from Kenya. Scientists have suggested that this was just another habilis, but this has not been confirmed. ^[8] The other is currently called Homo georgicus. It is from Georgia and may be an intermediate form between H. habilis and H. erectus,^[9] or a sub-species of H. erectus.^[10]

[change] Homo ergaster and Homo erectus

Homo erectus was first discovered on the island of Java in Indonesia, in 1891. The discoverer, Eugene Dubois originally called it Pithecanthropus erectus based on its morphology that he considered to be intermediate between that of humans and apes.^[11] Homo erectus lived 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. called Homo ergaster, or Homo erectus ergaster'.

In the Early Pleistocene, 1.5–1 mya, in Africa, Asia, and Europe, presumably, some populations of 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.^[12] 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. They reserve H. erectus only for those fossils found in the Asian region that meet certain requirements (as to skeleton and skull) which differ slightly from ergaster.

[change] Homo neanderthalensis

Reconstruction of a Neanderthal hunter, in the American museum of history

Homo neaderthalensis (usually called Neanderthal man) lived from about 250,000 to about 30,000 years ago. Also proposed as Homo sapiens neanderthalensis: There is still a discussion if it was a separate species Homo neanderthalensis, or a subspecies of H. sapiens.^[13] While the debate remains unsettled, evidence from mitochondrial DNA and Y-chromosomal DNA sequencing indicates that little or no gene flow occurred between H. neanderthalensis and H. sapiens, and, therefore, the two were separate species.^[14] 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." More investigation of a second source of Neanderthal DNA supported these findings.^[15] However, supporters of the multiregional hypothesis point to recent studies indicating non-African nuclear DNA heritage dating to one mya.^[16] The reliability of these studies has been questioned,though.^[17]

[change] Homo floresiensis

Homo floresiensis, which lived about 100,000–12,000 years ago has been nicknamed hobbit for its small size. Its size may be a result of insular dwarfism.^[18] H. floresiensis is intriguing both for its size and its age. It is a concrete example of a recent species of the genus Homo 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 (considered small for a chimpanzee and less than a third of the H. sapiens average of 1400 cm³).

There is an ongoing debate over whether H. floresiensis is indeed a separate species.^[19] Some scientists presently believe that H. floresiensis was a modern H. sapiens suffering from pathological dwarfism.^[20] This hypothesis is supported in part, because the modern humans who live on Flores, the island where the skeleton was found, are pygmies. This coupled with pathological dwarfism could indeed create a hobbit-like human. The other major attack on H. floresiensis is that it was found with tools only associated with H. sapiens.^[20]

[change] Homo sapiens

Homo 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, its skull grew and technolgies based on stone tools developed. This gives evidence that a transition between H. erectus to H. sapiens occurred. The direct evidence suggests there was a migration of H. erectus out of Africa, then a further development of H. sapiens from H. erectus in Africa. 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. The DNA of individuals is more alike than usual for most species. This 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. 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.

[change] References

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