Human evolution

Human evolution is about the origin of human beings. All humans belong to the same species, Homo sapiens, which appeared first in Africa but has spread to almost all parts of the world. Fossils found in Africa prove that humans first appeared there.

The word 'human' in this context means the genus Homo. However, studies of human evolution usually include other hominids, such as the Australopithecines. This group diverged (split) from the genus Homo in Africa by about 2.3 to 2.4 million years ago. The first Homo sapiens, the ancestors of today's humans, evolved around 200,000 years ago.

People have known for centuries that man and the apes are related. Clearly, even though they look different, their anatomy is similar. For this reason, during the 18th century, Buffon and Linnaeus put men and apes together in a single family. In the 19th century, Charles Darwin suggested that animals have very similar anatomies when they share a common ancestor. In fact, humans and apes are close relatives. Both are primates: the order of mammals which includes monkeys, apes, lemurs and tarsiers.

The great apes live in tropical rainforests. It is thought that human evolution started when a group of apes (now called the australopithecines) began to live more in the savannah. 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 simple tools. This process began at least 5 million years ago. We have fossils of two or three different groups of walking apes, and one was the ancestor of humans.

The biological name for "human" or "man" is Homo. The modern human species is called Homo 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 a skull of "Neanderthal man" in 1856.

Humans are similar to great apes 

By 1859, zoologists had known for a long time that humans are, in their anatomy, similar to the great 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.

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 was On 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, the implication of the theory was clear to readers at the time.

Several people discussed the evolution of humans. Among them were Thomas Huxley and Charles Lyell. Huxley convincingly showed 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, the idea of human evolution was already well-known. The theory was controversial. Even some of Darwin's supporters (such as Alfred Russel Wallace and Charles Lyell) did not like the idea that human beings have evolved their impressive mental capacities and moral sensibilities through natural 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 either chimpanzees or gorillas. Both live in central Africa in tropical rainforests. In fact, chimpanzees are closest to us. Biologists believed that humans share a common ancestor with other African great apes and that fossils 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 of proteins and genes in apes and man. These studies showed that man shares about 95 to 98% of these structures with chimpanzees. 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".

Distinguishing features 

Primates have diversified in habitats such as trees and bushes. They have many features which are adaptations to their environment. Here are some of those traits:
 Shoulder joints which allow high degrees of movement in all directions.
 Five digits on the fore and hind limbs with opposable thumbs and big toes; hands can grasp, and usually big toes as well.
 Nails on the fingers and toes (in most species).
 Sensitive tactile pads on the ends of the digits.
 Sockets of eyes encircled in bone.
 A trend towards a reduced snout and flattened face, attributed to a reliance on vision at the expense of smell.
 A complex visual system with binocular (stereoscopic) vision, high visual acuity and color vision.
 Brain with a well developed cerebellum for good balance.
 Brain large in comparison to body size, especially in simians (old world monkeys and apes).
 Enlarged cerebral cortex (brain): learning, problem solving.
 Reduced number of teeth compared to primitive mammals;.
 A well-developed cecum: vegetable digestion.
 Two pectoral mammary glands.
 Typically one young per pregnancy.
 A pendulous penis and scrotal testes.
 Long gestation and developmental period. and
 A trend towards holding the torso upright leading to bipedalism.

Not all primates have these anatomical traits, nor is every trait unique to primates. Primates are frequently highly social, live in groups with 'flexible dominance hierarchies'.

Other similarities 
Closely related animals almost always have closely related parasites. This usually comes about because parasites evolve with their hosts, and when host populations split, their parasites split also. It is also possible for parasites to get from one species to another. Two of the most serious parasitic infections of humans in Africa have originated in apes. Each may have been transferred to humans by a single cross-species event.

There are several species of mosquito, and several species of the malarial parasite Plasmodium. The most serious type, P. falciparum, which kills many millions of people each year, originated in gorillas. It is now virtually certain that chimpanzees are the source of HIV-1, the major cause of AIDS. This information is got by the sequence analysis of ape and human viruses.

The relevance of this to evolution is that our physiology 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.

Immediate ancestors of the genus Homo 

It was not until the 1920s that hominid fossils were discovered in Africa. In 1924, Raymond Dart described Australopithecus africanus. The specimen was called 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 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 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 ability to walk on two feet more or less upright. It was thought that intelligence 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 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 cousin. The australopithecines were originally classified as either gracile or 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.

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.

The genus Homo 

It was Carolus Linnaeus who chose the name Homo. 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 of H. sapiens. Many were likely our "cousins", they developed away from our ancestral line.

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 the Homo genus.

The evolution of the genus Homo took place mostly in the Pleistocene. The whole genus is characterised by its use of stone tools, initially crude, and becoming ever more sophisticated. So much so that in archaeology and anthropology the Pleistocene is usually referred to as the Palaeolithic, or the Stone Age.

Homo habilis 
Homo habilils was likely the first species of Homo. It developed from the Australopithecus, about 2.5 million years ago. It lived until about 1.4 million years ago. It had smaller molars (back teeth) and larger brains than the Australopithecines.

Towards Homo erectus 
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 called Homo rudolfensis. It is known from a single incomplete skull from Kenya. Scientists have suggested that this was just another habilis, but this has not been confirmed. The other is currently called Homo georgicus. It is from Georgia and may be an intermediate form between H. habilis and H. erectus, or a sub-species of H. erectus.

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. 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. 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. 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.

 Neanderthal man Homo neaderthalensis (usually called Neanderthal man) lived from about 250,000 to about 30,000 years ago. Also, less usual, as Homo sapiens neanderthalensis: there is still some discussion if it was a separate species Homo neanderthalensis, or a subspecies of H. sapiens. 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. 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.

 Denisovan man 
A genetic analysis of a piece of finger bone found in Siberia 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 its mitochondrial DNA 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.

 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 island dwarfism, the tendency for large mammals to evolve smaller forms on islands. 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 This is 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. Some scientists believe that H. floresiensis was a modern H. sapiens suffering from pathological dwarfism. Modern humans who live on Flores, the island where the skeleton was found, are pygmies. This fact is consistent with either theory. One line of attack on H. floresiensis is that it was found with tools only associated with H. sapiens.

Stone artifacts have been found on Flores which can be dated to a million years ago.  These artifacts are proxies; which means there were no skeletons of humans, but only a species of Homo 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.

 Homo sapiens Homo sapiens has lived from about 250,000 years ago to the present. Between 400,000 years ago and the second warm period in the Middle Pleistocene, around 250,000 years ago, its skull grew and more sophisticated technologies based on stone tools developed. One possibility is that a transition between H. erectus to H. sapiens occurred. The evidence of Java Man suggests there was an initial migration of H. erectus out of Africa. Then, much later, a further development of H. sapiens from H. erectus in Africa. Then a subsequent migration within and out of Africa eventually replaced the earlier H. erectus.

 Out of Africa 

Studies of the human genome, especially the Y-chromosome DNA and mitochondrial DNA, have supported a recent African origin. Evidence from autosomal 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 they had moved into Eurasia and the Middle East. This was the area where Neanderthals, Homo neanderthalensis, had been living for a long time (at least 500,000 years in western Europe).

By about 42 to 44,000 years ago Homo sapiens had reached western Europe, including Britain. 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 time Homo sapiens arrived in Australia (However a recent archaeological dig in Kakadu, Northern Territory, may suggest Homo sapiens arrived in Australia 65,000 years ago). Their arrival in the Americas was much later, about 15,000 years ago. All these earlier groups of modern man were hunter-gatherers.

Current research has established that human beings are genetically rather homogenous (similar). The DNA of individuals is more alike than usual for most species. This may have resulted from their relatively recent evolution or from the Toba catastrophe. Distinctive genetic have arisen as a 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 shape, and internal characteristics such as the ability to breathe more efficiently at high altitudes.H. sapiens idaltu, from Ethiopia, about 160,000 years ago, is a proposed subspecies. It is the oldest known anatomically modern human.

 Species list 
This list is in chronological order by 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 floresiensis''

Related pages 
ENCODE: the complete analysis of the human genome
Paleoanthropology

References 

Evolution
Anthropology