Human evolution

**Human evolution** is about the origin of human beings. All humans belong to the same species, _Homo sapiens_. Fossils found in Africa prove that humans first appeared there. Later, they spread to almost all parts of the world.

"Human" means the genus _Homo_. However, studies of human evolution usually include other hominids too, like 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 classified 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 (this is called bipedalism). 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.

According to a 2006 paper:> 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.
- 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

Like most closely related animals, apes and humans 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 jump 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 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 virus that causes AIDS. Scientists determined this by doing sequence analysis of ape and human viruses.

This shows that human 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 that the individual could walk on two feet. 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. In the 1930s, when the robust specimens were first described, the _Paranthropus_ genus was used to describe them. 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_

Carolus Linnaeus 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", and developed away from our ancestral line.

Anthropologists are still investigating the exact line of descent. They do not all agree on which specimens should count as a separate species, and which are subspecies. 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. Stone tools were so central to the Pliestocene that it is usually called the Palaeolithic, or the Stone Age, in archaeology and anthropology.

### _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 _H._ _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, which he considered to be intermediate between that of humans and apes. _H. 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). _H. erectus_ may have used fire to cook their meat.

A famous example of _H. erectus_ is Peking Man; other specimens 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 _H. ergaster_.

### Neanderthal man

_Homo neaderthalensis_ (usually called Neanderthals) lived from about 250,000 to about 30,000 years ago. It is less commonly called _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 dated it 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. This suggests that the bone belongs to a previously unknown species, called the Denisovans. 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 fossil 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 cm<sup>3</sup> This is small for a chimpanzee and less than a third of the _H. sapiens_ average of 1400 cm<sup>3</sup>.

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 were dated to a million years ago.  These artifacts are _proxies,_ which means no skeletons of humans were found, but only a species of _Homo_ could have made the artifacts. 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 300,000 years ago to the present. Between 400,000 years ago and the second warm period in the Middle Pleistocene, 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 between _H. erectus_ to _H. sapiens_ occurred. The evidence of Java Man suggests there was a 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_.

_H sapiens_ shows adaptations and skills called behavioural modernity by some anthropologists.

#### Out of Africa

Studies of the human genome, especially the Y-chromosome DNA and mitochondrial DNA, suggest that humans originated relatively recently in Africa. Evidence from DNA supports this view. By about 90,000 years ago modern humans 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 42,000 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. Their arrival in the Americas was much later, about 15,000 years ago. All these earlier groups of modern man were hunter-gatherers.

Research has shown that human beings are genetically rather similar. The DNA of individuals is more alike than usual in most species. This may have resulted from their relatively recent evolution or from the Toba catastrophe. Skin colour is an adaptation to differing climates. These traits are a very small component of the _Homo sapiens_ genome and include such outward 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