Human

Biologists classify humans (Homo sapiens) as a species of primates and the only surviving species of the genus Homo. The species is commonly referred to as "humankind" or "humanity" and its members as "humans", "human beings" or "people". (Also sometimes called "man" or "mankind", but that usage is discouraged these days on the grounds of gender neutrality). Man is a male human being and woman is a female human being. All current humans, from across all areas of the Earth, are of this species: attempts to divide the human population into "races" are now often considered to be spurious.

According to mainstream biology, the closest living evolutionary relatives to humans are the two species of chimpanzee Pan troglodytes[?] ("common chimp") and Pan paniscus ("pygmy chimp" or "Bonobo"), and to a lesser degree other hominoids such as orangutans and gorillas. Biologists have compared a sequence of DNA base pairs between humans and chimpanzees, and estimated an overall genetic difference of 5% [1] (http://www.pnas.org/cgi/content/abstract/99/21/13633). It has been estimated that the human lineage diverged from that of chimpanzees about 5 million years ago, and from gorillas about 8 million years ago. However, recent news reports of a hominid skull approximately 7 million years old already showing a divergence from the ape lineage strongly suggests an earlier divergence. Some scientists argue that bonobos, chimpanzees and, possibly, gorillas should be lumped into the genus Homo, but this is currently a minority opinion.

Various religious groups have raised objections and controversy concerning the theory of humanity's evolution from a common ancestor with the other hominoids. See creationism and argument from evolution for opposing points of view.

Physical characteristics

The body of humans is described in the human anatomy group of articles.

The evolution of Homo sapiens is characterized by a number of important trends:

• expansion of the brain cavity[?] and brain itself, which is typically about 1,400 cm³ in volume, well over twice that of a chimpanzee or gorilla. Some physical anthropologists argue that a reorganization of the structure of the brain is more important than cranial expansion itself.
• canine reduction.
• bipedal locomotion[?]
• descent of the larynx (which makes possible the production of the complex sound known as vocal language).

How these trends are related, in what ways they have been adaptive, and what their role is in the evolution of complex social organization and culture, are matters of ongoing debate among physical anthropologists.

Although body size is highly heritable, it is also significantly influenced by environmental and cultural factors such as diet. The mean height of an American adult female is 162 centimetres and the mean weight is 62 kg. Males are typically heavier - 175 cm and 78 kilogram. Humans vary substantially around these means, and the means themselves have varied depending on locality and historical factors.

Human children, typically weighing 3-4 kilograms and 50-60 centimetres in height, are born after a nine-month gestation period. Helpless at birth, they continue to grow for some years, typically reaching sexual maturity[?] at around 12-15 years of age. Boys continue growing for some time after this, often only reaching their maximum height around the age of 18. The average human lifespan is approaching 80 years in wealthy nations, with the assistance of medicine.

See also human physical appearance.

Homo sapiens compared to other species

Humans often consider themselves to be the "dominant" species on Earth, and the most advanced in intelligence and ability to manage their environment. This belief is especially strong in Western culture, and is based in part in the Biblical Creation story in which Adam is explicitly given dominion over the Earth and all of its creatures.

Biologists and scientists in general, though, do not consider "dominant" to be a useful term, because the adaptive value of any trait or complex of traits depends on the niche and is highly mutable. From a scientific standpoint, Homo sapiens certainly is among the most generalized species on Earth. Smaller and simpler animals such as bacteria and insects greatly surpass humans in population size and diversity of species, but few single species occupy as many diverse environments as humans. Many other species, for example, are adapted to specific environments, whereas humans rely on tools such as clothing and manufactured shelter, which are themselves often produced and used through complex social interactions.

The use of tools and the ability to alter their environment (building shelter, weaving fabrics for clothing, language, and the development of complex social relationships and structures, etc.) has been cited as a characteristic which distinguishes humans from other animals. This difference, however, is not absolute, as ethologists have recorded such behaviors in many species. Apes and even birds, for example, are known to "fish" for insects using blades of grass or twigs, and even to shape the tools for that purpose. No other animal uses tools to the same degree or with the same flexibility as Homo sapiens. Similarly, other animals often have simple methods of communication, but the degree to which humans create and use complex grammar and abstract concepts in language has not been seen in any other species, despite much effort to find it.

Chomskian linguistics holds that a distinguishing feature of humans is that we are the only extant species with a language instinct - a genetic predisposition that produces a brain mechanism whose function is to acquire a language by observing those around us.

Some anthropologists think that these readily observable characteristics (toolmaking and language) are based on a less easily observable mental process that might be unique among humans: the ability to think symbolically. That is, humans can think abstractly about concepts and ideas. They can question, use logic, understand mathematical concepts, and so on in ways that no other animals are known to do, although several species have demonstrated some ability in this area. Nor have other animals demonstrated any remotely comparable ability to plan their actions. This belief is why the species was named Homo sapiens, sometimes translated as "Man the Thinker". Note, however, that the extinct species of the Homo genus (eg, Homo neanderthalensis, Homo erectus) were also adept tool users and there is some evidence that they may have had linguistic skills. Moreover, there are many other animals alive today which use tools, so the idea that making and using tools is often considered outdated.

While humans have all these characteristics, from the biological viewpoint "what distinguishes humans from all other animals?" is an odd question: there's no one thing that makes cats, dolphins, or song sparrows unique. Finding other species that shape tools or can use sign language may shed light on human evolution; it doesn't erase the differences between humans and related species.

See also: Evolution of Homo sapiens, human condition, man, woman, child

External link

• A Look at Modern Human Origins (http://www.modernhumanorigins.com/)