Humans (Homo sapiens) are a species of Great Ape and the only surviving species of the genus Homo. The species is commonly referred to as man, mankind or humanity and its members as humans, human beings, persons or people. Adult males are known as men and adult females as women. There is only one extant subspecies, H. sapiens sapiens. Humans are notable for their increased intelligence and the ability to use language.
The closest living evolutionary relatives to humans are the two species of chimpanzeePan troglodytes ("common chimp") and Pan paniscus ("pygmy chimp" or "Bonobo"), and to a lesser degree other hominoids such as orangutans and gorillas. It is important to note however, that humans only share a common ancestor with these and are not descended directly from them. Biologists have compared a sequence of DNAbase pairs between humans and chimpanzees, and estimated an overall genetic difference of 5% . 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.
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.
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 cm (64 in) and the mean weight is 62 kg (137 lb). Males are typically larger: 175 cm (69 in) and 78 kilograms (172 lb). 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 (6-9 pounds) and 50-60 centimetres (20-24 inches) 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.
Religions maintain that, beyond having a physical and mental nature, mankind also has a spiritual nature; many hold that this spiritual nature distinguishes mankind from other creatures. Most non-religious maintain that mankind has no spiritual aspect, and is not thus distinguished.
The original habitat in which humans evolved is the African savannah (see Vagina gentium, Environment of Evolutionary Adaptedness). Culturally transmitted technology has allowed humans to colonize all of the continents and adapt to all climates. Within the last few decades, humans have been able to temporarily inhabit Antarctica, the ocean depths, and outer space, although permanent habitation of these three environments is not yet possible. Humans, with a population of about six billion, are one of the most numerous mammals on Earth.
Being primates, humans' original life style is hunting/gathering, which is adapted to the savannah where they evolved. Other human life styles are nomadism (often linked to animal herding) and permanent settlements made possible by the development of agriculture. Humans have a great capacity for altering their habitats by various methods, such as agriculture, irrigation, urban planning and construction, and activities accessory to those, such as transportation and manufacturing goods.
Permanent human settlements are dependent on proximity to water and, depending on the lifestyle, other natural resources such as fertile land for growing crops and grazing livestock or, seasonally by populations of prey. With the advent of large-scale trade and transportation infrastructure, immediate proximity to these resources has become less necessary, and in many places these factors are no longer the driving force behind growth and decline of population.
Humans living on Antarctica, under the ocean, or in space are part of scientific, military, or industrial expeditions, and habitation of these environments is temporary.
Life in space has thus far been temporary living, with up to ten humans in space at a given time (seven on the Space Shuttle, three on Mir) and currently around three in the International Space Station. This is a direct result of humans' vulnerability to ionizing radiation. Prior to 1961, all humans were restricted to the earth; Yuri Gagarin was the first human to travel into space. At various periods between 1969 and 1974, up to two humans spent varying amounts of time on the Moon. As of yet, residencies or human explorations on other planets have not come to be.
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 the use of fire (see lifeform) and on tools such as clothing and manufactured shelter, which are themselves often produced and used through complex social interactions.
Various attempts have been made to identify a single behavioral characteristic that distinguishes humans from all other animals, e.g. the ability to make and use tools (building shelter, weaving fabrics for clothing); the ability to alter the environment; language; and the development of complex social relationships and structures. Considered in isolation, however, these differences are 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. For these reasons, the idea that making and using tools is a defining characteristic of humans is often considered outdated, though of course no other animal uses tools to the same degree or with the same flexibility as Homo sapiens. Similarly, other animals often have 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.
Chomskianlinguistics holds that a distinguishing feature of humans is that they 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. Dolphins may also have this trait as they show dialect.
Some anthropologists think that these readily observable characteristics (tool-making and language) are based on less easily observable mental processes 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 greater than other animals are known to do, although several species have demonstrated some abilities in these areas. In any case, the idea that these abilities distinguish humans from other species is the basis of the name Homo sapiens, sometimes translated as "Man the Thinker". It should be noted, however, that the extinct species of the Homo genus (e.g. Homo neanderthalensis, Homo erectus) were also adept tool makers and there is some evidence that they may have had linguistic skills.
While humans have all these characteristics, from the biological viewpoint the question "What single characteristic distinguishes humans from all other animals?" is an odd one: it is not a question that is usually asked of cats, dolphins, or song sparrows. Finding other species that shape tools or can use sign language may shed light on human evolution, but it doesn't erase the differences or similarities between humans and other species.