|by Marcia Malory|
Humans (Homo sapiens) are great apes.
The closest living relatives of humans are bonobos and chimpanzees.
Humans, chimpanzees and bonobos had a common ancestor that lived around 6 million years ago.
The next closest relatives of humans are gorillas, followed by orangutans.
Anatomically modern humans first appeared about 200,000 years ago, in East Africa.
According to fossil and DNA evidence, humans left Africa between about 125,000 and 40,000 years ago.
Today, humans can be found almost everywhere on our planet.
Because humans have migrated to many different places on Earth, they have developed many different physical adaptations in order to cope with different environmental conditions. Therefore, humans vary greatly in appearance.
For example, humans that migrated to parts of the world that do not have much sunlight developed lighter hair and eye color so that they could absorb more ultraviolet light from the sun, which is essential for the formation of Vitamin D.
Nevertheless, because interbreeding between different groups of humans has never been completely restricted, there are no living human subspecies.
When humans divide each other into "races", they are creating cultural divisions that have no biological basis.
Humans can be distinguished from other great apes by their bipedal stance, their short, fine hair and their larger brains in comparison to body size.
Although other great apes do walk on two legs from time to time, adult humans are the only great apes for whom bipedalism is the usual mode of locomotion.
There are different theories that explain why humans became bipedal.
Bipedalism would have freed up humans' hands, which would have made it easier for humans to transport food after a hunt.
A bipedal stance would make a human appear larger and more threatening; this could have prevented attacks by predators.
Standing on two limbs rather than four would have reduced the surface area of the body exposed to the sun, and so decrease the chance of overheating.
The reduction in the length and thickness of body hair, as well as the development of more efficient sweat glands, is also thought to have helped prevent overheating.
In the human brain, the prefrontal cortex and the temporal lobes are greatly enlarged in comparison with these brain regions in other primates.
The increase in size of these areas is associated with advanced language skills, the ability to think abstractly and the ability to maintain complex social relationships.
Humans have very complex social groups that take on many different forms. Language can be used to codify the behavior that is expected within a social group.
Mating and Reproduction
Humans often have sex that does not result in reproduction.
They have sex for pleasure, to cement relationships, to obtain resources and to assert dominance.
Humans do not have a breeding season, and they will have sex whether or not the female is fertile. They can be monogamous or polygamous.
Usually, human females give birth to one offspring at a time, but they can have multiple births, particularly twins.
Human babies require parental care for a long time.
In general, humans are omnivorous, although some humans survive on herbivorous diets. Humans sometimes restrict their diets because of cultural rules.
Some scientists believe that an increase in meat and fat consumption among the ancestors of humans was responsible for the growth of the human brain.
Humans change the composition of their food by cooking it. One hypothesis about the origin of cooking says that cooking food is easier to digest than raw food. By cooking their food, humans saved energy that would have been used for digestion and used that energy for hunting and other activities. The increase in energy could also have helped to increase brain size.
About 10,000 years ago, humans began developing agriculture and domesticating animals. This resulted in an increase in the amount and variety of food available.
Human diets vary considerably according to culture and the availability of different foods in different environments.
The wide variety of food resources has led to subpopulations of humans evolving different dietary adaptations. For example, a minority of humans have developed the ability to digest lactose (milk sugar) without any difficulty well into adulthood.
While all great apes have some tool-making ability and some language ability, in humans, these skills are exceptionally advanced.
Human technological ability has enabled humans to modify their environments so that they can live almost anywhere on Earth, and even in space. The invention of clothing has enabled humans to live in cold climates despite a lack of thick hair.
Humans have complex languages with which they can express many different abstract concepts. They use language to discuss their own thoughts and the thoughts of other humans.
They develop and communicate hypotheses about the things they observe around them. This has led to the development of religion, philosophy and science.
Humans have an aesthetic sense. They create art, music, literature and fashion.