Biologists explain the similarity between humans and other hominoids by their descent from a common ancestor. In 2001, a hominid skull was discovered in Chad. The skull is about 7 million years old, and has been classified as "Sahelanthropus tchadensis". This skull may show that the date at which humans started to evolve (develop differently) from other primates is 2 million years earlier than scientists had previously thought.
Humans are part of a subfamily called the Homininae (or hominins), inside the hominids or great apes.
Long ago, there used to be other types of hominins on Earth. They were like modern humans, but not the same. "Homo sapiens" are the only type of hominins who are alive today. The earliest known fossils of genus "Homo" have been called "Homo habilis" (handy man). The first fossils of "Homo habilis" were found in Tanzania. "Homo hablilis" is thought to have lived about 2.2 to 1.7 million years ago. Another human species thought to be an ancestor of the modern human is "Homo erectus". There are other extinct species of "Homo" known today. Many of them were likely our 'cousins', as they developed differently than our ancestors. A theory called the Sahara pump theory has been used to tell how different species of plants and animals moved from Africa to the Middle East, and then elsewhere. Early humans may have moved from Africa to other parts of the world in the same way.
The first truly modern humans seem to have appeared between 300,000, and 200,000 years ago in East Africa. In paleontology, 200,000 years are a "short" time. So, scientists speak of a "recent single origin" of humans. These early humans later moved out from Africa. By about 90,000 years ago they had moved into Eurasia. This was the area where Neanderthals, "Homo neanderthalensis", had been living for a long time (at least 350,000 years).
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 this event 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.
Early human history is commonly divided into three ages. The time periods are labeled with the material used for tools.
The "Stone Age" is commonly subdivided into the Paleolithic, Mesolithic, and Neolithic periods.
Up to about 10 thousand years ago most humans were hunter-gatherers. They did not live in one place, but moved around as the seasons changed. The start of planting crops for food, called farming made the Neolithic revolution. Some people chose to live in settlements. This also led to the invention of metal tools and the training of animals. About 6000 years ago the first proper civilizations began in places like Egypt, India, and Syria. The people formed governments and armies for protection. They competed for area to live and resources and sometimes they fought with each other. About 4000 years ago some states took over or conquered other states and made empires. Examples include ancient Greece and the Roman Empire.
Some modern day religions also began at this time such as Judaism and Hinduism. From the Middle Ages and beyond humanity saw an explosion of new technology and inventions. The printing press, the car, the train, and electricity are all examples of this kind of invention. As a result of the developments in technology, modern humans live in a world where everyone is connected, for example by telephone or by internet. People now control and change the environment around them in many different ways.
In early times, humans usually settled near to water and other natural resources. In modern times if people need things they can transport them from somewhere else. So basing a settlement close to resources is no longer as important as it once was. Since 1800, the number of humans, or population, has increased by six billion. Most humans (61%) live in Asia. The rest live in the Americas (14%), Africa (14%), Europe (11%), and Oceania (0.5%).
Most people live in towns and cities. This number is expected to get higher. In 2005 the United Nations said that by the end of that year, over half the world would be living in cities. This is an important change in human settlement patterns: a century earlier in 1900 only 14% of people lived in cities, in 2000 47% of the world's population lived in cities. In developed countries, like the United States, 80% of the population live in cities.
Humans have a large effect on the world. Humans are at the top of the food chain and are generally not eaten by any animals. Humans have been described as super predators because of this. Because of industry and other reasons humans are said to be a big cause of global climate change.
Human body measurements differ. The worldwide average height for an adult human male is about , and the worldwide average height for adult human females is about . The average weight of an adult human is for females and for males. Body weight and body type is influenced by genetics and environment. It varies greatly among individuals.
Human hair grows on the underarms, the genitals, legs, arms, and on the top of the head in adults of both genders. Hair will usually grow on the face of most adult males, and on the chest and back of many adult males. In human children of both genders, long hair grows only on the top of the head. Although it might look like humans have fewer hairs than most primates, they actually do not. The average human has more hair "follicles", where hair grows from, than most chimpanzees have. Human hair can be black, brown, red or blond. When humans get older hair can turn grey or white.
Human skin colors vary greatly. They can be a very pale pink all the way to dark brown. There is a reason why people in tropical areas have dark skins. The dark pigment (melanin) in the skin protects them against ultraviolet rays in sunlight. The damage caused by UV rays can and does cause skin cancer in some people. Therefore, in more sunny areas, natural selection favors darker skin color. Sun tanning has nothing to do with this issue, because it is just a temporary process which is not inherited. In colder climates the advantage of light-colored skin is that it radiates less heat. Therefore, in less sunny areas, natural selection favors lighter skin color.
Humans are not as strong as other primates of the same size. An average female orangutan is at least three times as strong as an average human.
The average human male needs 7 to 8 hours sleep a day. People who sleep less than this are generally not as healthy. A child needs more sleep, 9 to 10 hours on average.
The human life cycle is quite like most other mammals. The young grow inside the female mother for nine months. After this time the baby is pushed out of the woman's vagina.
However, unlike most other mammals, human childbirth is somewhat dangerous. Babies' heads are large, and the mothers pelvis bones are not very wide. Since people walk on two legs, their hips are fairly narrow. This means that birth can be difficult. Rarely, mother or baby may die in childbirth. The number of mothers dying in childbirth is less in the 21st century. This is because of better medication and treatment. In many poor countries the number of mothers dying is higher. Sometimes it is up to 10 times as many as richer countries.
The average human baby weighs 3–4 kg at birth and is 50–60 cm tall. Babies are often smaller in poorer countries. Babies in poor countries may die early because of this.
Humans have four stages in their lives: childhood, adolescence, adulthood and old age.