Before the 2002 announcement that Sahelanthropus chadensis was 7 to 6 million years old, the oldest known human was thought to be Australopithecus afarensis, which is 4 million years old. The fossil of a female named "Lucy" excavated in Ethiopia in 1974 indicates that her brain size was about the same as that of a chimpanzee (about 400 mL) and that she was a good tree ^climber2). 2) These ancestors are sometimes called "ape-men" because they possessed characteristics of both apes and humans.

Apes were able to stand upright and walk on two legs. This "upright bipedal walking" is a major event in human history. The reason is that walking on two legs enables us to do many ^things3). For example, they can see farther, making it easier to spot enemies; they look bigger and stronger than if they stood on four legs; they have an advantage in hot grasslands because the area exposed to the sun is smaller when they stand on two legs; and they can walk on their hands. Even better, they can carry things and use tools with both ^hands*. One can imagine that the use of their hands allowed them to be more dexterous, and their brains developed as well.

Two million years ago, a group called "proto-humans" emerged, with a brain size of about 1,000 mL. They made tools made of stone (stone tools) and were able to use fire, which allowed them to live in Europe and Asia, which were colder than Africa.

Similarly extinct were the "Neanderthals," who lived in Europe from 400,000 to 40,000 years ago. Their brain size was larger than that of present-day humans, about 1500 mL, and they wore clothes and accessories made of animal teeth and shells, and painted in ^caves4). Although Neanderthals are thought to have died out due to the cold, traces of their lives remain today. A genetic study of the present humans has revealed that they inherited Neanderthal ^genes5).

Homo sapiens, which appeared in Africa 300,000 years ago, ⁶⁾ lived at the same time as Neanderthals and eventually spread around the world in place of Neanderthals. The Homo sapiens "Cro-Magnon" found in France in 1868 is thought to be part of the ancestors of modern Europeans. What differentiates Homo sapiens from earlier humans is that it became fluent in language. They were able to think in complex ways and adapt to different environments. Eventually, they left Africa for Europe, Asia, Australia, and the Americas.

Thus, since humans parted from the chimpanzee genus and began to evolve, several species of humans have been born and disappeared. This is a very long path, and the only way to solve the riddle is to infer it from fossils, remains, genes, and so on. Therefore, if new fossils or remains are found, or if new information is obtained from genes, history will change dramatically. Homo sapiens means "wise man" or "wise man" in Latin. We can be sure that smart people will continue to use science to solve the riddles of evolution.