Humans did not evolve from monkeys or chimpanzees as was once thought. We did have common ancestors, but present day monkeys and chimpanzees have evolved from those ancestors just as the human linage has evolved. The way we should conceptualize our relationship to primates is that some time ago (estimates range from 5 to 15 million years ago), the primates and hominids split. The primary difference was that primates walk on all fours while hominids walk on two legs.
What is the advantage of walking on two legs? First, your other two limbs, or arms, are free to carry and throw objects. Both have important implications for gathering and hunting food. It is also an advantage for living in non forested areas. The great apes, which walk on all four legs, are found only in forests.
There are today, over 3,000 fossils of species more closely related to humans than any of the living primates. Fossils of the earliest known hominids are referred to as Australopithecus. This classification includes several different species whose fossils range from 5 million to 1 million years old. While walking upright, they had some ape-like features including brains (450 grams) about the size of chimpanzees (400 cc.). All fossil remains of Australopithecus come from Eastern and Southern Africa. There is no evidence any of these species ever migrated out of Africa. They probably did not use fire and while they may have used rocks and sticks as tools it does not appear they made stone tools.
The next group of hominids to show up in the fossil records are a genus known as Homo. Early Homo species appear in the fossil record about 2½ million year ago. The Homo line had larger brains than the Australopithecus. Unlike the Australopithecus, some Homo's made it out of Africa and into Asia and Europe. Early Homo species had brains of about 700 grams. Homo erectus 800 - 1200 grams. Several of these species used fire and made tools.
Fire offers a number of advantages.
A European species referred to as Neanderthals had brains as large as modern humans (1500) and lived as recently as 28,000 years ago, coexisting with modern humans for over 70,000 years. The Neanderthals' used fire, made tools and buried their dead.
Modern humans are the only hominid species to have survived. The oldest fossils of modern humans are found in Africa and are estimated to be approximately 100,000 years old. Like earlier Homo species, modern humans moved out of Africa into Asia and Europe. Estimates are that humans migrated to Australia between 40 and 80,000 years ago and finally to the Americas 15,000 years ago. Homo sapiens now inhabit every continent and have been to the moon.
Language: From one brain to another
For further reading on the evolution of hominids:
Foley, Robert. 1995. Humans before Humanity. Cambridge:Blackwell Publishers, Inc.
Tudge, Colin. The Time Before History: 5 Million Years of Human Impact. New York:Scribner.