Scientists today estimate that the Earth is 4.6 billion years old, but that number has shifted more than a few times over the last century. The earliest signs of human activity did not appear until 2.7 million years ago, in the early Stone Age (or Lower Paleolithic era), which lasted until 200,000 years ago, although the species called Australopithecus dates back more than 4 million years ago. Africa is often referred to as "the cradle of humanity," and all the fossil discoveries were made there.
The earliest human ancestors--called hominids--walked upright on two legs and were scavengers. Homo erectus, another early hominid dating from 1.5 million years ago, evolved in Africa but migrated to Eurasia, the first human species to do so. These very early humans used fire (but probably could not make it), created stone weapons and tools, and were successful in occupying a wide range of habitats. Nevertheless, all non-African populations of Homo erectus eventually died out without leaving descendants.
Our own species, Homo sapiens, evolved from Homo erectus through various transitional stages in the savannah lands of eastern Africa about 150,000 years ago. Homo sapiens is a highly social and adaptable species, fully capable of using complex language. Modern humans moved out from the original species' homeland on the eastern plains of Africa to occupy much of eastern, northern, and southern parts of the continent. The special challenges of the rain forest environment slowed the movement of humans into the western regions of Africa.
Around 105,000 years ago, modern humans migrated northward through Egypt and out of Africa via the Sinai Peninsula to the Middle East. There they apparently met and coexisted with humans of a different and older species - Neanderthals (Homo neandertalensis)-- that had a simpler, less flexible culture and technology.