Ever since Charles Darwin, most scholars have come to view human beginnings in the context of biological change on the planet. In considering this enormous process, we operate on a timescale quite different from the billions of years that mark the history of the universe and of the earth. According to archeologists and anthropologists, the evolutionary line of descent leading to Homo sapiens diverged from that leading to chimpanzees, our closest primate relatives, some 5 million to 6 million years ago, and it happened in eastern and southern Africa. There, perhaps twenty or thirty different species emerged, all of them members of the Homininae (or hominid) family of human-like creatures. What they all shared was bipedalism, the ability to walk upright on two legs. In 1976, the archeologist Mary Leakey uncovered in what is now Tanzania a series of footprints of three such hominid individuals, preserved in cooling volcanic ash about 3.5 million years ago. Two of them walked side by side, perhaps holding hands.

Over time, these hominid species changed. Their brains grew larger, as evidenced by the size of their skulls. About 2.3 million years ago, a hominid creature known as Homo habilis began to make and use simple stone tools. Others started to eat meat, at least occasionally. By 1 million years ago, some hominid species, especially Homo erectus, began to migrate out of Africa, and their remains have been found in various parts of Eurasia. This species is also associated with the first controlled use of fire.

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Eventually all of these earlier hominid species died out, except one: Homo sapiens, ourselves. With a remarkable capacity for symbolic language that permitted the accumulation and transmission of learning, we too emerged first in Africa and quite recently, probably no more than 250,000 years ago (although specialists constantly debate these matters). For a long time, all of the small number of Homo sapiens lived in Africa, but sometime after 100,000 years ago, they too began to migrate out of Africa onto the Eurasian landmass, then to Australia, and ultimately into the Western Hemisphere and the Pacific islands. The great experiment of human history had begun.