The Peopling of the World

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.

The art and beauty of the brain

Imagine the brain, that shiny mound of being, that mouse-gray parliament of cells, that dream factory, that petit tyrant inside a ball of bone, that huddle of neurons calling all the plays, that little everywhere, that fickle pleasuredrome, that wrinkled wardrobe of selves stuffed into the skull like too many clothes into a gym bag. The neocortex has ridges, valleys, and folds because the brain kept remodeling itself though space was tight. We take for granted the ridiculous-sounding yet undeniable fact that each person carries around atop the body a complete universe in which trillions of sensations, thoughts, and desires stream. They mix privately, silently, while agitating on many levels, some of which we're not aware of, thank heavens. If we needed to remember how to work the bellows of the lungs or the writhing python of digestion, we'd be swamped by formed and forming memories, and there'd be no time left for buying cute socks. My brain likes cute socks. But it also likes kisses. And asparagus. And watching boat-tailed grackles. And biking. And drinking Japanese green tea in a rose garden. There's the nub of it-the brain is personality's whereabouts. It's also a stern warden, and, at times, a self-tormentor. It's where catchy tunes snag, and cravings keep tugging. Shaped a little like a loaf of French country bread, our brain is a crowded chemistry lab, bustling with nonstop neural conversations. It's also an impersonal landscape where minute bolts of lightning prowl and strike. A hall of mirrors, it can contemplate existentialism, the delicate hooves of a goat, and its own birth and death in a matter of seconds. It's blunt as a skunk, and a real gossip hound, but also voluptuous, clever, playful, and forgiving.