(FORTUNE Magazine) – Ever since the first stone tools appeared more than two million years ago in East Africa, humanity has evolved in tandem with the tools and machines it has invented.

But now the evolutionary tracks of humankind and technology are beginning to overlap so completely that the very meaning of "human being" may change. In this new relationship, technology is expanding humankind beyond the limits of flesh and blood, spawning a futuristic species that sees farther, runs faster, even lives longer than the standard, unalloyed biological human.

That may sound like science fiction--but it's not, says Bruce Mazlish, professor of history at the Massachusetts Institute of Technology and author of The Fourth Discontinuity: The Co-Evolution of Humans and Machines. "We cannot think realistically anymore of the human species without machines," he says. "Human nature is now absolutely and indissolubly connected to the machines we create."

If Mazlish is right, then humankind may soon experience its biggest identity crisis since Darwin. "The struggle we now face goes back centuries in which human beings have had fantasies about machines--about what form machines will take, whether human beings or machines will dominate, and whether humans will end up becoming like machines," says Mazlish. Genetic engineering and advances in computer intelligence will only further confuse humankind's sense of self, he adds. "A lot of this is reality now. If you like, our best and worst dreams have come true."

On the following pages, photographer James Balog documents what he calls Techno sapiens: fusions of humans and machines that can be found today in American research labs and hospitals, and even on the streets. Add up the images, says Balog, and it's not hard to envision a race of flesh-and-technology beings with electric hands, legs of steel that run a two-minute mile, and perceptual powers unknown in nature. "Imagine you are a traveler from another galaxy," Balog says. "You land in North America today and look around carefully, with fresh eyes. This is what you might see."

The prospect of Techno sapiens may seem ominous. But Mazlish, for one, sees the soul in the new machine: "I think, 'My God! What a magnificent creature mankind is to be able to make these machines. We are the creature that creates machines.'"