Jared Diamond (author of the indispensible, highly recommended Guns, Germs, and Steel: The Fates of Human Societies and the fascinating Why Is Sex Fun?) has the scoop, and the answer may surprise you: it's agriculture.

Some good stuff here:

To people in rich countries like the U. S., it sounds ridiculous to extol the virtues of hunting and gathering. But Americans are an élite, dependent on oil and minerals that must often be iimproted from countries with poorer health and nutrition. If one could choose between being a peasant farmer in Ethiopia or a bushman gatherer in the Kalahari, which do you think would be the better choice?

And:

Archaeologists studying the rise of farming have reconstructed a crucial stage at which we made the worst mistake in human history. Forced to choose between limiting population or trying to increase food production, we chose the latter and ended up with starvation, warfare, and tyranny.

Hunter-gatherers practiced the most successful and logest-lasting life style in human history. In contrast, we’re still struggling with the mess into which agriculture has tumbled us, and it’s unclear whether we can solve it. Suppose that an archaeologist who had visited from outer space were trying to explain human history to his fellow spacelings. He might illustrate the results of his digs by a 24-hour clock on which one hour represents 100,000 years of real past time. If the history of the human race began at midnight, then we would now be almost at the end of our first day. We lived as hunter-gatherers for nearly the whole of that day, from midnight through dawn, noon, and sunset. Finally, at 11:54 p. m. we adopted agriculture. As our second midnight approaches, will the plight of famine-stricken peasants gradually spread to engulf us all? Or will we somehow achieve those seductive blessings that we imagine behind agriculture’s glittering façade, and that have so far eluded us?

And the real question:

Are twentieth century hunter-gatherers really worse off than farmers? Scattered throughout the world, several dozen groups of so-called primitive people, like the Kalahari bushmen, continue to support themselves that way. It turns out that these people have plenty of leisure time, sleep a good deal, and work less hard than their farming neighbors. For instance, the average time devoted each week to obtaining food is only 12 to 19 hours for one group of Bushmen, 14 hours or less for the Hadza nomads of Tanzania. One Bushman, when asked why he hadn’t emulated neighboring tribes by adopting agriculture, replied, "Why should we, when there are so many mongongo nuts in the world?"

The guy makes a lot of sense.

(from Discover Magazine, May 1987. I found this link on a Metafilter post about the coming bird flu plague, which will surely kill us all -- that is, if the asteroids don't get us first.)