What has proved an unsustainable burden to the life of the planet is also proving unsustainable for the planet's dominant species. In China a recent shift to meat-heavy diets has been linked to increases in obesity, cardiovascular disease, breast cancer and colorectal cancer. U.S. and World Health Organization researchers have announced similar findings for other parts of the world. And then there are the growing concerns about what happens to people who eat the flesh of animals that have been pumped full of genetically modified organisms, hormones and antibiotics.

These concerns may seem counterintuitive. We evolved as hunter-gatherers and ate meat for a hundred millenniums before modern times. It's natural for us to eat meat, one might say. But today's factory-raised, transgenic, chemical-laden livestock are a far cry from the wild animals our ancestors hunted. When we cleverly shifted from wildland hunting and gathering to systematic herding and farming, we changed the natural balances irrevocably. The shift enabled us to produce food surpluses, but the surpluses also allowed us to reproduce prodigiously. When we did, it became only a matter of time before we could no longer have the large area of wildland, per individual, that is necessary to sustain a top-predator species.

By covering more and more of the planet with our cities, farms and waste, we have jeopardized other top predators that need space as well. Tigers and panthers are being squeezed out and may not last the coming century. We, at least, have the flexibility--the omnivorous stomach and creative brain--to adapt. We can do it by moving down the food chain: eating foods that use less water and land, and that pollute far less, than cows and pigs do. In the long run, we can lose our memory of eating animals, and we will discover the intrinsic satisfactions of a diverse plant-based diet, as millions of people already have.

I'm not predicting the end of all meat eating. Decades from now, cattle will still be raised, perhaps in patches of natural rangeland, for people inclined to eat and able to afford a porterhouse, while others will make exceptions in ceremonial meals on special days like Thanksgiving, which link us ritually to our evolutionary and cultural past. But the era of mass-produced animal flesh, and its unsustainable costs to human and environmental health, should be over before the next century is out.

Ed Ayres is editorial director of the Worldwatch Institute and author of "God's Last Offer: Negotiating for a Sustainable Future."

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