The Gaia hypothesis, proposed by the British scientist James Lovelock in the 1970s, states that life preserves the conditions for its own survival. Just as plants breathe out oxygen and humans breathe it in, the whole biosphere keeps the chemistry of air, oceans and soil in a balance that allows life to flourish. The entire planet works together as a giant living organism.

Peter Ward, a paleontologist at the University of Washington who specializes in mass extinctions, this year expressed a dimmer view of life on earth. He sees not a self-optimizing biosphere but a tangle of organisms that have evolved to starve their competitors and pollute their surroundings, behaving in ways that are ''inherently selfish and ultimately biocidal.'' In his book ''The Medea Hypothesis,'' named after the Greek mother who slaughtered her own children, Ward argues that for billions of years the biosphere has been its own worst enemy. ''Life seems to be actively pursuing its own demise,'' he wrote recently in New Scientist, ''moving earth ever closer to the inevitable day when it returns to its original state: sterile.''

According to Ward, the mayhem started soon after the emergence of bacteria billions of years ago, which choked the earth's atmosphere first with a heat-reflecting haze of methane and then, a billion years later, with dangerous levels of oxygen, which at the time was toxic to life. Soon after, plants sucked up so much carbon that temperatures plummeted, creating a pair of deep ice ages. Of the five great extinctions since the rise of animals, Ward claims, four were caused not by volcanoes or by meteors but by life itself. To top it off, biomass has been declining for the last billion years.

After the current round of man-made global warming, both Ward and Lovelock predict that our descendants will eventually confront a long-term drop in carbon dioxide that threatens to wipe out all plant life within roughly a billion years. Grim as it may seem, a Medean perspective could help us avoid environmental guilt and nostalgia as we face these crises. ''We must overcome nature,'' Ward writes, and later continues, ''We do not want to go 'back to nature.'''