An all-embracing thesis about life on Earth, eye-catchingly named after an ancient myth? That would be James Lovelock's Gaia hypothesis, with its appealing image of a gigantic, self-regulating system, maintaining the conditions for the biosphere to flourish.

Enter Peter Ward, palaeontologist, popular author and expert on mass extinctions. The ideas behind the Gaia theory are far from the truth, he suggests. Instead, we should accept the directly contrary notion: life is, in the end, suicidal. Hence another label - the Medea hypothesis, for the tragic Greek heroine who murdered her children.

Medea is Gaia's evil twin, embodying the idea that organisms are driven to look after their own reproductive interests, and to hell with the consequences. This is not just an incidental property of life. It is an evolutionary requirement.

Ward's position thus hews to an interpretation of Darwinism that excludes everything except the strictest competition for survival. But the really important relationships are not so much between organisms as between life and the land, air and seas, and this is the greatest insight that came from Lovelock's original thinking about Gaia.

The way we now try and understand the whole Earth system takes account of life's effects on ocean and atmospheric composition, rock weathering, clouds and a complex of biogeophysical cycles linking them together. Gaia's more respectable offspring, earth systems science, is interested in the effects of all these interactions, and whether they help stabilise the planet.

Again, Ward is clear that they do not. As he puts it, "aggregates of species interacting with the physical environment as well as other life ... have effects not selected for - lethal effects". His evidence comes mainly from his analyses of past mass extinctions. With one exception, he maintains, these were a result of climatic effects induced by burgeoning life.

Ironically, the odd one out is the episode 65 million years ago that saw the death of the dinosaurs in the wake of a large asteroid impact, and that has been Ward's own area of special study from his University of Washington base. However, he draws on others' work to make a good case that the rest are Medean. Thus, the two spells when life suffered extreme cold on a "snowball Earth" were probably triggered by wildly proliferating plants mopping up atmospheric carbon dioxide, and the consequent global cooling. An earlier mass extinction, by contrast, was induced by the advent of oxygen producers, poisoning their microbial forebears; another - earlier still - by the outpourings of methane producers.

In each case, there were interactions with the rest of the Earth's systems that shifted things further in a direction harmful to life. Ward catalogues other climate shifts in similar terms. Reconstructions of such long-ago events are always open to argument, and he probably overstates the case when he says Medea's handiwork is everywhere.

Ward is persuasive in claiming that the Earth damps down fluctuations in environmental conditions less often than it amplifies them. That relates to his other main aim, which is to dispel any faith in a natural balance. He suggests that this view is at the core of modern environmentalism, which interprets Gaia theory to mean that, if we just messed about with it less, harmony would be restored to an essentially benign nature. But his chapter on environmental philosophy - which he mentions twice that he hated writing - is pretty superficial.

Part of the reason for its inclusion is that Ward thinks this all ties in with urgent current concerns about global climate change. Here, he is less convincing than in his overall critique of Gaian self-regulation. It is already pretty certain that there are more positive feedbacks (making a bad situation worse) than negative ones (making it better) waiting to kick in as global temperatures rise. Whether that is because the system is inherently unstable, or because we have overwhelmed any capacity it may have to even out big changes, makes little difference to the outcome.

Similarly, opinion is already shifting towards exploring the possibility of geo-engineering to avert catastrophe, in view of the likelihood that rhetoric about reducing carbon-dioxide emissions will fall short of reality. Ward's long view tells him that some such measures will be needed again in perhaps half a billion years' time to keep the Earth habitable. That is intriguing, but not that relevant to the nearer-term prospect of melting ice caps and rising seas in the next few hundred years. This provocative book would have worked better if he had stuck with the geological timescales where he is most at home.