The always excellent Dragon Tales science blog has a nice summary with some discussion of the Medea hypothethis and past extinction events in Earth's history attributed to being caused by life.

The "Medea Hypothesis is that life is suicidal and will exterminate itself by resource overuse and that it produces far more, or induces far more, positive feedbacks than correcting negative feedbacks. The negative feedbacks, he (author Peter Ward of 'Rare Earth' fame) feels, are absolutely necessary for the Gaia Hypothesis to work. The Medea Hypothesis is a reaction to the Gaia Hypothesis and proposed as an alternate.

Ward proposes that certain events in the evolution of life, in deep time, that support the Medea Hypothesis. I am going to run through them now. I will quote as much as possible and try to summarize otherwise."

VERY interesting reading.