NPR has introduced me to the Medea Principle; just as the Gaia Principle states that a planet can be thought of as a single living organism, this idea states that the single biggest threat to life on other planets…is life:
Drawing on a detailed study of the geological record [University of Washington paleontologist Peter] Ward argues that rather than James Lovelock’s “Gaia” – the supportive Earth-mother – a better image to carry in thinking about life and the planet is Medea the mother in Greek Myth who killed her own children.
A principle point Ward deploys for his unusual thesis is the simple fact that most of the planet’s dramatic and dangerous mass extinctions were caused not by outside events but by life itself changing (polluting?) the environment on which it depends. In Ward’s view life has “biocidal” tendencies, which over time have led to sharp declines in biomass and diversity.
Life, in composite, does have a way of screwing with numbers. Ward’s book is here on Amazon, and more thoroughly reviewed yonder.