Anthropocentrism holds that the only things valuable in themselves are: human beings, their desires and needs, and the satisfaction of those. In turn, Gaia theory holds that the Earth and all creatures on it constitute something akin to a vast living being. Many layfolk maintain that Gaia theory implies the falsity of anthropocentrism, and thus puts the kibosh on that doctrine. But philosophical writers deny this implication. This paper therefore argues for what we may call “the Kibosh Thesis”—that Gaia theory, when correctly understood, does indeed put the kibosh on anthropocentrism. It defends this thesis by appealing to “the Part-Whole Thesis”—that no parts of a living being which do not constitute the whole being can have as much intrinsic value as the being itself has. Since the evidence supporting Gaia theory is mounting, this thesis appears to provide a fairly strong argument against anthropocentrism. In arguing for this position, I show why anthropocentrism is a plausible doctrine, specify Gaia theory’s main claims, meet the main philosophical objections to the Kibosh Thesis, and develop the argument from the Part-Whole Thesis.