Astrobiology. 2016 Jan;16(1):7-22. The Case for a Gaian Bottleneck: The Biology of Habitability. Chopra A1, Lineweaver CH1. The primary idea of the Gaia Hypothesis - Lynn Margulis - is controversial, so I expect this will be. But it may also be a way to answer to Fermi's question: 'Where are they?' - meaning why have we not detected intelligent life elsewhere in the galaxy.