Anthropic constraint on transient variations of fundamental constants

The anthropic principle implies that life can emerge and be sustained only in a narrow range of values of fundamental constants. Here we show that anthropic arguments can set powerful constraints on transient variations of the fine-structure constant α over the past 4 billion years since the appearance of lifeforms on Earth. We argue that the passage through Earth of a macroscopic dark matter clump with a value of α inside differing substantially from its nominal value would make Earth uninhabitable. We demonstrate that in the regime of extreme variation of α, the periodic table of elements is truncated, water fails to serve as a universal solvent, and protons become unstable. Thereby, the anthropic principle constrains the likelihood of such encounters on a 4-billion-year timescale. This enables us to improve existing astrophysical bounds on certain dark matter model couplings by several orders of magnitude.