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 {\em transient} variations of the fine-structure constant $\alpha$ 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 $\alpha$ inside differing substantially from its nominal value would make Earth uninhabitable. We demonstrate that in the regime of extreme variation of $\alpha$, 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.