  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.
