From Free to Freeloading

No parasite was born a parasite. Each one descends from a free-living ancestor that fended for itself. The shift began innocently: an organism living near a host, then on it, then quietly skimming its food. Generation by generation, natural selection favored the ones that took more and gave less. There was no single leap from guest to thief. It was a slow slide, stretched across millions of years, until taking from a host became a way of life.

Parasitism is not a rare accident. It evolved independently hundreds of times across the tree of life. Plants, fungi, insects, worms, single-celled protists, and bacteria each arrived at it on their own, with no shared blueprint. When the same strategy keeps reinventing itself in lineages that never met, that is nature's verdict: living off a host is one of the most profitable bets in evolution.