Brocks, JochenBobrovskiy, Ilya2024-10-142024-10-140036-8075https://hdl.handle.net/1885/733721486Cholesterol is a flat, rigid molecule that slots into animal cell membranes, controlling physical properties such as membrane rigidity, fluidity, permeability, and curvature. Other eukaryotes have taken sterol biosynthesis further by adding alkyl groups to the side chain, such as phytosterols found in plants. However, it remains an enigma why the membranes of nearly all animals use cholesterol alone, whereas most other eukaryotes, including plants and fungi, contain complex mixtures of various sterols in their membranes. On page 520 of this issue, Michellod et al. (1) report the discovery of gutless worms (Olavius algarvensis) that endogenously produce cholesterol as well as phytosterols that are usually found in plants. By searching for the remains of such sterols in fossils, it may be possible to map the evolution of sterol biosynthesis in the animal kingdom and eventually understand why humans exclusively produce cholesterol.application/pdfen-AU© 2023 The authors202310.1126/science.adh80972024-02-18