Sterols are important molecular components of the plasma membranes of eucaryotic cells. Using deuterium NMR spectroscopy in conjunction with statistical mechanical modelling, we present a unifying picture of how the evolution-engineered differences in molecular chemistry between cholesterol and its precursor lanosterol are manifested in the physical properties of model membranes in terms of molecular order and phase equilibria. Cholesterol optimizes the stability of a particular membrane phase, the liquid-ordered phase, that is a liquid and at the same time exhibits high molecular conformational order.