We connect evidence that 20 °C is the most stable temperature for cellular processes with macroecological observations. Examples show that temperatures warmer than ~20 °C result in decreases in: aquatic species’ tolerance to low oxygen; marine pelagic and benthic algal productivity; pelagic and benthic predation rates; global species richness in pelagic fishes, plankton and benthic invertebrates; and genetic diversity; but increased extinctions in the fossil record. The realised thermal niche of reef fishes and invertebrates globally is narrowest among species with distributions centred on 20 °C, as also seen in microbes. While many species have evolved to live at warmer and colder temperatures, most species live at, and extinctions in the fossil record across seven phyla were lower at, 20 °C. The mathematical “Corkrey” model, which predicts that thermal breadth should be minimized and species richness maximised at 20 °C across all Domains of life, provides an explanation for this “20 °C effect”. A literature search found highest species richness at ~20 °C across life in air and water, including animals, plants and microbes. That life seems centred around ~20 °C implies fundamental constraints that compromise the ability of extant tropical species to adapt to higher temperatures.