The essentializing of ethno-racial groups has been suggested to be driven by an evolved system for biological reasoning. Data examining essentialism in Western, industrial (WEIRD) populations is often consistent with this hypothesis. However, other groups exhibit different patterns: some essentialize more, others less, and often patterns of essentialism do not match the expectations of biological analogy. Here, we outline a cultural evolutionary hypothesis for how WEIRD populations came to reason similarly about ethno-racial groups and biological species. Our argument is as follows: (i) until relatively recently Western thought did not reliably essentialize even biological species; (ii) the discovery of species fixism and later theories of hard heredity engendered species essentialism; (iii) mutationist theories, Mendelian inheritance and pure lines research programs extended essentialism to variation within species, including human populations; and (iv) this model of biological variation became embedded in WEIRD educational and economic systems, allowing it to persist even as biological theories moved on. We conclude that although essentialism in general may be an evolved feature of cognition, its relationship to biological thinking is not and is instead a product of WEIRD cultural evolution.