Exaptation in Human Evolution. How to test adaptive vs. exaptive evolutionary hypotheses

Palaeontologists Stephen J. Gould and Elisabeth Vrba introduced the term “ex-aptation” (Gould, Vrba, 1982; Vrba, Gould, 1986) with the aim of improving and enlarging the scientific language available to researchers studying the evolution of any useful character, instead of calling it an “adaptation” by default, coming up to what Gould named an “extended taxonomy of fitness” (Gould, 2002). With the extension to functional co-optations from non-adaptive structures (“spandrels”), the notion of exaptation expanded and revised the neo-Darwinian concept of “pre-adaptation” (which was misleading, for Gould and Vrba, in suggesting foreordination). Exaptation is neither a “saltationist” nor an “anti-Darwinian” concept and, from 1982 to date, was adopted by many researchers in evolutionary and molecular biology, and particularly in human evolution. Exaptation was also addressed with some theoretical objections. With respect to one of these - the “non-operationality objection” - we analyze the possible operationalization of the concept in two recent meaningful studies, and identify six directions of empirical research, necessary to test “adaptive vs. exaptive” evolutionary hypotheses. We then write on a comprehensive survey (available online) of the literature, and on the basis of this we make a quantitative and qualitative evaluation of the adoption of the term among scientists who study human evolution.We discuss the epistemic conditions that may have influenced the adoption and appropriate use of exaptation, and comment on the benefits of an “extended taxonomy of fitness” in present and future studies on human evolution.