The Evolutionary Accident Unique to Homo Sapiens

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IIn 1979, renowned evolutionary biologists Stephen J. Gould and Richard Lewontin published “The San Marco Spandrels and the Panglossian Paradigm,” a relatively short treatise pushing back against what they saw as a disconcerting trend in their field.

Evolutionary biologists, both men wrote, had become enamored with atomizing organisms into distinct traits and telling “little stories” about how these individual aspects had been optimized by natural selection to achieve their ideal form.

To illustrate the error of this way of thinking (what they called “the adaptationist program”), they used the example of the “spandrels” of St. Mark’s Cathedral in Venice. A “spand” is the curved triangular space between two arches perpendicular to each other, and at St. Mark’s they are decorated with detailed mosaics.

It would be wrong to say that the spandrels were created for the purpose of displaying mosaics, Gould and Lewontin explained. Instead, they were architectural by-products of arched constructions, decorated after the fact. And just as spandrels are byproducts of the constraints of architectural space, many biological traits are byproducts of the constraints of physical and developmental processes, Gould and Lewontin argued.

Since the publication of their paper, the term “spand” has become shorthand among evolutionary biologists for a fortuitous trait or result of a byproduct of evolution. Now, a new study published in PLOS One studies the potential extent of a human trait: the chin.

Read more: “What made the first humans intelligent”

Absent in other primates – and even in Denisovans and Neanderthals – the bony, protruding chin is a uniquely human feature and often used to identify members of our species in the fossil record. As such, it is tempting to indulge in another uniquely human trait (storytelling) and find a reason why it was perfected by natural selection. Among other things, supporting the lower jaw to facilitate chewing or acting as a secondary sexual characteristic to announce maturity to partners, are two such stories.

To study theories of chin evolution, researchers at the University at Buffalo examined the genetic sequences involved in head and jaw development for evidence of evolution. Specifically, the team examined whether the sequences involved in producing the chin itself were subject to direct selection, whether they arose neutrally due to genetic drift, or whether they were simply a byproduct of evolution acting on other traits (a spandrel). They found that the evidence indicated that the chin was a spandrel.

“The chin evolved largely by accident and not by direct selection, but as an evolutionary byproduct resulting from direct selection on other parts of the skull,” study author Noreen von Cramon-Taubadel said in a statement.

The traits that natural selection most likely acted on included a reduction in the size of anterior teeth and other cranial changes linked to the evolution of bipedalism, the authors said.

“Although we find evidence of direct selection on parts of the human skull, we find that traits specific to the chin region fit the spandrel model better,” explained von Cramon-Taubadel. “The changes since our last common ancestor with the chimpanzee are not due to natural selection on the chin itself but to selection on other parts of the jaw and skull.”

It’s an important reminder that traits must be considered in context and that not all differences between species are the result of an all-powerful force of natural selection.

“Generating empirical evidence against this reasoning is an important goal of this study and of biological anthropology in general,” von Cramon-Taubadel said. “The results highlight the importance of assessing the evolution of physical characteristics with trait integration in mind. »

Enough to stroke your chin and think.

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Main image: cybermagician / Shutterstock

• Jake Currie

  Published on February 17, 2026

  Jake Currie is a writer based in Brooklyn, New York.