Are human chins a spandrel? New research suggests the chin is a byproduct of jaw and skull evolution

In this Science Quickly episode, researchers explore the human chin, a trait unique to our species. Lauren Schroeder explains that the chin is likely a byproduct of broader evolutionary changes in the jaw and cranium, rather than a trait directly selected for its chin. The discussion covers competing hypotheses, the methods used to test them, and how dental reduction appears to drive the chin as a byproduct of jaw evolution. The result challenges the idea that chins evolved primarily for chewing, speech, or sexual selection, and instead positions the chin as a spandrel-like outcome of integrated craniofacial evolution.

Overview

The podcast examines a long-standing question in human evolution: why do we have chins? Lauren Schroeder, a paleoanthropologist at the University of Toronto, Mississauga, provides a concise summary of the issue, noting that chins are a distinctly human feature and that Neanderthals did not share this trait. The episode situates the chin within the broader morphological landscape of the skull and jaw, emphasizing that to understand the chin one must view it as part of an integrated craniofacial system.

Defining the chin and its uniqueness

The chin, scientifically termed a mental protuberance, is described as a bony projection at the lower jaw where the jaw meets. The host and Schroeder discuss the chin’s uniqueness to Homo sapiens and the absence of a chin in our closest relatives, such as Neanderthals. This sets the stage for exploring evolutionary explanations beyond simple functional necessity.

"The chin is a bony protuberance at the lower part of the jaw. We are the only species to have one. Even our closest ancestor, Neanderthals, did not have a chin." - Lauren Schroeder, paleoanthropologist at the University of Toronto, Mississauga

The competing hypotheses

The paper tested three broad ideas: direct selection on the chin (adaptive meaning or function), a byproduct of natural selection acting on other jaw or skull structures, and genetic drift (neutral evolution). Schroeder explains the logic behind these hypotheses and the importance of analyzing chin traits within the context of the entire craniofacial system rather than in isolation.

Methods and data

The researchers compared a wide range of primates, including humans, and traced the evolutionary changes in jaw and cranium over time using an evolutionary tree. The aim was to detect signatures of directional selection specifically on chin traits versus other craniofacial features. The approach emphasizes integration of morphology, measuring how different parts of the skull and jaw co-evolve across species and through time.

Key findings and interpretation

The episode highlights a central finding: there is strong evidence that direct selection affected the cranium and jaw, but the chin itself does not show a clear directional selection signature. This suggests that the chin is more likely a byproduct of evolution in other regions of the skull and jaw, rather than a trait that was directly selected for its own benefit. Schroeder summarizes the conclusion as a byproduct of evolution in other spaces, with dental reduction playing a pivotal role in generating lower-jaw forces that lead to chin development.

"The strongest directional selection signatures tied to dental reduction suggest that as the jaws and alveolar region shrink, byproduct forces in the lower jaw contribute to chin formation." - Lauren Schroeder, paleoanthropologist at the University of Toronto, Mississauga

The spandrel analogy and broader implications

The researchers invoke the spandrel concept, borrowed from architecture and popularized by Gould and Lewontin, to illustrate how evolution can produce traits that are not direct adaptations. A spandrel is a byproduct of two arches coming together, not something designed for its own sake. The chin is presented as a potential biological spandrel, arising from the interplay of jaw reduction and cranial restructuring rather than from direct selection for a chin itself.

"A spandrel is the concept of it in evolutionary biology comes from Stephen Jay Gould and Richard Lewontin. They argue that not every trait is an adaptation, and some traits arise as a byproduct of other evolutionary forces on other structures. The spandrel analogy is from architecture, where arches create triangular spaces that are a byproduct. Evolution can behave the same way." - Lauren Schroeder, paleoanthropologist at the University of Toronto, Mississauga

Broader context and future directions

While the chin appears to be a byproduct, the discussion underscores a broader shift in evolutionary biology toward recognizing byproducts in morphological integration. The team points to other traits where direct selection may be present on one structure while another trait rides along as a byproduct. Schroeder also notes interest in pursuing further research on different traits and explaining how the craniofacial system evolves as a whole. The interview ends with a reflection on the ongoing nature of chin research and the possibility of studying additional integrated traits in the future.

"In recent years, there’s been a push to not explain every trait as having functional significance. Testing morphological integration of the cranium and mandible sits within this shift, and the chin as a potential byproduct has been hypothesized before." - Lauren Schroeder, paleoanthropologist at the University of Toronto, Mississauga