Only humans have chins: Study shows it's an evolutionary accident. Really!

Does it not provide any significant benefits to humans? How about speech and eating?

So far this paper is only published on PLOS.

The lead author Noreen von Cramon-Taubadel has only less than 4900 lifetime citations!

Why these authors even used the term "spandrel" (the space between the shoulders of adjoining arches) in the title of their paper?

"... "The chin evolved largely by accident and not through direct selection, but as an evolutionary byproduct resulting from direct selection on other parts of the skull," says Noreen von Cramon-Taubadel, Ph.D., professor and chair of the UB Department of Anthropology in the College of Arts and Sciences. ..."

From the abstract:

"Humans are unique among primates in possessing a chin, yet it is currently unclear whether the form of the symphyseal region of the mandible where the chin is located is the product of direct selection or a by-product of evolutionary pressures on other craniomandibular features.

Here, we conduct an evolutionary analysis of hominoid craniomandibular traits to test three hypotheses: symphyseal mandibular traits evolved

(1) neutrally due to genetic drift,

(2) under direct selection, and 

(3) as a by-product (or “spandrel”) of selection on other craniomandibular traits. Evolutionary rates of morphological change, via Lande’s generalized genetic distance, were estimated along each branch of a fully-resolved hominoid phylogeny to reveal patterns of neutral, stabilizing and directional selection.

Directional selection was detected along the branch between humans and the last common ancestor of chimpanzees and humans, against a backdrop of pervasive stabilizing selection and neutral evolution in hominoids. Significant directional selection was found on cranial traits reflecting increased basicranial flexion, neurocranial expansion, and reduction in lower facial prognathism, and on mandibular traits that generate a more parabolic-shaped, gracile mandible with a smaller ramus and shallower corpus.

In contrast, of the nine mandibular “chin” traits, only three were under significant direct selection, while the other six were either under no selection or indirect selection. Thus, the results are consistent with the hypothesis that the symphyseal morphology that forms the human chin evolved largely as a by-product (i.e., spandrel) [???] of direct selection for reduced anterior dental size and the craniofacial changes correlated with the evolution of bipedalism in hominins, rather than as a specific adaptation."

Only humans have chins: Study shows it's an evolutionary accident

Is the human chin a spandrel? Insights from an evolutionary analysis of ape craniomandibular form

Fig 2. Craniomandibular landmarks with all 22 cranial and 24 mandibular interlandmark distances considered here.

Landmark definitions can be found in S1 Table. Solid colored lines show the expected selection gradients to increase (red) and to decrease (blue) for the nine mandibular symphyseal traits related to the evolution of a chin under a hypothetical model of direct selection along the Homo sapiens branch since the last common ancestor with chimpanzees. [Notice, the authors forgot to explain the meaning of the capital letters!]