DOI: 10.5061/dryad.ncjsxkt7k
title: Data from: Is the human chin a spandrel? Insights from an evolutionary analysis of ape craniomandibular form

description: 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.
description: # Data from: Is the human chin a spandrel? Insights from an evolutionary
 analysis of ape craniomandibular form Dataset DOI:
 [10.5061/dryad.ncjsxkt7k](10.5061/dryad.ncjsxkt7k)
 "S3_Table.xlsx" contains the logged cranial and mandibular
 traits (interlandmark distances) for all ape taxa used as input data for
 this study. ## Description of the data and file structure Logged cranial
 and mandibular interlandmark distances. ### Files and variables #### File:
 S3_Table.xlsx **Description:** Logged interlandmark distances #####
 Variables * *Taxon*: Current valid scientific taxon name (genus, species,
 subspecies) * *Sex*: Male (M) or Female (F) * *Cranial traits*: These
 codes represent the logged Euclidean distances between pairs of cranial
 landmarks labelled using the following codes: BA: Basion: The point where
 the anterior margin of the foramen magnum intersects the midsagittal plane
 HOR: Hormion: The midline point of attachment of the vomer and sphenoid
 bones IS: Intradentale superior (Prosthion): The most anterior midline
 point on the maxillary alveolus between the two central incisors PNS:
 Posterior nasal spine (Alveolon): The point on the interpalatal suture in
 line with the posterior margins of the alveolar process NA: Nasion: The
 point of intersection of the nasofrontal suture and the midsagittal plane
 SB: Sphenobasion: The midline point on the sphenooccipital suture SUB:
 Subspinale: The most inferior midline point where the nasal spine
 intersects with anterior edge of the maxilla APET: Anterior petrous
 temporal (Petrosal): The most anterior point of the petrous element of the
 temporal bone AM1: Anterior M1 (Alveolare): The most anterior point on the
 alveolus of the first molar C/P3: C/P3: The most inferior point on the
 external surface of the alveolar margin between the canine and P3 EAM:
 External auditory meatus (ant): The most anterior point on the margin of
 the external auditory meatus MT: Maxillary tuberosity: The most posterior,
 inferior point on the maxilla SP: Sphenion: The most anterior extent of
 the sphenoparietal suture (pterion) ZI: Zygomaxillare inferior
 (Zygomaxillare): The most inferior, anterior point on the
 zygomaticomaxillary suture ZS: Zygomaxillare superior (Zygoorbitale): The
 point where the zygomaticomaxillary suture intersects with the inferior
 orbital margin * *Mandibular traits*: These codes represent the logged
 Euclidean distances between pairs of mandibular landmarks labelled using
 the following codes: GNA: Gnathion: The most inferior midline point on the
 mandibular symphysis INFR: Infradentale: The most superior midline point
 on the buccal surface of the alveolus LIN: Linguale: The most
 superior-posterior point on the lingual superior transverse torus MO:
 Mandibular orale: The most superior midline point on the lingual surface
 of the alveolus ALV: Alveolus (pos): The most superior, posterior point on
 the alveolus CONL: Condylion (lateral): The most lateral point on the
 superior surface of the mandibular condyle CONM: Condylion (medial): The
 most medial point on the superior surface of the mandibular condyle COR:
 Coronion: The most superior point on the coronoid process GON: Gonion: The
 point of maximum curvature where the posterior ramus and the basal corpus
 intersect MC/P3: (Mandibular) C/P3: The most lateral point on the alveolus
 between the canine and P3 MEN: Mental foramen (ant): The most anterior
 point on the lateral edge of the mental foramen MFO: Mandibular foramen:
 The most anterior, inferior point on the medial edge of the mandibular
 foramen M3: M3 (lat-pos): The most lateral point on the alveolus posterior
 to M3 PM1: (Posterior) M1: The most superior, posterior point on the
 alveolus RAMA: Ramus (ant): The most anterior point on the ascending ramus
 in line with the alveolus RAMP: Ramus (pos): The most posterior point on
 the ascending ramus in line with the alveolus SIG: Sigmoid notch: The most
 superior point of maximum inflection in the depth of the sigmoid notch