Towards the restoration of ancient hominid craniofacial anatomy: Chimpanzee morphology reveals covariation between craniometrics and facial soft tissue thickness

Abstract

In modern humans, facial soft tissue thicknesses have been shown to covary with craniometric dimensions. However, to date it has not been confirmed whether these relationships are shared with non-human apes. In this study, we analyze these relationships in chimpanzees (Pan troglodytes) with the aim of producing regression models for approximating facial soft tissue thicknesses in Plio-Pleistocene hominid individuals. Using CT scans of 19 subjects, 637 soft tissue, and 349 craniometric measurements, statistically significant multiple regression models were established for 26 points on the face and head. Examination of regression model validity resulted in minimal differences between observed and predicted soft tissue thickness values. Assessment of interspecies compatibility using a bonobo (Pan paniscus) and modern human (Homo sapiens) subject resulted in minimal differences for the bonobo but large differences for the modern human. These results clearly show that (1) soft tissue thicknesses covary with craniometric dimensions in P. troglodytes, (2) confirms that such covariation is uniformly present in both extant Homo and Pan species, and (3) suggests that chimp-derived regression models have interspecies compatibility with hominids who have similar craniometric dimensions to P. troglodytes. As the craniometric dimensions of early hominids, such as South African australopithecines, are more similar to P. troglodytes than those of H. sapiens, chimpanzee-derived regression models may be used for approximating their craniofacial anatomy. It is hoped that the results of the present study and the reference dataset for facial soft tissue thicknesses of chimpanzees it provides will encourage further research into this topic.

Relationships between the hard and soft dimensions of the nose in Pan troglodytes and Homo sapiens reveal the nasal protrusions of Plio-Pleistocene hominids

“…Given that chimpanzee and modern human means differ significantly, it is not possible to approximate the position of pronasale for these species using averages. More and more forensic studies are revealing that linear regression models actually outperform averages (Campbell et al, 2021a;Dinh et al, 2011) Based on the results of the out-of-group interspecies compatibility tests, we suggest that it can be formulated as a general rule that hominids with longer cranial base lengths tend to have longer nasal cavities, and that hominids with maxillae tilted further down from the Frankfurt horizontal plane tend to have axes of the nasal cavity also directed further downwards. Furthermore, since this has been identified in two extant species of hominid, which feature quite distinct skull morphologies, and that the regression models can reliably approximate pronasale position and nasal protrusion in other African great apes (i.e.…”

“…Recently, Campbell et al (2021a) showed that facial soft tissue thicknesses covary with craniometric dimensions in chimpanzees. However, Campbell et al (2021a) only analyzed . CC-BY 4.0 International license perpetuity.…”

“…Recently, Campbell et al (2021a) showed that facial soft tissue thicknesses covary with craniometric dimensions in chimpanzees. However, Campbell et al (2021a) only analyzed chimpanzees and their formulae provide no information about the nasal protrusions of archaic hominids. The present study begins to alleviate this, at least in part.…”

“…The material used in this study consists of 19 computed tomography (CT) scans of chimpanzee (P. troglodytes) heads, previously analyzed in Campbell et al (2021a) KUPRI (dmm.pri.kyoto-u.ac.jp). The sex ratio for the chimpanzee sample was 1:1.71 (7 male and 12 female) and the mean age 30.9 years (minimum = 9; maximum = 44; SD = 10.1).…”

“…The material used in this study consists of 19 computed tomography (CT) scans of chimpanzee (P. troglodytes) heads, previously analyzed in Campbell et al (2021a), and 72 lateral cephalometric radiographs of humans. The chimpanzee sample was collected as Digital Imaging and Communications in Medicine (DICOM) format bitmap files from the Digital Morphology Museum, KUPRI (dmm.pri.kyoto-u.ac.jp).…”