Morphological modularity in the anthropoid axial skeleton

The study of morphological modularity among skeletal regions is important as modular structure can confine the effect of mutation or selection to limited sets of traits (Hallgrímsson et al., 2009; Armbruster et al., 2014; Goswami et al., 2014; Klingenberg, 2014), thus influencing the potential pathways of evolutionary change. Modular structure refers to higher interactions or correlation within skeletal regions (termed ‘modules’) with relatively less connectivity between-modules (Klingenberg, 2014). In other words, a modular structure suggests stronger within-module integration than between-module integration. As noted by Armbruster et al. (2014: 1), “integration and modularity refer to the patterns and processes of trait interaction and independence”. In this regard, modularity and integration are not antonyms but complementary concepts. In the present study, modularity was used to refer to relative degrees of connectivity within and between a priori defined modules. Organisms with more modular body plans are considered to have different levels of within- and between-module constraints on morphological evolution as modules tend to have greater evolutionary independence compared with other body regions. Therefore, trait covariation may have to be dissociated or structured in differential ways to facilitate the evolution of novel skeletal morphology (Armbruster et al., 2014; Goswami et al., 2014; Klingenberg, 2014). For instance, Young et al. (2010) reported that hominoids have stronger degrees of modularity (i.e., lower covariation) between limb elements than other anthropoids. Thus, Young et al. (2010) suggested that there was dissociation of trait covariation between limb elements in hominoid ancestors, which allowed for the evolution of novel limb proportions among hominoids, including the human lineage in relation to obligate bipedal locomotion (i.e., short forelimbs and long hindlimbs).

In the vertebral column, the results of Williams et al. (2019) suggest that antipronograde and dorsostable hominoids may have experienced reduced biomechanical and developmental/genetic constraints compared to pronograde and dorsomobile anthropoids in terms of the number of presacral vertebrae. Although the axial skeleton is traditionally separated into the vertebral column and the skull, it is important to note that the basicranium shares developmental origins and genetic pathways with vertebral elements (Burke et al., 1995; Wellik, 2007) and acts as the ‘central integrator’ of the skull (Lieberman, 2011), suggesting the existence of modular structure across the skull and vertebral column. However, despite its importance for understanding the evolutionary history of the human and nonhuman primate bauplan, only a few studies have investigated modularity (or covariation) among axial skeletal elements in primates (e.g., Villamil, 2018; Arlegi et al., 2018, 2020, 2022; Villamil and Santiago-Nazario, 2021). Moreover, these studies have tended to focus on limited axial skeletal regions, such as the cranium and/or cervical vertebrae, and only examined morphological integration/modularity in hominoids (Villamil, 2018; Arlegi et al., 2018, 2022; Villamil and Santiago-Nazario, 2021). Although Arlegi et al. (2020) investigated covariation across almost all vertebrae, the analyses were limited to humans. Thus, it is unknown whether axial skeletal elements of hominoids also have stronger degrees of modularity than other anthropoids as has been found to be the case for limb elements (Young et al., 2010). Knowing this is important as it may suggest that hominoid ancestors had weaker genetic constraints among traits or lower evolutionary constraint (i.e., constraint on the evolution of diverse torso morphology in response to diverse locomotor repertoires) than other anthropoid groups, thus allowing for the evolution of novel axial skeletal morphologies in the hominoid lineage, including the unique skeletal morphology associated with human bipedalism.

It is well understood that humans display novel characteristics in the skull (Lieberman, 2011; Gómez-Robles et al., 2017; Schroeder and von Cramon-Taubadel, 2017; Veneziano et al., 2018, von Cramon-Taubadel et al., 2021) and vertebrae (Latimer and Ward, 1993; Shapiro, 1993a; Been et al., 2019) related to our upright body posture and bipedal form of locomotion. For instance, Schroeder and von Cramon-Taubadel (2017) found that humans showed a strong signal of directional selection on basicranial flexion, facial retraction, and cranial vault expansion among ape lineages. Directional selection for brain expansion may have been one of the major forces that shaped human skull morphology, given that facial retraction is also associated with basicranial flexion and brain size expansion (Lieberman et al., 2000; Lieberman, 2011; Neaux et al., 2018). In terms of the vertebral column, there is a mosaic of primitive and novel characteristics observable among australopiths (Johanson et al., 1982; Lovejoy et al., 1982; Shapiro, 1993b; Meyer et al., 2015, 2017; Williams et al., 2018; Williams and Meyer, 2019) and fossil Homo specimens (Latimer and Ward, 1993; Meyer, 2005; Gómez-Olivencia et al., 2013, 2017; Arsuaga et al., 2015; Williams et al., 2017; Meyer and Williams, 2019; Gómez-Olivencia and Been, 2019). Although australopiths show more primitive vertebral morphologies than early and late Homo specimens, australopiths have certain degrees of lumbar lordosis, which is thought to be an adaptation for bipedal locomotion to move the center of the torso over the sacroiliac and the hip joints (Latimer and Ward, 1993; Shapiro, 1993a,b; Gómez-Olivencia and Been, 2019; Meyer and Williams, 2019; Williams and Meyer, 2019; Williams et al., 2021). Also, early and late Homo specimens showed lumbar lordosis although the Sima de los Huesos (SH) Homo specimens and Neanderthals had lesser degrees of lumbar lordosis than modern humans (Latimer and Ward, 1993; Gómez-Olivencia et al., 2013; Gómez-Olivencia and Been, 2019; Meyer and Williams, 2019; but see Williams et al., 2022). Taken together, the evolution of the novel characteristics and morphological diversification in the skull and vertebral column of hominins suggest that a high evolutionary potential may have pre-existed in hominoid ancestors and/or that the human lineage presents very different patterns of modularity compared with other hominoids.

In this regard, the purpose of the present study was to investigate both whether hominoids have relatively stronger modularity (i.e., weaker covariation) among axial skeletal elements than other anthropoids and to compare patterns of modularity between humans and other hominoids, by testing the following two hypotheses: 1) hominoids will have stronger degrees of modularity between axial skeletal elements (i.e., cranium, mandible, vertebrae, and sacrum) than other anthropoids as was previously reported to be the case for limb elements (Young et al., 2010); 2) humans will have different patterns of modularity in the axial skeleton compared to other hominoids.

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