Anatomical network analysis shows decoupling of modular lability and complexity in the evolution of the primate skull.

Modularity and complexity go hand in hand in the evolution of the skull of primates. Because analyses of these two parameters often use different approaches, we do not know yet how modularity evolves within, or as a consequence of, an also-evolving complex organization. Here we use a novel network theory-based approach (Anatomical Network Analysis) to assess how the organization of skull bones constrains the co-evolution of modularity and complexity among primates. We used the pattern of bone contacts modeled as networks to identify connectivity modules and quantify morphological complexity. We analyzed whether modularity and complexity evolved coordinately in the skull of primates. Specifically, we tested Herbert Simon's general theory of near-decomposability, which states that modularity promotes the evolution of complexity. We found that the skulls of extant primates divide into one conserved cranial module and up to three labile facial modules, whose composition varies among primates. Despite changes in modularity, statistical analyses reject a positive feedback between modularity and complexity. Our results suggest a decoupling of complexity and modularity that translates to varying levels of constraint on the morphological evolvability of the primate skull. This study has methodological and conceptual implications for grasping the constraints that underlie the developmental and functional integration of the skull of humans and other primates

Phylogenetic relations of the 20 taxa studied.

<p>Calibration of branch length follows molecular dating [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127653#pone.0127653.ref032" target="_blank">32</a>].</p

Network parameters quantifying modularity and complexity.

<p>Pearson’s product-moment correlations show a significant positive correlation between the modularity and the complexity measured as the number of bones (N: <i>r</i> = 0.691, <i>p</i> = 5.28e^–4), as predicted by the near-decomposability hypothesis. However, the other parameters used as measures of complexity lack this positive correlation with modularity; instead we observe a negative correlation between modularity and complexity (K: <i>r</i> = –0.442, <i>p</i> = 0.029; D: <i>r</i> = –0.701, <i>p</i> = 4.12e^–4; C: <i>r</i> = –0.409, <i>p</i> = 0.041). Finally, disparity or anisomerism does not correlate at all with modularity (H: <i>r</i> = 0.149, <i>p</i> = 0.729).</p><p>Network parameters quantifying modularity and complexity.</p

Schema of the skull of primates formalized as a network.

<p>An anatomical network represents the bones and physical joints (i.e. sutures and synchondroses) of the skulls as nodes and links in a network model. Because these physical joints are primary sites of bone growth and diffusion of stress forces, topological relations also capture developmental and functional co-dependences among bones [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127653#pone.0127653.ref027" target="_blank">27</a>]. The analysis of these anatomical networks helps uncover the morphological organization of the skull regardless of variation in its shape and size. <i>Labels</i>: <i>eth</i>, <i>ethmoid; fro</i>, <i>frontal; lac</i>, <i>lacrimal; max</i>, <i>maxilla; nas</i>, <i>nasal; nch</i>, <i>nasal concha; occ</i>, <i>occipital; pal</i>, <i>palatine; par</i>, <i>parietal; pmx</i>, <i>premaxilla; sph</i>, <i>sphenoid; tem</i>, <i>temporal; vom</i>, <i>vomer; zyg</i>, <i>zygomatic; l</i>, <i>left; r</i>, <i>right</i>.</p

Connectivity modules identified in the skull of outgroup taxa and Strepsirrhini.

<p>The four main types of modules: midfacial (in <i>blue</i>), palatal (in <i>green</i>), premaxillary (in <i>yellow</i>) and neurocranial (in <i>red</i>) are already present in the skull of <i>Tupaia</i> and <i>Cynocephalus</i>. The skulls of Strepsirrhini (<i>left</i>) show a conserved composition of the cranial module: occipital, sphenoid, parietals, and temporals. The midfacial module is divided into left and right specular modules. Strepsirrhini vary in the formation of palatal and premaxillary modules.</p

Connectivity modules identified in the skull of Platyrrhini and <i>Tarsius</i>.

<p>In contrast to Strepsirrhini, the frontal bone is unpaired in the skull of Haplorrhini (but see [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127653#pone.0127653.ref033" target="_blank">33</a>]). In <i>Tarsius</i> the frontal bone is included in the cranial module (in <i>red</i>), while in Platyrrhini the frontal belongs to the midfacial module (in <i>blue</i>) or to the premaxillary module (in <i>yellow</i>) in <i>Colobus</i>. Platyrrhini (<i>right</i>) show a conserved bone composition of the cranial module: occipital, sphenoid, parietals, temporals, and zygomatics. The three types of facial modules are also present, but they vary in their bone composition from one skull to another.</p

Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia.

Our understanding of the origin of the genus Homo has been hampered by a limited fossil record in eastern Africa between 2.0 and 3.0 million years ago (Ma). Here we report the discovery of a partial hominin mandible with teeth from the Ledi-Geraru research area, Afar Regional State, Ethiopia, that establishes the presence of Homo at 2.80 to 2.75 Ma. This specimen combines primitive traits seen in early Australopithecus with derived morphology observed in later Homo, confirming that dentognathic departures from the australopith pattern occurred early in the Homo lineage. The Ledi-Geraru discovery has implications for hypotheses about the timing and place of origin of the genus Homo

Hominin track assemblages from Okote Member deposits near Ileret, Kenya, and their implications for understanding fossil hominin paleobiology at 1.5 Ma

Tracks can provide unique, direct records of behaviors of fossil organisms moving across their landscapes millions of years ago. While track discoveries have been rare in the human fossil record, over the last decade our team has uncovered multiple sediment surfaces within the Okote Member of the Koobi Fora Formation near Ileret, Kenya that contain large assemblages of ∼1.5 Ma fossil hominin tracks. Here, we provide detailed information on the context and nature of each of these discoveries, and we outline the specific data that are preserved on the Ileret hominin track surfaces. We analyze previously unpublished data to refine and expand upon earlier hypotheses regarding implications for hominin anatomy and social behavior. While each of the track surfaces discovered at Ileret preserves a different amount of data that must be handled in particular ways, general patterns are evident. Overall, the analyses presented here support earlier interpretations of the ∼1.5 Ma Ileret track assemblages, providing further evidence of large, human-like body sizes and possibly evidence of a group composition that could support the emergence of certain human-like patterns of social behavior. These data, used in concert with other forms of paleontological and archaeological evidence that are deposited on different temporal scales, offer unique windows through which we can broaden our understanding of the paleobiology of hominins living in East Africa at ∼1.5 Ma