Sexual Dimorphism in Homo erectus Inferred from 1.5 Ma Footprints Near Ileret, Kenya

Sexual dimorphism can be one of the most important indicators of social behavior in fossil species, but the effects of time averaging, geographic variation, and differential preservation can complicate attempts to determine this measure from preserved skeletal anatomy. Here we present an alternative, using footprints from near Ileret, Kenya, to assess the sexual dimorphism of presumptive African Homo erectus at 1.5 Ma. Footprint sites have several unique advantages not typically available to fossils: a single surface can sample a population over a very brief time (in this case likely not more than a single day), and the data are geographically constrained. Further, in many cases, the samples can be much larger than those from skeletal fossil assemblages. Our results indicate that East African Homo erectus was more dimorphic than modern Homo sapiens, although less so than highly dimorphic apes, suggesting that the Ileret footprints offer a unique window into an important transitional period in hominin social behavior

Pleistocene footprints show intensive use of lake margin habitats by Homo erectus groups

Reconstructing hominin paleoecology is critical for understanding our ancestors’ diets, social organizations and interactions with other animals. Most paleoecological models lack fine-scale resolution due to fossil hominin scarcity and the time-averaged accumulation of faunal assemblages. Here we present data from 481 fossil tracks from northwestern Kenya, including 97 hominin footprints attributed to Homo erectus. These tracks are found in multiple sedimentary layers spanning approximately 20 thousand years. Taphonomic experiments show that each of these trackways represents minutes to no more than a few days in the lives of the individuals moving across these paleolandscapes. The geology and associated vertebrate fauna place these tracks in a deltaic setting, near a lakeshore bordered by open grasslands. Hominin footprints are disproportionately abundant in this lake margin environment, relative to hominin skeletal fossil frequency in the same deposits. Accounting for preservation bias, this abundance of hominin footprints indicates repeated use of lakeshore habitats by Homo erectus. Clusters of very large prints moving in the same direction further suggest these hominins traversed this lakeshore in multi-male groups. Such reliance on near water environments, and possibly aquatic-linked foods, may have influenced hominin foraging behavior and migratory routes across and out of Africa

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