The contribution of angiogenesis to variation in bone development and evolution

Background The craniofacial skeleton reflects many important evolutionary trends of primates, including derived orbit morphology (Ross, 1995; Ravosa et al., 2000), cranial base shape (Lieberman et al., 2000), and increased relative cranial vault size (Isler et al., 2008). Similar to findings in many other clades that are not reviewed here, differences in primate dentition (Lambert et al., 2004) and the morphology of the semicircular canal system (Spoor et al., 2007) provide evidence of diet and locomotion, respectively, while the degree of sexual dimorphism provides hints about social behavior (Plavcan, 2001). Morphology of the cranial vault, including cranial volume and relative neurocranial height, are important characteristics that help to distinguish different primate clades (Fleagle et al., 2010). In particular, the human skull is highly derived and cranial elements are useful for determining phylogenetic relationships among hominins (Lahr, 1996; Schwartz and Tattersall, 2003), with vault morphology being an important diagnostic feature in operational definitions of Pleistocene hominin species and descriptions of new fossils (Athreya, 2009). The skull houses structures that enable many important functions associated with the human condition, including cognition (Holloway, 1969; Falk, 1992; Sherwood et al., 2008), vocalization (Kay et al., 1998; MacLarnon and Hewitt, 1999), and thermoregulation (Beals et al., 1984; Weaver, 2009), and as such, human craniofacial evolution raises particularly intriguing questions about our species’ origins (Lieberman et al., 2008). Ontogenetic analysis of fossil and extant primates allows anthropologists to explore the developmental bases of morphological variation, the contribution of mechanical stresses to derived morphology, and ontogenetic shifts that are associated with phylogeny (Lieberman et al., 2002; Lovejoy et al., 2003; Zollikofer and Ponce de León, 2010). Since Stephen J. Gould’s discussion of the relationship between ontogeny and phylogeny (Gould, 1977), ontogenetic shifts, including changes in life-history characteristics, have been used to explain the origin of morphological differences between modern humans and other hominid species including chimpanzees (Leigh, 2004; Robson and Wood, 2008), Ardipithecus ramidus (Suwa et al., 2009), Homo erectus (Dean et al., 2001; Smith, 2004), and Neanderthals (Tillier, 1995; Ponce de León and Zollikofer, 2001), typically using postnatal morphological data. While studies of postnatal craniofacial development are important, studies of prenatal developmental mechanisms are necessary to complete our understanding of the ontogenetic bases of many important craniofacial features.