Looking over a Neandertal's shoulder07 Jan 2012
A study by Di Vincenzo, Steven Churchill and Giorgio Manzi has fallen into the early drawer of the Journal of Human Evolution: “The Vindija Neanderthal scapular glenoid fossa: Comparative shape analysis suggests evo-devo changes among Neanderthals”
The glenoid fossa is the part of the scapula that articulates with the head of the humerus. It’s the base of the “socket” in the ball-and-socket joint of the shoulder – indeed, “glenoid” comes from the Greek word for “socket”. Roughly shaped like a rounded teardrop, the glenoid is narrower in early hominins and relatively broad in recent people. Neandertals have an intermediate form compared to earlier and later humans.
The main point of the study is that the Vindija glenoid specimen, Vi-209, has a more humanlike form than other Neandertals. Another conclusion based on the comparative sample is that the sample of glenoids from late Neandertals is intermediate between early Neandertals and recent people. Likewise, Upper Paleolithic and Mesolithic-era European specimens are intermediate between late Neandertals and recent people. Here’s a graph with the first and second principal components of the variation; I’ve highlighted these groups.
Therefore, it seems reasonable that heterochronic factors related to the prolonged developmental pattern of our species (Smith et al., 2007a), which contrasts with the faster growth rates of Neanderthals and other archaic hominins (Smith et al., 2007b; but see; Guatelli-Steinberg et al., 2005), led to longer periods of bone deposition along the inferior-lateral edge of the SGF [scapular glenoid fossa]. This could explain the observed variation along PC1 (and/or CV1) for different morphs of the genus Homo, reaching in H. sapiens the greatest extent in width of the SGF and, particularly, of its scapular portion. This is also consistent with the observation by Churchill and Trinkaus (1990) that much of the variability of the glenoid surface is a function of size variation of the joint itself, which can be viewed as forming a single functional matrix sensu Moss and Young (1960). Thus, the overall reduction in developmental rates in the genus Homo (relative to those of other hominoids) across the Pleistocene may account for the general evolutionary trend in SGF shape seen in the fossils, with more marked changes in developmental rates between archaic (including Neanderthals) and early modern humans, producing somewhat more dramatic differences between these groups in joint shape. Green et al. (2010) suggest that some of the differences between Neanderthals and modern humans in shoulder and thoracic morphology (particularly those related to clavicular length) are attributable to differences in the RUNX2/CBFA1 gene. The temporal pattern observed here would suggest that, with respect to SGF shape at least, that some differences are due to overall differences in developmental schedules (rather than specific differences in genes controlling development of the shoulder, such as RUNX2/CBFA1 or HoxC6).By suggesting at least one actual genetic substitution in recent humans, they lend some plausibility to the idea. I am more hesitant to accept the assumption that Neandertals had faster developmental schedules than recent people, although it could be true. This specific assumption is not necessary to support the idea of heterochronic change in the glenoid, which could be caused by much more focused developmental processes. If glenoid shape reflects heterochronic developmental changes, the data suggest that those changes were ongoing in global populations during the Holocene. Indeed, the difference between recent people in the study and Upper Paleolithic Europeans is as great as the difference between late Neandertals and Upper Paleolithic Europeans. The study's recent human sample covers a broad geographic distribution but is relatively small in numbers; a fuller comparison of recent people might uncover a more interesting pattern of change. The scapula has long figured in discussions of Neandertal genetic persistence. Neandertal scapulae often have a sulcus (groove) on the dorsal (back) aspect of the axillary border, and this feature is also found in a high fraction of early Upper Paleolithic skeletons
The morphology of the SGF [scapular glenoid fossa] is unlikely to be under the genetic control of a single locus. Thus, it is more likely that regulatory genes controlling developmental rates overall produce pleiotropic effects throughout the skeleton. The introduction of these and other (non-regulatory) alleles into the Neanderthal populations of the Near East, and their movement by gene flow across Neanderthal demes into southern Europe (well in advance of the actual in-migration of modern humans) could account for mosaic morphology seen in the Vindija G3 Neanderthals, including the Vi-209 scapula. Introgression and subsequent gene flow would not be expected to have affected early Neanderthal populations (those predating the admixture), nor late Neanderthal populations from western (trans-Alpine) Europe, because they were separated by geographic barriers ( [Fabre et al., 2009] and [Degioanni et al., 2011] ), and/or protected from gene flow by distance (as hypothesized by Voisin, 2006).There is as yet no evidence that the Vindija Neandertal genomes have genetic introgression from the African populations from which present non-Africans derive most of their genetic heritage. Green and colleagues
Fabio Di Vincenzo and colleagues analyzed the shape of the outline of the glenoid fossa on the scapula (not to be confused with the glenoid on your skull), from Australopithecus africanus to present day humans. The glenoid fossa is essentially the socket in the ball-and-socket joint of your shoulder. The authors found that there is pretty much a single trend of glenoid shape change from Australopithecus through the evolution of the genus Homo: from the fairly narrow joint in Australopithecus africanus and A. sediba, to the relatively wide joint in recent humans. The overall size and shape of the joint influences/reflects shoulder mobility, so presumably this shape change hints that more front-to-back arm motions became more important through the course of human evolution (authors suggest throwing in humans from the Late Pleistocene onward).I think Cofran takes this in an interesting direction with respect to his own dissertation work on development in earlier hominins.