[QUOTE=racer72]
I can sympathize. It wasn’t until I had shaved my full beard down to a circle beard 3 weeks ago that I remembered that one of the reasons I decided to grow a beard was that I don’t have much in the way of a jawline any more.
Needless to say, I now have about 3 weeks worth of full beard growth.
Waenara
October 23, 2007, 10:51pm
42
I googled around and managed to find this incredibly detailed paper (43-page, 6.6MB PDF!) titled The Human “Chin” , with a detailed analysis of the variation in chins in humans and Pleistocene hominids (including Neandertals).
I don’t have time right now to read it in depth, but basically I came away with the idea that the “chin” is not a well-defined technical term, and is often used to describe different morphological characteristics. There is a broad range of normal variation in human chins, and the characteristics of human or hominid “chins” can change as they mature (juveniles to adults).
Starting on page 36:
Morphologically, the primary implication of our observations is that a bulging symphyseal region is not equivalent to a chin. When humans and elephants can both be described as having chins (e.g., Enlow, 1982), it is probably time to reconsider the applicability of the term. Consequently, we recommend that the term ‘‘chin’’ no longer be used in discussions of mandibular morphology and its phylogenetic and systematic significance . If, however, one feels the need to retain this term, it should be restricted in usage only to extant H. sapiens and those fossils displaying the constellation of symphyseal features of this species. Since it is obvious that bulges and swellings as well as subalveolar depressions of differing degrees of expression and different morphologies have come to adorn the symphyseal regions of a diversity of hominids, we further recommend that the terms (and the synonyms of) ‘‘mentum osseum,’’ ‘‘incurvatio mandibularis,’’ ‘‘tuberculum laterale,’’ and even ‘‘trigonum mentale’’ also be limited, or even dropped from usage. As anatomical terms, they are used to refer both to sometimes superficially similar and to sometimes markedly different structures in a diversity of adult specimens that may have arisen from totally different juvenile configurations . As for children, the sometime presence of a gomphotic scar especially low down along the midline of the mandible is a naturally occurring result of symphyseal fusion. Unless it is distinctly raised and part of an everted, inverted ‘‘T’’-shaped structure that is associated with mental fossae, it is not a ‘‘hint’’ or a ‘‘trace’’ of a mental trigon as seen in H. sapiens. Thus, these terms have become charged with a scenario of ‘‘chin’’ evolution which should itself be discarded .
Throughout this paper, we have attempted to keep our description of morphology apart from a phylogenetic and, especially, functional interpretation of it. We have, however, stressed the importance of understanding the morphology of the adult by starting first with an appreciation of the details of juveniles precisely because this is the most fundamental way in which to appreciate the similarities or differences that exist among individuals and between the average morphologies of taxa.
Basically, however, the facts are that H. sapiens has a distinctive symphyseal configuration from its ontogenetic outset and that many other hominids, especially Neanderthals, do not share any version of these apomorphies. From this it would seem reasonable to conclude that Neanderthals and those specimens that are morphologically similar to them in symphyseal morphology are not members of the species H. sapiens. But if one was committed to the interpretation that there was only a single species, which encompassed a diversity of morphologically distinctive specimens— such as all or most of those we discuss here—then one would be obliged to explain how members of the same species could have come to be morphologically different from one another. We, however, are trying to understand as best we can what the details of difference and similarity are from the beginning. Only afterward can we bring this information to bear on our phylogenetic and systematic hypotheses, and, if we so choose, to offer explanations for how and even perhaps why certain features came to be the way they are. Our appreciation of development also differs significantly from the more prevalent one in the literature: essentially, if development is taken into consideration at all, it is done secondarily, after specimens of adult individuals have been sorted into a Haeckelian scale naturae that is regarded as reflecting a transformation sequence . This license is prevalent in paleoanthropology, as is indicated in the oft-published diagrams of fossil adult crania arranged in a presumed phylogenetic sequence.
Clearly symphyseal morphology, especially as considered within a developmental framework, lends support to previous suggestions of notable taxic diversity within the genus Homo (e.g., Howell, 1994; Santa Luca, 1978; Schwartz & Tattersall, 1996b; Tattersall, 1986; Zollikofer et al., 1995): a diversity that is at present substantially underestimated.
[QUOTE=WF Tomba]
The “Jay Leno/Quentin Tarintino” genes.