Even if the article is behind a paywall, the abstract is self-explanatory enough to be worth a mention:
Emiliano Bruner et al., Geometric variation of the frontal squama in the genus homo: Frontal bulging and the origin of modern human morphology. American Journal of Physical Anthropology, 2013. Pay per view → LINK [doi: 10.1002/ajpa.22202]
Abstract
The majority of studies of frontal bone morphology in paleoanthropology have analyzed the frontal squama and the browridge as a single unit, mixing information from different functional elements. Taking into account that the bulging of the frontal bone is often described as a species-specific trait of Homo sapiens, in this article we analyze variation in the midsagittal profile of the genus Homo, focusing on the frontal squama alone, using landmark-based superimpositions and principal components analysis. Our results demonstrate that anatomically modern humans are definitely separated from extinct human taxa on the basis of frontal bulging. However, there is minor overlap among these groups, indicating that it is necessary to exercise caution when using this trait alone to make taxonomic inferences on individual specimens. Early modern humans do not show differences with recent modern humans, and “transitional” individuals such as Jebel Irhoud 1, Maba, and Florisbad, show modern-like frontal squama morphology. The bulging of the frontal squama in modern humans may represent a structural consequence of more general cranial changes, or it could be a response to changes in the morphology of the underlying prefrontal brain elements. A subtle difference between Neandertals and the Afro-European Middle Pleistocene Homo sample is associated with flattening at bregma in the former group, a result that merits further investigation.
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| Frontal bone: the frontal squama or scales is labeled as 1 Source: Atlas of Human Anatomy. |
I wonder how much this confusion between frontal squama (rising of the forehead) and browridge (facepalm!) has caused the strange theories of Trinkaus and followers?
I wonder how much this confusion between frontal squama (rising of the forehead) and browridge (facepalm!) has caused the strange theories of Trinkaus and followers?
ReplyDeleteWhat is meant by this comment, more specifically?
....
In cranio-facial terms, a bulging frontal squama is caused by an (at least locally) large brain in relation to anterior cranial base, as in achondroplasia (e.g. google achondroplasia and frontal bossing), while a flat browridge comes from a low distance between the most anterior point of the orbit and the most anterior point of the frontal bone (also the most anterior point of the anterior cranial base), in line with the accepted spatial model of browridge formation.
Frontal bone flattening in early modern homo sapiens comparable to current homo sapiens, with the presence of larger brow ridges suggests that their brains had a comparable size to ours relative to the cranial base, but that their orbits had a more anterior position to the cranial base than ours do (in some ways more comparable to other homo, in the relative, if not absolute, size compared to the cranial base).
"What is meant by this comment, more specifically?"
DeleteThat school tends to blur the differences between the various human species existent in the past and to make Neanderthals and Sapiens stem from highly speculative common ancestors only some 100-300,000 years before the coalescence of our species and the first divergences such as that of Souther African Aborigines (Khoisan). For that they have no material support such as the spread of Acheulean, which happened some one million years ago and is IMO the real point of split between Sapiens and Neanderthals, but just confusing anthropometry (and in some cases also even more confusing molecular-clock-o-logy, which I am certain that it is totally speculative and producing chronologies that can be well a mere fourth of what actual material and archaeogenetic data demands - see for example the comment I made yesterday at Ethio Helix blog on this matter).
"Frontal bone flattening in early modern homo sapiens comparable to current homo sapiens, with the presence of larger brow ridges"...
No need to go back to early Homo sapiens. There is people today, even if in the minority, who do retain extremely marked browridges. They seem to be more common in some relatively isolated areas like the Baltic Countries or Aboriginal Australia but they do seem to exist in all populations.
Not sure what your point is anyhow. Nobody claims that archaic Homo sapiens or even Neanderthal were less intelligent, I'm just saying that Neanderthals diverged from us a million years ago or so and not just a few hundred thousand years. And that is evident also in cranial structure (where our kin is much more innovative - but has brains of equal size or even smaller than Neanderthals - it's not about brains but bones and general body plan what I mean).
There is people today, even if in the minority, who do retain extremely marked browridges. They seem to be more common in some relatively isolated areas like the Baltic Countries or Aboriginal Australia but they do seem to exist in all populations.
ReplyDeleteSure, modern day humans are very variable. However there is still a large shift between early modern homo sapiens and current humans.
Not sure what your point is anyhow. Nobody claims that archaic Homo sapiens or even Neanderthal were less intelligent
I'm not really saying anything about intelligence. Just that it is interesting that the species had these likely divergences in cranial shape, whatever that may mean. There is no other point. I was simply trying to ellucidate why it would seem to me that the patterns of frontal bulging and supraorbital torus projection exist in the species, with frontal bulging as a likely consequence of building a large brain on a short anterior cranial base (as in achondroplasia), and flattening as the inverse and with supraorbital torus due to the anterior position of the orbit relative to the frontal bone, on the basis you or your readers might be interested. And please don't take this as me trying to say anything about chronology.
Idaltu, some 160,000 years ago is totally modern.
DeleteI am of the punctuated evolution school and I think that most evolutionary jumps happen in small periods of intense change, while most of the time things are relatively stable. That seems to apply to our species indeed.
As for the why behind the anatomical changes, a key may be in the chin, which allows us to chew as strong as any ape if not more but without needing anymore prominent jaws or the lever support of supercilliary arcs. The chin is the kind of nimious but radical revolutionary change that really allows for extreme cranial shape changes.
Of course these shape changes may have some relation on how our brains are organized and what not but this is far from established.
I think the bent beam model for supraorbital torus supporting chewing is pretty widely considered dead though isn't it? I'm pretty sure the spatial model is the dominant model.
ReplyDeletee.g. 2012 paper discussing torus formation, and reviewing the various models concludes -
http://www.isita-org.com/jass/Contents/2012vol90/e-pub/22781585.pdf
"Lieberman (2000) has provided one of the most comprehensive reviews of sources of variation in frontal bone morphology. In particular he assessed the ontogenetic and architectural basis for a prominent supraorbital torus in archaic Homo. He also compared supraorbital morphology in early hominins to that of anatomically modern humans. He ultimately found that the model first proposed by Weidenreich in 1941 was the most successful at explaining browridge formation.
In essence, prominent browridges are a byproduct of upper facial projection, which occurs due to the differential patterns of development of the inner vs. the outer tables of the frontal bone (which, he notes, are part of the neurocranium and face, respectively) (Lieberman, 2000).
He concluded that browridge formation is not affected by in vivo response to masticatory strain, nor is simply an allometric consequence of cranial size.
While allometric differences do explain an element of browridge morphology, they cannot entirely explain variation in torus robusticity. Rather, allometric differences in the upper face and projection secondarily affect browridge morphology, including influencing sexual dimorphism in modern humans (Rosas & Bastir, 2002)."
The spatial model has the benefit in particular of explaining why orangutans don't have a brow ridge - "the splanchnocranium in Pongo, while maintaining the same shape as the African apes, is positioned differently: it is dorsally inclined relative to the braincase" "the proximal relationship between the orbital cavities and braincase in orangutans eliminates the need for a bony torus"
Still, even if the bent beam masticatory model is totally wrong, you might still be right about the chin, because if a chin increases masticatory efficiency, then the need for a large facial size (in terms of bones and muscles) is decreased (a large maxilla also contributing the chewing efficiency, even if differently / less so than a large mandible), and as the face decreases, total facial projection decreases and thus relatively decreased brow ridge.
I don't know if the chin does this though - I am not sure whether the chin has a functional role in mastication rather than just being a leftover from changes in bony deposition in the face, or whether it may be involved in facial muscle movements for facial expressions or what.
Of course these shape changes may have some relation on how our brains are organized and what not but this is far from established.
The neurocranium does pretty much form in response to brain shape (being also slightly influenced by the cranial base, which is influenced by the face), in line with it being formed by intramembranous ossification around the brain.
But brain shape does not really reflect organization that well - it does not tell you that much about which regions are getting relatively larger and smaller and even that only tells you very little about brain organization.
Most interesting commentary, Matt, thanks. I think that you are right in nearly all accounts (I never even considered this issue of the torus in any depth and just now realized that orangutans don't have it - facepalm!)
Delete"I don't know if the chin does this though - I am not sure whether the chin has a functional role in mastication rather than just being a leftover from changes in bony deposition in the face"...
I am positive that there are quite conclusive studies on this matter. Just searching for references via Wikipedia I found these:
Flora Gröning, "Why do humans have chins? Testing the mechanical significance of modern human symphyseal morphology with finite element analysis". AJPA 2011. PPV → LINK.
Compared to Neanderthals, the modern human symphysis shows higher strains during dorsoventral shear and wishboning, but is as effective as the Neanderthal symphysis in resisting vertical bending in the coronal plane and the loads resulting from simulated incision and unilateral molar biting.
And also:
David J. Daegling, "Functional morphology of the human chin". Evolutionary Anthropology 1993. PPV → LINK.
When the morphology of the chin is considered in light of experimental data on mastication, its evolution can be interpreted as a consequence of recent changes in mandibular proportions that have altered the relative importance of different masticatory stresses.
"... brain shape does not really reflect organization that well"...
True.
interesting page
ReplyDeleteGlad to read that! Good luck with the paper.
ReplyDeletefrontals