Osteology Everywhere: Bacon or first rib?

I went to a cafe today to eat breakfast and get some work done. Write, write, write. It’s important to be properly nourished to ensure maximal productivity.

The Ron Swanson diet.

The Ron Swanson diet.

But I was aghast to behold the food they placed before me:

More bacon, please.

What on earth is this?

First of all, this is not a sufficient amount of bacon.

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Secondably, this bacon is a spitting image of a first rib:

First ribs, from left to right: Human, chimpanzee, bacon. First two images from eSkeletons.org.

First ribs from the right side of the body, viewed from the top. From left to right: Human, chimpanzee, bacon. First two images from eSkeletons.org.

At the top of the ribcage, just beneath the clavicle and subclavian artery and vein, the first rib is much shorter and flatter than the rest of the ribs. As Jess Beck at Bone Broke points out, “The first and second rib give something of an awkward ‘slow song at a middle-school dance’ kind of a hug, while the lower ribs provide a more comfortable and self-assured embrace.” I mean, just lookit how sheepishly the bacon dances with the eggs in the first picture, it has ‘middle-school dance’ written all over it.

But the bacon is not totally identical to the human and chimpanzee counterparts. It’s missing their anteromedially sweeping arc, and the distal portion reaching out to the egg is fairly straight. This suggests we’re probably missing much of the original distal end. Posteriorly or dorsally (toward the bottom in the pic), it also appears to be missing much of the lateral portion including the vertebral facet. In this regard, this bacon rib looks a lot like the first rib of Homo naledi:

Full stack of ribs. From left to right: Human, bacon, Homo naledi, Dmanisi Homo erectus, Australopithecus sediba (x2), Australopithecus afarensis specimen "Lucy," Ardipithecus ramidus, and chimpanzee. Images not to scale except Lucy and Ardi.

Full stack of ribs. Left to right: Human, bacon, Homo naledi, Dmanisi Homo erectus, Australopithecus sediba (x2), Australopithecus afarensis specimen “Lucy,” Ardipithecus ramidus, and chimpanzee. Images not to scale except Lucy and Ardi. Image credits given below.

It is hard to make good homologous comparisons among these fossils and bacon, since so many are so incomplete. But it looks like the hominins are relatively longer (front to back, or dorsoventrally) compared to the chimpanzee. That is, oriented along the rib “neck,” the ventral/distal end projects far more medially beyond the proximal vertebral facet in the chimp, while in the hominins the two ends are more flush.  Ardi is really incomplete and so very hard to orient, but it may be more like the chimp (I think it needs to be rotated to the right more, to make the lateral edge more vertical like all the other specimens).

It will be interesting to see what my colleagues working on the Homo naledi thorax have to say about rib shapes and their functional importance, hopefully not too long from now.

Anyway, I really wish I had more bacon.

Fossil rib sources
ResearchBlogging.orgDmanisi Homo erectus: Lordkipanidze D, Jashashvili T, Vekua A, Ponce de León MS, Zollikofer CP, Rightmire GP, Pontzer H, Ferring R, Oms O, Tappen M, Bukhsianidze M, Agusti J, Kahlke R, Kiladze G, Martinez-Navarro B, Mouskhelishvili A, Nioradze M, & Rook L (2007). Postcranial evidence from early Homo from Dmanisi, Georgia. Nature, 449 (7160), 305-10 PMID: 17882214

Australopithecus sediba: Schmid P, Churchill SE, Nalla S, Weissen E, Carlson KJ, de Ruiter DJ, & Berger LR (2013). Mosaic morphology in the thorax of Australopithecus sediba. Science, 340 (6129) PMID: 23580537

Homo naledi: Morphosource.

Australopithecus afarensis and Ardipithecus ramidus: White TD, Asfaw B, Beyene Y, Haile-Selassie Y, Lovejoy CO, Suwa G, & WoldeGabriel G (2009). Ardipithecus ramidus and the paleobiology of early hominids. Science, 326 (5949), 75-86 PMID: 19810190

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Osteology Everywhere: Why we number our premolars 3-4

Portishead* came on the radio the other day, making iTunes display the cover of their album, Third. My inner osteologist rejoiced to see it prominently features a tooth!

Third album cover by Porthishead (2008). Image from Wikipedia

Well not a picture, but rather the name, of a tooth. In each quadrant of your mouth (most likely) are two premolars, commonly referred to as “bicuspids.” In the biz, we usually call these pals,  “P3” and “P4.”

UW 101-1277 mandible, part of the Homo naledi holotype skull. Modified from the Wits media gallery.

UW 101-1277 mandible, part of the Homo naledi holotype skull. Each capital letter stands for the tooth type (incisor, canine, premolar, and molar). Modified from Wits’ image gallery.

You might be wondering why we call them P3 and P4, when there are only two premolars per quadrant — what happened to P1 and P2?  Homology to the rescue!

The ancestral mammalian condition was to have four premolars (and a 3rd incisor) in each side of the jaw. This is a “dental formula” of 3-1-4-3, indicating the numbers of each tooth type from front to back. Over time, different groups of animals have lost some of these teeth. Baleen whales have lost all of them.

P1 and an incisor were lost early in the evolution of Primates. Most Strepsirrhines and New World monkeys retain this primitive”2-1-3-3″ dental formula :

Ring tailed lemur (left) and woolly monkey (right) maxillae, showing the primitive primate dental formula including a P2. For scale, gridlines are 10 mm (left) and 20 mm (right).

Ring tailed lemur (left) and woolly monkey (right) maxillae, showing the primitive primate dental formula including a P2. For scale, gridlines are 10 mm (left) and 20 mm (right). Images from this boss database.

The last common ancestor of catarrhines (living humans, apes and Old World monkeys) lost the P2, and so we have only two premolars left in each side of the jaw. These are homologous with the third and fourth premolars of the earliest mammals. And that’s why we call them P3-4.

*The song was “The Rip.” It’s a very good song with an insanely creepy and trippy video:

More convergence and arboreality: Knuckle-walking and the African apes

As long as we’re on the topic of homoplasy, a recent study suggests that knuckle-walking evolved independently in chimpanzees (Pan) and gorillas (Gorilla). If true, this suggests that hominins did not evolve from a knuckle-walking ancestor. Interesting.

Take-home points from the paper include:

  • Many purported ‘knuckle-walking’ features of the hominoid wrist might rather indicate arboreal wrist postures
  • Knuckle-walking in Pan and Gorilla are biomechanically distinct, and may thus have evolved independently in each lineage
  • More tentatively: Humans may not have evolved from a knuckle-walking ancestor, lending further credence to the idea that Pan is not a great model for the Pan-human common ancestor
  • This may be another example of one of Futuyma’s Principles of Evolution: HOMOPLASY IS COMMON IN EVOLUTION

“Knuckle-walking” refers to the mode of locomotion employed by most Pan and Gorilla when on the ground. Whereas most terrestrially quadrupdal primates use either the palmar surfaces of their ‘fingers’ or their palms to contact the ground, Pan and Gorilla‘s hands contact the ground with the back surface of the middle of their fingers (their intermediate phalanges, in technical terms). It is a very unusual posture–so far as I know, among all animals it is unique to these apes. So, it is perfectly sensible to assume that that knuckle-walking in chimpanzees and gorillas is homologous, represents the ancestral posture in African apes, and that humans evolved from a knuckle-walking ancestor.

But Tracey Kivell and Dan Schmitt present evidence from the wrist that suggests knuckle-walking in Pan and Gorilla are biomechanically and developmentally distinct. They point to several features of the wrist bones (carpals) that have traditionally been assumed to reflect knuckle-walking behavior. The expression of these features does not fit expectations given size and maturation differences between the two African apes. In fact, most of the features are more common/pronounced in Pan, and sometimes even other primates, more so than in Gorilla. The authors thus posit that many of the hitherto-knuckle-walking features of the wrist are actually indicative of arboreal wrist postures, and not knuckle-walking.

That authors acknowledge that it is possible that the wrist differences between Pan and do not necessarily preclude the possibility that knuckle-walking in the two apes has a common, ancestral origin, and that the differences accumulated after the evolutionary split between Gorilla on the one hand and Pan-humans on the other. That is to say, the behavior in the apes is homologous (as in common ancestry) but non-identical. Another possibility, which would also indicate that humans did evolve from a knuckle-walking ancestor, is that the behavior evolved separately in the Gorilla lineage, and in the Pan-hominin lineage before the split between Pan on the one hand and hominins on the other. The only way to test such a hypothesis is with fossils, fossils which so far as I know we do not have (yet).

Reference
Kivell T and Schmitt D. Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor. Proceedings of the National Academy of Sciences, in press.

More LB1 stuffs, but no real news

New analyses of the Flores “hobbit” material have been published recently, including a geometric morphometric (GM) study of the LB1 cranium (Baab and McNulty, in press). Readers will recall that this particular small-brained specimen has fueled the controversy over whether the Flores material is a unique, small-bodied hominin species, or whether this and other specimens were pathological (humans). At this stage in the game, I still do not know what we can say exactly is going on until we get more fossils. Admittedly, I haven’t yet read the recent analyses of the postcranial material. Maybe they will change my mind. So, I’d like to keep this more descriptive and comparative, than speculative on phylogeny versus pathology. Some neat things from the study:

Coordinates of landmarks were recorded off a “stereolithographic model” of LB1. This is a neat technology in which a plastic 3D model is ‘printed out’ based on data from a CT scan. While this might not give the same resolution as the real thing (or a good cast of the real thing), it’s a great way to make physical models of delicate fossils. Additionally, measuring/collecting landmark data from fossils can always potentially damage specimens, and stereolithography is a way to circumvent this problem. However, I don’t know that it would have been any more difficult to digitize landmarks from the actual CT scan (i.e. with a computer software package), and it almost certainly would have been cheaper. Oh well, smoke them if you’ve got them, right?

The study used GM to compare the shape of LB1 cranium to the shapes of apes, fossil hominins and modern humans. I like that the study included the analysis in Procrustes shape space (with nuances of location, rotation and scaling omitted) and in form space (omits only location and rotation, leaving size/scaling in the analysis). By examining the specimens in form space, the authors were able to study the relationship between shape and size, i.e. the effects of allometry: what cranial shape would “an LB 1-sized individual” of each taxon have? The results of the form space analyses are kind of neat, although I can’t interpret them too readily since I’m not sure what exactly the first two PC scores indicate, other than size. In a nutshell, though, LB1 appears most similar to what would be expected of a small-bodied fossil Homo individual. Of course, this is nothing new: Gordon et al (2008) came to the same conclusion last year, but using inter-landmark distances as variables (as opposed to landmarks as variables). Additionally, last year we found that the microcephalics in our lab appear more similar in shape to modern than fossil humans (I wonder if we should have tried to publish that…).

The authors also examined asymmetry as a way to address the allegation of pathology in LB1 (left-right asymmetry is believed to be indicative of developmental disruptions arising from environmental or genetic stress). LB 1 is more asymmetrical than the human mean (but within the range of variation), and roughly in the middle of the range of variation for fossil hominins. This suggests to the authors that LB1 is not pathologically asymmetrical, and that asymmetry in the fossil is rather likely due to effects of fossilization. Unfortunately, however, the issue of pathology-related asymmetry is not wholly adequately addressed, so this result must be taken with a grain of salt (fluctuating asymmetry is generally believed to indicate developmental disruptions, and there are two other types of asymmetry that must be factored out…). It is at this point that some GM qualms I have come to the fore. Really there’s one issue specifically: homology and measurement error.

As mentioned above, GM statistically analyzes shapes using landmark coordinates as variables. Of course when comparing things, you want to make sure you’re actually comparing equivalent things (“homologous” structures). So on a cranium there are a few, which Bookstein identified as “Type I,” and are often the intersections of bony sutures, such as the point where your two nasal bones contact your frontal bone. But beyond that, many landmarks are extrema (“Types II-III), i.e. points of widest breadth, which are not necessarily (ever?) homologous structures. The trouble, then, is a large risk of comparing non-homologous structures, which can render biological analyses nearly meaningless. Additionally, many of these such points must be found fairly arbitrarily, which means that even if given landmarks are homologous (i.e. biologically meaningful), there is a good chance of measurement error confounding the study–this is especially true in fluctuating asymmetry studies.

Use of these Type II-III landmarks is not terrible, it’s just a reason to be wary. One must ask, based on how arbitrarily a landmark was found, and the likelihood of measurement error, how meaningful will the results be? One landmark that comes to mind from this study is alare, “the most lateral point on the margin of the nasal aperture.” How significant is ‘lateral-most’ as a criterion–is there something functionally important about this point, and is it very variable within a given taxon (I really do not know, but now I’m interested…)? Moreover, the margins of the nasal aperture (“nose hole”) are not always sharply delineated, but can be rounded, making the margin itself difficult to identify: for example, OH 5 has very rounded margins, but KNM-ER 406 seems to have sharp margins–both specimens are Australopithecus boisei. In this study, alare contributed second most to individual asymmetry in all taxa. Is this because of measurement error, or the fact that this is not a biologically significant point, or both/neither? Other fun landmarks include “malar root origin,” and “Frontomalare temporale… The point where the frontozygomatic suture crosses the temporal line (or outer orbital rim)” (my emphasis). So throughout the study, you took either one intersection or the other?! When did you use which one, and why? Homology FAIL. Maybe I (or someone else) will come up with a new (hopefully better) way to study asymmetry… Now I’ve lost my train of thought.

References
Baab K and McNulty K. Size, shape and asymmetry in fossil hominins: The status of the LB1 cranium based on 3D morphometric analyses. J Hum Evol, in press.

Gordon A, Nevell L, and Wood B. 2008. The Homo floresiensis cranium (LB1): Size, scaling, and early Homo affinities. Proc Nat Acad Sci 105: 4650-4655.