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.
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.