Data, development and diets

As mentioned briefly but repeatedly on this blog, my dissertation is about growth of the lower jaw in Australopithecus robustus (right), comparing it with jaw growth in recent humans. This is important because we don’t really know exactly how skeletal-dental (especially skeletal) maturation of our fossil relatives compares with us today. From a developmental perspective, it is also important to know how and when adult form arises during growth, and how these processes vary within and between species.

It’s not easy to examine ontogeny in fossil samples. In a post a few weeks ago I included a drawing of some of the A. robustus juvenile jaws. At the time, I was pointing out variation in dental maturity (which is a nice thing when studying growth), but the picture also reveals a bigger bugbear – variable preservation of features (which is a terrible thing if you’re trying to study growth).

For example, the youngest individual in the fossil sample (right, viewed from above, front is at the top of the picture) includes only the second baby molar tooth, a bit of the bone surrounding the sides and back of the tooth, and a small portion of the ascending ramus. The oldest subadult in the sample (SKW 5), on the other hand, is almost entirely complete. In between these ages, jaws variously preserve different parts. Under these circumstances (i.e. lots of missing data), growth cannot be studied by traditional (namely, multivariate) methods (how I will deal with this is a topic for another day).

So while studying the fossils in South Africa, in order to maximize the number of comparisons I could possibly make, I measured just about every single linear dimension conceivable on these jaws. I thus have a spreadsheet with 300 columns of measurements I could take on each specimen. Most of the cells are empty : (

What’s a boy to do?! In order to compare A. robustus with humans, I need to take the same measurements on a growth series of human jaws, too. But life is short, and if I want to finish this project before I succumb to some sinister signature of senescence, I really can’t take hundreds of measurements on a human sample which is much larger than the fossils. Plus, a lot of the individual measurements are a bit redundant: some of the distances overlap, many of the variables can be taken on the right and the left sides, etc.

If I am to finish collecting data in a reasonable time frame, I need to cull my variables from 300 to however many (a) maximizes the comparisons I can make within the less-complete A. robustus sample, and (b) are not too repetitive. Boo. Plus I have to get these spreadsheets ready to be read and analyzed in the program R, which for whatever reason is always a pain in the ass.

Again, the statistics of the overall comparisons is a topic for another day, and I haven’t had the opportunity yet to write the analytical program(s). But I have looked at some individual traits in A. robustus compared with a subsample of humans. For example, at the left is a plot of changes in height of the jaw at the baby second molar or adult second premolar (which replaces the baby molar). Obviously my human sample is way to small at the moment to make any really meaningful statements about how growth compares between the two species. Note also that these are absolute measures and not size-corrected, and that these are stages of dental eruption rather than chronological ages. But from this preliminary view, the two species are very similar up to around when the first adult molar comes in (“stage 4” here). Thereafter, the A. robustus individuals dramatically increase in size rather fast, whereas humans only slowly increase in size.

Again, this is a very preliminary result, and only for a single measurement. But it is interesting in light of a recent study by Megan Holmes and Christopher Ruff (2011). These researchers compared jaw growth recent humans who differed in the consistency of their diets. They found that kids in the two populations were not too different, but the samples became more different with age to become fairly different as adults. Now, A. robustus seems to have eaten a diet with lots of hard objects (see recent review by Peter Ungar and Matt Spohneimer), but humans’ diet (and technology) really obviates the need for chewing as powerful as seen in A. robustus. So this dietary divergence may well be reflected in the growth difference suggested above, but it may not be the sole factor. PLUS I NEED TO INCREASE MY HUMAN SAMPLE.

Stay tuned for more analyses and results!

ResearchBlogging.orgReferences to make you smarter and stronger
Holmes, M., & Ruff, C. (2011). Dietary effects on development of the human mandibular corpus American Journal of Physical Anthropology, 145 (4), 615-628 DOI: 10.1002/ajpa.21554

Ungar, P., & Sponheimer, M. (2011) The Diets of Early Hominins. Science 334(6053), 190-193. DOI: 10.1126/science.1207701  

Field Update 1: Things Fall Apart

So, I’ve been here for a week now, pretty much left to my own devices. What have I learned from my first real trip to the field (that’s right, the field in a building)?

Well, the project I was so psyched about before I came will not work out. The sample is just too small, that is, not enough specimens preserve all the traits I need. I contacted Dr. Miriam Zelditch at UM who is a pretty amazing scientist working on integration, among other things. She confirmed my suspicion that even with resampling, such a small empirical sample size will not work out. I still think it’s a neat idea, so I’m contemplating heading over to the Mammals collection to test it out on larger samples of extant primates. So Life Lesson #1 (which I’ve learned elsewhere): often, things don’t work out they way you’d intended.

Everyone at the museum has been lovely so far. Very friendly and very helpful. And the fossils! I’ve only examined things Swartkrans and Kromdraai (I found out that some of the Kromdraai teeth I looked at today might be early Homo instead of Australopithecus robustus). But the real fossil are amazing! I never realized the extent to which the fossils were reconstructed–I don’t think I’ve ever seen so much glue before. In fact, one specimen had a note from the late Dr. Charles Lockwood pointing out that it appears to have been reconstructed (glued together) slightly incorrectly. So it’s not all taphonomy after all…

But then the taphonomy is incredible, too. Taphonomy literally means “burial laws” in Greek, and it’s the study of how fossils come to look the way they do–that is, what happened after the organism died, how long did it take to be buried, what geological processes affected it, etc? The South African cave systems have interesting taphonomy: Many, or most, Swartkrans specimens may have been victims of carnivores like leopards, that subsequently fell into the caves. Once in the caves, they were buried by other debris and bones that fell in the caves. Many of the specimens have been distorted–squished flat or contorted in other weird ways. COB 101, for example, is part of a cranium that has been flattened on itself, such that the forehead is right next to the hard palate (I’m told they call the specimen “Pancake” around here).

So even where there are fairly complete specimens, many have been ‘morphed’ from having spent over 1 million years under the moving earth, further preventing quantitative analyses. Milford likes to note that with fossils, the data don’t speak for themselves. Earlier in the week he asked me if the fossils had talked to me yet. I replied that they seem to be whispering, “try harder.” Really, though, they were yelling at me, “You suck! You’ll never figure us out, you hack.” Milford replied that the fossils, in their condition, are telling me (and other paleontologists) to be innovative. Which I believe is true. So, here’s Life Lesson #2 (I haven’t tasted the fruits of the lesson, but I sincerely believe it): Most research questions are answerable; however, some require more creativity than others.