I’m going to do my best to keep up with the blog during by Big Summer Adventure, and one thing I’d like to do is “F-ing Fossil Friday!” in which I focus on fossils for a bit. We’ll see if I can make this pan out.
Today I got out the rest of the Australopithecus robustus mandibles at the Transvaal Museum (above), save for I think maybe 1. As you can see from the picture, taphonomy (what happens to an animal’s remains between death and our digging them up) creates a serious challenge for the study of variation in this species. I’m focusing on ontogenetic variation – differences associated with growth and development. In spite of its fragmentary nature, so far as I know this is the best ontogenetic series of any fossil hominid (I should probably look more into A. afarensis here, too). In the bottom left you’ll see SK 438, the youngest in the sample, whose baby teeth haven’t quite come in all the way. Poor little guy! At the top right corner is SK 12, probably the oldest individual and also a big bugger.
One thing that I’ve noticed so far, only a preliminary observation that I need to actually run some numbers on, is that as individuals get older, the length of their tooth row (molars and premolars) gets shorter. This is because of the tendency for teeth to move forward during growth – “mesial drift” – and for adjacent teeth to literally wear into one another, their ends becoming flatter and flatter. While I should have realized this, it was surprising at first to find some dimensions of the lower jaw actually decreasing during growth. Now, I still have to run some tests to see if this is a biologically significant phenomenon. But it’s always nice to learn something new, even after just 2 days back with my best extinct buddies.
Stay tuned to future eFfing fossil Fridays!

Good olde dentistrie

I’m reading up on mandibular rotation, which is the change in orientation of the mandibular corpus relative to the rest of the skull during growth (the corpus is the horizontal part of your jaw that holds up your teeth; check out the shape changes in the mandibles in the blog header). So far as I can tell, the original classic paper on the topic is by Bjork (1955). Growth was studied by implanting metal pins into the jaws, then seeing how they move across ontogeny via X-rays (which were once called “roentgenograms,” neat-o!) Here’s a picture of the procedure, from Bjork (1955):
HOLY GOD WHAT DID THAT KID DO TO DESERVE THIS?! And although there must be a third person there, it sorta looks like there’s a three-handed dentist wielding a hammer, a nail, and a kid’s face. No wonder so many people are afraid of the dentist.
BJORK A (1955). Facial growth in man, studied with the aid of metallic implants. Acta odontologica Scandinavica, 13 (1), 9-34 PMID: 14398173

Growing a Homo erectus kid, sort of

A paper, given at this year’s Physical Anthropology meetings, was just published online in the Journal of Human Evolution, with a re-evaluation of the height and possible growth pattern of a subadult skeleton of Homo erectus (KNM-WT 15000, aka “Nariokotome boy,” aka “Stripling youth”). When initially described, it was estimated that this young chap would gave grown to be around 6 feet tall. However, controversy around the skeleton’s age at death and probable growth pattern have made this quite a contentious topic. In the recent paper, Ronda Graves and colleagues used a South African human growth pattern and a pattern from “naturally-reared captive” chimpanzees to devise a series of intermediate growth patterns that might have characterized H. erectus. Using the pattern they felt most likely reflected the Nariokotome skeleton’s estimated life history parameters, the authors estimate the potential adult height of the youth to have been closer to about 5′ 4″.

All I’d like to say about this is that deciding how tall a subadult skeleton like this would have grown to be is inherently tricky. First, the individual’s height when he died has to be estimated, and this will always be an estimate, so there’s one level of error there. Next, in order to determine the duration of growth remaining had he lived, one must estimate the skeleton’s age at death. This has been debated for the Nariokotome skeleton, because the pattern of tooth eruption and tooth enamel formation seem to point toward an age of around 8-10 years; but the pattern of long-bone closure suggests an age closer to 13 or so years. If anything, this means we cannot assume a ‘human-like’ or ‘chimpanzee-like’ pattern of skeletal and dental development for H. erectus. But our final step in figuring out how tall this kid would’ve been is to use the previous 2 estimations to infer how much longer, and at what rate, he would have grown. Crap.
The authors tried to circumvent this issue by averaging/modifying the human and chimpanzee mathematical growth curves. The curves themselves come from averages of respective species samples, which then were combined (sort of like averaging) to create intermediate growth curves. They then also multiplied the intermediate curves by various constants, in attempt to model different life history patterns in the growth curves. Incidentally, one of these ‘altered life history’ models provided their preferred height estimate of 5′ 4″. I think this is a clever and interesting way to tackle the question of how to estimate height from fossils; but I think it’s important to bear in mind how far-removed from both their models they had to get to do this. How many assumptions and potential sources of error should be permissible? Are there any biological constraints that might actually render a human-chimp average growth pattern to be unrealistic? I dunno!
Anyway, an interesting paper, and with a pretty good literature review, too. As stated above, the model they preferred gave a much shorter height than traditionally accepted for this individual. The model (and so H. erectus) also lacks a modern human-like growth spurt, which this specimen would have ‘needed’ to attain a tall adult height like has been traditionally thought. Other researchers have used the cranial remains to argue that Nariokotome kid would have had a good amount of growth left to have a more characteristically H. erectus-like skull, though it is unclear if this necessarily means an adolescent growth spurt. So, this was a very interesting and thorough study, but I’m sure it’s not the last we’ll hear about growth and development in H. erectus.
The paper
Graves RR, Lupo AC, McCarthy RC, Wescott DJ, and Cunningham DL. Just how strapping was KNM-WT 15000? Journal of Human Evolution, in press.