Afarensis

Over at UD there is a post bashing paleoanthropology. For example, one commenter (GilDodgen) says:

The methods and concepts of evolutionary anthropology often consist of making up stories, presenting them as facts, and arriving at silly conclusions.

This is the first of a series examining that issue.

Most of you are familiar with broad outlines of human evolution, starting with Ardipithecus and extending to us., but have you ever wondered how it is we can talk about such things? How can we, as scientists, take a fragment of a bone (contrary to creationist rhetoric one can learn valuable things from fragments - depending on what piece of bone it is), an entire bone or a complete skeleton and learn anything about our evolutionary history? Take teeth for example, anthropologists love teeth. From teeth we can learn about different kinds of stress the individual faced while thier teeth were growing - by examing various kinds of hypoplasias for example. We can also do some interestings by studying fluctuating and directional asymmetries (both of which have a strong environmental basis). We can even look at human variation and, following Turner, learn about the spread of modern humans across the globe. As interesting as these subjects are, they don't get us very far. What we want to know is something like how Glenn Conroy can say:

It is interesting to note that most of the early Miocene hominoids (dryomorphs) had thin-enameled teeth and most of the middle Miocene hominoids (ramamorphs) had thick enameled teeth. This suggests that enamel thickness increased coincidently with major ecological changes that were taking place in East Africa during this perod, namely the change from wetter, more forested conditions to drier, more woodland-bushland environments.

on page 208 of Primate Evolution (1st ed)? We also want to know what the relationship is between thin enamel/ wet environments and thick enamel/dry environments is, and how does it all relate to later human evolution? Let's start by looking at the picture below.

It is a cross section of a molar. Note the pulp cavity surrounded by dentin and capped by enamel. Obviously, we are concerned with the enamel and would like some means of measuring it. So let's look at Gary Schwartzs' "Taxonomic and Functional Aspects of Patterning of Enamel Thickness Distribution in Extent Large Bodied Hominoids" published in the AJPA in 2000. The study examined seven measurments taken on the molars (of both male and female) of eight Orangs, six chimps, nine gorillas and 39 tenth century humans from Austria. Since there are size differences between the species mentioned Schwartz had to figure out an appropriate way to eliminate size effects and an appropriate statistical technique. He settled on various measures of cervical width and discriminant function analysis as the best way of performing the analysis. When talking about enamel thickness a pretty simple equation has always been used. Generally, thicker enamel was taken to indicate a reliance on harder foods, while thinner enamel has been interpreted as an indicator of a softer diet. But this is kind of basic and studies have shifted towards studying the distribution of enamel across the crown which should give clues to the type of masticatory stress the tooth is undergoing. Schwwartz was interested in two questions. First, the distribution of enamel really related to functional concerns. Second, is enamel thickness a reliable indicator of taxonomy. Some of the results were highly interesting. Pongo and Homo displayed a wide degree of variation in enamel thickness and overlapped in all measurments (both are considered to be thick enameled species) while Pan and Gorilla displayed smaller degrees of variation and rarely overlapped. Overall, the pattern was complicated enough for Schwartz to conclude:

These...seem to indicate a more complicated pattern of enamel thickness distribution than is recognized by comaprable indices of "relative" or "average" enamel thickness.... As a result, a comparitive dichotomy of "thin enameled" and "thick enameled" based on such indices...may underestimate proportional differences in enamel thickness in teeth of hominoid taxa, which may prove critical in investigating the taxonomic, functional, and phylogenetic importance of enamel thickness patterning.

In terms of taxonomic utility, the discriminant function reclassified hominoids with 90% accuracy (although I am skeptical of this because they are trying to classify specimens used to create the discriminant function).
Scwartz does address the issue of what enamel thickness indicates about diet but let's approach it from a slightly different angle.

On the left is a scanning electron microscope phot of a gorilla molar on the right is a the same for Ouranopithecus.
A paper by Merceron et al published in Palaios in 2005 (A New Method of Dental Microwear Analysis: Application to Extent Primates and Ouranopithecus macedoniensis [Late Miocene of Greece]) reported on the results of dental microwear analysis of 216 primates (3 species of baboon, gibbons, orangs, chimps and gorillas) plus 15 specimens of Ouranopithecus.

A total of six variables (numbers of scratches, wide scratches, cross scratches, pits, large pits, and length of scratches) were analyzed. One of the questions the study sought to answer was if microwear analysis could discriminate between the main dietary catagories. One cluster contained primates with low pitting - mainly the gibbon and gorilla. A second cluster contained primates with high abrasion - namely the three species of baboon. A third cluster - with values intermediate to the other two - contained orangs and chimps. But what of Ouranopithecus? Ouranopithecus had high numbers of scratches, pits and large pits. In fact the microwear on Ouranopithecus' teeth linked it most strongly with Papio hamadryas hamadryas indicating that it was a hard food feeder, most likely feeding on tubers roots and rhizomes. This also suggests that Ouranopithecus has thick enamel.

Finally, we can study the microstructure of enamel to learn some interesting things about the timing of tooth development in different species. T. M. Smith, in a paper called Experimental Determination of the Periodicity of Incremental Features of Enamel (published in 2006 in the Journal of Anatomy) studied 98 sections of maxillary and mandibular teeth from 17 juvenile pigtailed macaques. What they wished to address was the periodicity (or lack thereof) of intradian lines, laminations, retzius lines (B in the picture below):

and cross-striations:

Results indicated that cross striations are the result of a 24 hour secretion rhythm, intradian lines are the result of a 12 hour secretion cycle, Retzius lines represent about a 4 day interval and laminations are the result of a 24 hour rhythm. Smith mentions an intriguing notion in the paper in discussing the above findings it is pointed out that two or more circadian rhythms could be responsible.

There are other approaches that could be mentioned, analysis of secretion rates in extant hominoids, or the effects of masticatory stress on the shape of the mandibular corpus spring to mind but this post is already too long so I will leave that for another time. My next post in this series will be on the cranial base and will, probably, be posted the day after tomorrow.