Wednesday, January 21, 2015

Post Lecture Recap: The Tale of the Teeth

Last night, I headed to the beautiful Bell House, in the Gowanus section of Brooklyn, for this month's Secret Science Club lecture, featuring Dr Shara Bailey, of NYU's Physical Anthropology Department and the Center for the Study of Human Origins. Dr Bailey's, er, bailiwick, is dental paleoanthropology, the study of hominin teeth.

Dr Bailey began the lecture by stating that teeth are an important part of the anthropological toolbox- the display of one's teeth can inform others of one's emotional state, one's health, and one's wealth. An examination of an individual's teeth can reveal the approximate age of that person- the first permanent molars (the "school teeth") typically erupt at the age of six years, the second permanent molars (known as the "factory teeth" in the days before child labor laws) typically erupt at the age of twelve, and the third permanent molars (the "wisdom teeth") typically erupt at age eighteen. If one were actually to cut a tooth in half (something that is generally not allowed to occur with teeth in museum collections), a more refined estimate of age can be made by counting the layers of weekly enamel secretion. The physiological stress of birth is evident by an enamel layer known as the neonatal line.

Enamel secretion is affected by diet- one dramatic example of this is the explosion of dental caries and abscesses in North American populations after maize was adopted as a dietary staple. In a less dramatic example, an examination of microwear on tooth enamel can reveal what an individual ate within the last years of its life.

Tooth morphology can also reveal the population affinity of an individual- Northern Europeans often have a feature on their first molars known as "Carabelli's cusp", Native American and East Asian populations often have "spatulate or shovel-shaped" incisors, African populations often have a feature called cusp 7, and indigenous Australians have a divergent cusp 5. By examining teeth, an expert can assess biological affinities and gauge migrations.

Dr Bailey noted that teeth have a high (97%) mineral content and quipped that they are "fossils in your mouth". While the roots of teeth need to fossilize, the teeth are "already there". Because of this, teeth are very common fossils. Because of the prevalence of fossil teeth, and the variety of distinct tooth morphologies, a study of Paleolithic hominin teeth can help to determine how much interbreeding there was between Neanderthals and early modern humans.

The lecture then shifted to a quick introduction to the Neanderthals ("H" optional, and silent, at any rate), a geographically and temporally distinct archaic hominin. The Neanderthals ranged from Europe to Siberia from approximately 250,000 years ago to approximately 40,000 years ago. Neanderthals were distinguishable by large brow ridges, a long "lemonhead", an occipital bun at the back of the skull, and midfacial prognathism (the middle of the face extends outward). The Neanderthals are commonly associated with an assemblage of tools known as the Mousterian industry.

Dr Bailey then asked why modern humans survived while the Neanderthals did not. Were Neanderthals subsumed into the modern human gene pool? Were they challenged dietarily, perhaps by an inability to catch birds or fish? Were they just culturally less sophisticated to the extent that they couldn't compete with early modern humans? Examining Neanderthal teeth could help to answer these questions. On the subject of diet, were Neanderthals scavengers? Primates typically don't scavange carcasses. An analysis of the isotopes in Neanderthal teeth and bones conducted about fifteen years ago revealed that meat made up much of the Neanderthal diet. Skeletal remains of Neanderthals reveal that they lived very tough lives- just about every Neanderthal skeleton found shows evidence of healed injuries. This combination of a lot of meat and a lot of injuries suggests that Neanderthals hunted a lot.

Among the Neanderthal remains found at the La Ferrassie site, there was evidence of drastic tooth wear in the incisors, some of the individuals having worn the enamel completely from their teeth. This pattern of wear is similar to that found in some modern Inuit populations- it is possible that this wear resulted from paramasticatory behaviors, such as using the teeth to hold tools or to work hides. The wear was evident in all adults, with no differentiation between males and females. There was also evidence to suggest that right-hand dominance occurred with the same frequency as it does in modern populations.

A topographical analysis of the enamel surface proved that Neanderthal diet varied over their geographic range- Neanderthals that inhabited open environments exhibited wear patterns similar to those of natives of Tierra Del Fuego, while those who inhabited wooded areas had wear patterns similar to those of modern forest dwellers. The amount of vegetation consumed correlated with the amount of tree cover. Individuals inhabiting similar environments tend to have similar diets.

Dr Bailey then described the use of calculus to determine diet- an examination of tartar can reveal phytoliths which provide proof of consumption. Phytoliths found in the dental calculus of Neanderthals found at the Shanidar III site in Iraq demonstrated that cooked barley was a component of their diet. Although samples came from a relatively limited geographic range, Neanderthals and early modern humans living in the same area ate the same things.

Neanderthals and early modern humans coexisted for 50,000 years in Europe, but determining the taxonomy of fossil teeth (the most commonly preserved part of the body) was beyond the capabilities of early researchers. It was originally thought that it was impossible to determine the taxonomic affinities of fossils solely from teeth, but this has turned out to be false. In order to determine the affinity of teeth, the morphology of teeth had to be studied and the scientific method had to be applied to the analysis of teeth of unknown origin. Dr Bailey then launched into a topic she dubbed "Neanderthal Teeth 101". Neanderthals had curved, shovel-shaped incisors. There then followed a flurry of information about hypocone cusps on the upper first molar, and the statistically significant differences between Neanderthal and modern human teeth. Neanderthal cusps tend to be "skewed"- they tend to form a more rhomboid pattern than is typical of modern human teeth, though there is some overlap. Despite the variation, the differences are statistically significant. The tips of Neanderthal cusps are more closely spaced, and it appears that Neanderthal tooth growth was more rapid. This rapid tooth growth is a derived trait, which Dr Bailey jokingly characterized as "whacko", extant apes and modern humans have more similar growth patterns.

There are 27-30 dental traits that point to Neanderthals- the combination of traits is significant, not one single trait. Again, there was a rapid-fire introduction to specific traits, a distinct lower 4th premolar, the high mid-trigonic crests common to 90% of Neanderthals. Dentally, Neanderthals were more different from humans than chimps and bonobos are.

Limiting one's observations to morphology, one would suggest that there was differentiation between Neanderthals and modern humans at the species level. Even though mammal species can have a divergence resulting from being separated for a million years, interbreeding can occur. The species concept does not apply except in the long term. It is believed that the Neanderthals and modern human populations separated morphologically over 400,000 years ago. That being said, modern humans of European and Asian descent have a genome characterized by 2-4% Neanderthal genes (the average being 2.9%). The presence of Neanderthal genes in the modern human genome can be explained by Neanderthal-modern human matings once ever 20-50 years throughout the years of population overlap. The persistence of these genes suggests that they played a significant role in the survival of these populations. A skeleton found at the Lagar Velho site in Portugal appeals to be a Neanderthal/modern human hybrid. The skeleton features short distal limbs, which could have proved to be an adaptive advantage in high altitude or cold weather conditions- the skeleton's teeth, though, offer no evidence of hybridization, necessitating further study of this specimen.

There may have been cultural factors limiting the interaction of modern humans and Neanderthals. There is a paucity of art associated with Neanderthals- most paleolithic art is associated with early modern humans, with the Aurignacian being perhaps the greatest flowering of paleolithic art created by modern humans. An analysis of teeth associated with Aurignacian sites shows that 89% of them belonged to early modern humans. Another Paleolithic "industry", the Châtelperronian, perhaps represents a Neanderthal population using tools more characteristic of early modern humans. It is unknown whether the Châtelperronian tools were made in imitation of modern human tools or were obtained through trade... it is possible that Neanderthals had the capacity to create as modern humans do, but that they tended not to do it.

The end of the Neanderthal era was marked by climate change- it was characterized by variable climatic trends, shifts between cold and temperate conditions, and the resultant shifts in biomes, such as tundras giving way to forests. Shrinkage of environment and the limitation of the Neanderthals' resource base would have resulted. At about 65,000 before present, the Neanderthal population appears to bounce back after a decrease, but by then early modern humans would have exerted pressures on Neanderthal populations. Small advantages for modern humans would have made an impact on the food base of Neanderthals, which resulted in their being supplanted.

Dr Bailey delivered a tour de force lecture, yet another triumph for the Secret Science Club. In the Q&A, some bastard asked about estimated caloric needs for Neanderthals- these people were built like short linebackers and had very large brains. Dr Bailey indicated that the general consensus is that an adult probably needed to consume 4,000 kilocalories worth of food per day. Said bastard also snuck in a question about the Denisovans- Dr Bailey said that there just wasn't enough evidence to make any definitive statements about them, but that they seemed to be more closely related to Neanderthals than modern humans.

Here's a video of an interview with Dr Bailey:

Pop open a beer, sit back, and listen to Dr Bailey drop science... it's the Tale of the Teeth, and it's a fascinating one.


M. Bouffant said...

Say, I'm going to get some new choppers next wk.

This I remember from anthropology: The Y-5 molar pattern. Maybe I should get monkey-style teef.

Big Bad Bald Bastard said...

We gotta get you on the stage to give a lecture. Ever think about a cross-country road trip?

mine az said...

Nice Post! Diet really affects our teeth. I love to eat sweets and due to this habit I got teeth ache problem. Then I decided to visit my dentist Hermosa Beach for treatment and luckily I got best treatment at reasonable prices.