Wisdom of Mann
Steve Mirsky caught up with Alan Mann for a recent Scientific American 60-second podcast: "Third Molars Illustrate Differential Reproduction". The result is a classic.
teeth
Tracing teeth troubles with fossil bacteria
Ed Yong has a great account today of some research from Alan Cooper's lab on the oral microbiome in pre-agricultural and post-agricultural Europeans: "Prehistoric Plaque and the Gentrification of Europe’s Mouth".
The hunter-gatherers had a diverse array of bacteria including several groups that are associated with good health. That fits with the relative absence of tooth decay or gum disease among modern or prehistoric hunter-gatherers. “They were at the end of a long period of happy co-evolution between us and oral bacteria,” says Cooper.
The advent of farming disrupted that tango. After the Agricultural Revolution, as humans began to chow down upon barley, wheat and other domesticated crops, the diversity of the mouth microbes fell, and species associated with oral diseases became more common. “Eating all this soft squishy carbohydrate and leaving it lying around the base of your teeth is effectively inviting in a whole new range of bugs to take up permanent residence in your mouth,” says Cooper.
I'll have some more comments on this new research when I can sit down to write them up. I've been waiting for this to come out for quite a long time -- I first heard about the research almost three years ago. The potential to characterize oral ecology across time is immense, and we have some excellent data on dental pathologies across the entire timespan. Caries and other dental pathologies are very new in human populations, and although starchy diets have been blamed, very little has been known about how oral bacteria themselves may have become more pathogenic over time. This study is really great because it opens a new door to looking at this evolution across time. We will need to compare this record with the evidence for morphological change in teeth across the same time span. Smaller teeth may have been a consequence of selection associated with dental pathology in agricultural peoples.
Next we will need to compare across space -- including greater sampling of oral microbiome variation among living humans. This is another new area in which we know more about prehistoric people than we do about living human variation!
Teeth and teaching
Razib Khan has a short but worthwhile post about dental health and heritability: "The moral measure of bad teeth".
As someone who is quite conscious of the power of genetics, I was quite taken aback by this blind spot. I realized that not only did I attribute my own rather fortunate dental health (so far) to my personal behaviors, but, I had long suspected those with dental issues of less than optimal habits. Obviously environment (e.g., high sugar diet) does matter. But apparently a great deal of the variation in the trait is heritable.
Tooth pathologies are a great example of heritability, because they provide a discrete character (cavities or no cavities) with an age-dependent component, a meristic (how many cavities) component, and several well-characterized environmental components. They're wonderfully multifactorial -- unlike, say, obesity, which is usually understood only along a single factor dimension. Plus, teaching students about this stuff actually helps them address their own health needs, as well as those of their future children.
Calculus microbially
Molecular archaeologist Christina Warinner gave a TED talk and the main ideas are now in a CNN article: "Why your dental plaque is valuable".
By applying advanced DNA sequencing and protein mass spectrometry technologies to ancient dental calculus, we can begin to reconstruct a detailed picture of the dynamic interplay between diet, infection and immunity that occurred thousands of years ago. This allows us to investigate the long-term evolutionary history of human health and disease, right down to the genetic code of individual pathogens, and it can teach us about how pathogens evolve and why they continue to make us sick.
This is really neat work, although the article doesn't go into any new results.
Tooth wear
Synopsis:Laboratory exercise discussing the basics of dental attrition.Teeth have a close association with longevity. Enamel is the hardest substance in the body, but it does break, wear out, and is sometimes attacked by microbes. In Westernized contexts, we are all familiar with cavities, caused by acid-emitting bacteria in the mouth. But in many natural human societies, cavities (called caries) are rare. Instead, a lifetime of eating abrasive natural foods usually causes the teeth to wear down, a process called attrition.
Dental attrition is very important in the anthropology of ancient peoples. It helps us to understand the food processing techniques — for example, the use of abrasive grinding stones to process grain in early agriculturalists. Dental wear also provides a way of understanding the ages at death of ancient skeletons. In Western societies, excessive tooth wear may be indicative of habitual behaviors such as grinding the teeth, or may result from biases in the chewing pattern to one side or part of the mouth.
Differential wear describes a dentition in which one tooth is worn significantly more than its neighbors. A normal process of tooth wear results in differential wear, as first molars erupt at age 5 and develop many years of wear before the third molars erupt in the mid- to late teens.
What to do: Examine the teeth at this station. How are they worn? Is there anything complicating their wear pattern, such as the presence of caries? Which individuals have differential wear? Which are worn the most?
Study terms:Deciduous teeth
Synopsis:Laboratory exercise introducing eruption of deciduous dentition in humans and primates.Like most mammals, humans have two sets of teeth. The first set is called the deciduous dentition, but you probably know these as "baby teeth."
The human deciduous dentition includes two incisors, one canine, and two molars in each quadrant. When people lose their deciduous molars, these are replaced by permanent premolars. The permanent molars do not have deciduous teeth in their places before them.
Deciduous teeth are abbreviated with a "d" and the tooth type and number in lowercase. For example, the deciduous lower first molar is a dm1; the upper left deciduous canine is luc.
What to do: Consider the series of models at this station. They represent the mandibular dentitions of children at different ages during their development. Can you determine the order that the permanent teeth erupt and replace the deciduous teeth? For example, are the permanent incisors the first to erupt? The permanent molars?
Part 2
There are several kinds of primate represented at this station. These primates have different adult body sizes, and grow at very different rates. Nevertheless, their teeth erupt in sequences that are very much like the human dental eruption sequence.
Yet, there are exceptions. Many primates erupt their canine teeth relatively late in their eruption sequence. In humans, the upper canine typically erupts before the second molars. In many primates, the canine is delayed in development compared to the second molars.
What to do: Examine the primate dentitions at this station. Identify the deciduous and permanent teeth that you see in each. Try to think about what age a human would likely be, with the same teeth present. Can you find aspects of tooth eruption that differ between humans and these primates?
Incisors
Synopsis:Laboratory exercise introducing incisors, including lemur tooth combs.The incisors are the front teeth. They are basically flat and have a blade-like occlusal surface. Each quadrant has two incisors.
In humans and other primates, the upper central incisor (called the I1) is typically larger, the lateral (the I2) smaller.
At this station you'll find casts of several primates, including some prosimians with tooth combs. Examine these mandibles. Some of the tooth combs include four teeth, and some six. The tooth combs with six teeth include the two incisors (I1 and I2) and the lower canines. The four-tooth combs are missing either the lateral incisor or the canine. Specialists disagree on this point. What do you think?
Canines
Synopsis:Lab exercise introducing canines, including sizes of maxillary canines in hominoids.The canine teeth in humans range from pointy-shaped to incisor-like in shape. There is only one canine in each quadrant, and it is the third tooth just distal to the incisors.
Upper canines are often denoted UC and lower canines are then LC (so that the left lower canine is LLC.
In many other primates, the canine teeth project out far beyond the others. There is often a large space, or \term{diastema} between the upper canine and the lateral incisor. For many species, the canine teeth are the largest difference between male and female skulls.
This station has the skulls of several kinds of primates. Measure the height of the right canine tooth in each maxillary dentition. This measurement is taken from the tip of the canine to the base of its enamel.
Then measure the breadth of the first molar.
Your assignment is to make a plot showing how canine height relates to molar breadth in this sample of primates. Are there any outliers in your plot?
Are there any other features of the mandibles that seem to correlate with canine breadth?
Wisdom teeth
Synopsis:Laboratory exercise to introduce third molar variation.Most humans have three molars, but many — especially in America — have their third molars (called wisdom teeth) extracted. Some people do not develop third molars at all, or they never erupt into occlusion.
Humans are not alone. Some other kinds of primates have entirely lost their third molars and normally erupt only two in each quadrant. The South American monkeys called callitrichids (marmosets and tamarins) are small-bodied monkeys who normally have only two molars in each quadrant of the jaw.
Many people begin to develop third molars within their jaws, but the teeth never erupt. Others don't have any development of the third molars at all. We may not know about this unless we learn it from X-ray images. Sometimes teeth are extracted before they emerge from the jaw, or erupt, but typically any problems become apparent at or after eruption.
We are interested in showing whether third molar eruption or extraction can be correlated with any of the measurements you took earlier in the semester. In the spreadsheet, for each of your third molars, indicate whether you have it in the tooth row now, whether it has been extracted, or whether it never erupted at all.
The two incisors, one canine, two premolars and three molars on both top and bottom are called the human dental formula. We write a dental formula as follows:
2 1 2 3
___________
2 1 2 3What is the dental formula of a human who has never erupted her upper wisdom teeth, but who has the lower ones?
Study terms:Molars
Synopsis:Laboratory exercise to introduce the terminology and anatomy of the molars.The most distal teeth are molars. Most humans have three molars, but many — especially in America — have their third molars (called wisdom teeth) extracted. Some people do not develop third molars at all, or they never erupt into occlusion. Molars have three or more cusps, and are used as grinding teeth.
The upper molars are typically labeled with superscript numbers M1, M2, and M3, the lowers with subscript numbers M1, M2 and M3. Hence, the left lower first molar becomes LM1.
Teeth have different directional terminology, referring specifically to the tooth row and the mouth. The direction toward the center front of the tooth row is mesial, and toward the rear of the tooth row is distal. For molars and premolars, the direction toward the cheek is buccal, and in toward the tongue is lingual.
The two incisors, one canine, two premolars and three molars on both top and bottom are called the human dental formula. We write a dental formula as follows:
2 1 2 3
___________
2 1 2 3
Pages
Neandertals
For years, I've worked on their bones. Now I'm working on their genes. Read more about the science studying these ancient people.
Denisova
From a finger bone of an ancient human came the record of a completely unexpected population. My lab is working on the science of the Denisova genome.
Acceleration
The advent of agriculture caused natural selection to speed up greatly in humans. We're uncovering some of the ways that populations have rapidly changed during the last 10,000 years.
Malapa
Just outside Johannesburg, the Malapa site is producing some of the most exciting finds in human evolution. This site is the headquarters of the Malapa Soft Tissue Project.