Saturday, July 26, 2014

Teeth ~ The Blame Game


When I ran my fingers over a small, complex tooth from a titanothere (an extinct, Eocene/Oligocene mammalian herbivore, a relative of horses and rhinos, among others), I was reminded of a point paleontologist Neil Shubin makes about our (human) teeth – they’re effective because they’re designed to fit together exactly, upper to lower ranks.  In his wonderful book, Your Inner Fish:  A Journey into the 3.5-Billion-Year History of the Human Body (2008), Shubin writes:
The most remarkable thing about our mouths is the precision with which we chew.  Open and close your mouth:  your teeth always come together in the same position, with upper and lower teeth fitting together precisely.  Because the upper and lower cusps, basins, and ridges match closely, we are able to break up food with maximal efficiency.  In fact, a mismatch between upper and lower teeth can shatter our teeth, and enrich our dentists.  (p. 6)
His point doesn’t apply just to humans but to many mammal species, past and present, with complex, differentiated teeth offering an array of cutting, sheering, and crushing surfaces on their crowns whose efficiency depends upon a fit between upper and lower ranks, as with the ancient animals who lost the now-fossilized teeth pictured below.  These teeth are shown first from the side and then looking down on the crown.  The first is from the titanothere (or brontothere) that prompted this posting (this specimen was found in the White River Badlands, South Dakota); the second is from an Oligocene rhinoceros, Hyracodon (the so-called “running rhino,” this example is also from the White River Badlands); and, the last is from a Pleistocene horse, Equus (I found this specimen in southwestern Florida).




Shubin’s passing reference to dentists and dental problems raises for me the question of “Why?”  Actually, “Why do we have an arrangement of teeth potentially subject to such problems?”  I guess I’m looking for a place to lay blame.

The broad outline of how teeth, in general, and mammal teeth, in particular, evolved is at the heart of paleoanthropologist Peter S. Ungar’s Teeth:  A Very Short Introduction (2014).  It actually is short (137 pages), but this is no dummied-down, science-lite, cursory piece.  Rather, it offers a richly detailed, nicely illustrated exploration of the science of teeth – what they do, how they do it, and how they came to be.  This slender volume is one of the latest additions to Oxford University Press’ Very Short Introductions (VSI) series which has been around for almost 20 years, but is new to me.  VSI offers some 350 titles on a wide array of topics in religion, science, history, politics, philosophy . . . . and on.  If Teeth is at all representative, the series is worth exploring.

My placing of blame for our dental issues begins, I suppose, with our distant, earliest vertebrate ancestor.  Ungar explains that teeth are unique to vertebrates because true teeth develop in the embryo from the neural crest layer of cells, a layer only we vertebrates have.  He considers which vertebrate taxon first sported teeth, but leaves us where the science seems to be – a bit uncertain (and, in my case, somewhat confused).    Did teeth develop from the inside-out (from elements in or around the throat), or the outside-in (from external placoid scales)?  Unclear.  But it is with the emergence of the earliest jawed fishes (gnathostomes) that true teeth are likely to have taken root (so to speak); today, jawed fish are all mostly toothed.

Mammals don’t hold a monopoly on tooth complexity.
And while we usually think of mammals when considering different kinds of teeth in one mouth, complex crowns, occlusion [bite], and chewing, many other vertebrates experimented with one or more of these things.  After all, teeth have been around twice as long as mammals.  (p. 54)
A system of a precise bite with complex, differentiated teeth doing a masterful job of chewing first emerged some 300 million years ago among “large, reptile-like” tetrapods called didectids.  Later, duckbill dinosaurs apparently took this to new heights, accomplishing not only the up-down motion of a hinged jaw, but side-to-side movement as well because of how their upper and lower teeth came together.

But, with the emergence of mammals, certain trends in teeth became more pronounced – moving from fish to amphibians and reptiles to mammals, we generally find fewer teeth in the mouth, a more restricted area in which teeth appear in the mouth, and a decline in the number of replacement teeth.  In fact, Ungar posits, “today, mammals own occlusion and chewing.”  (p. 3, emphasis added)  In the fossil record, the tell-tale signs of mammalians as chewers are: “ separation of front and back teeth into different types, a new jaw joint, reorganization of the chewing muscles, two generations of teeth, a bony palate, and prismatic [prism-shaped at the microscopic level] tooth enamel.” (p. 66)

Frankly, I've regretted that we mammals don't the multiple replacement teeth that other taxa do, but our occlusion and chewing preclude that.  Apparently, it's difficult to fashion a system of replacing different teeth several times over as they wear out or decay while still maintaining the precision of our bites.

Until Ungar laid it out for me, I wouldn’t have thought to lay blame for tooth difficulties on our warm bloodedness.  He outlines the intimate relationship between mammalian teeth, with their complexity and exact occlusion, to endothermy, our warm bloodedness which requires us to generate heat from the inside of our bodies.  Endothermy requires lots of energy and mammalian tooth structures, arrangements, and movements help to extract the most energy from the food consumed.  For instance, the shells of insects and seeds can be crushed in the mouth releasing the nutritious material inside, otherwise these would pass through undigested.  Breaking food down into small pieces exposes more surface area on which digestive enzymes can work.  This is a good thing – “the earliest mammals were able to spread farther and into colder places because chewing allowed them to squeeze the energy needed to fuel an internal furnace.  New habitats meant new potential resources, which fed back into selective pressures for even more new teeth.”  (p. 122)

I began by asking why we have a complex, carefully orchestrated arrangement of teeth in our mouths that can go wrong.  But, upon reflection, I realize my question assumed our system of occlusion and chewing was somehow at fault.  A more neutral question would be, “Why have humans developed tooth problems?”  Provocatively, Ungar suggests that it may not be the arrangement that’s at fault, but, rather, we are.  Indeed, tooth decay and occlusion issues are largely the province of Homo sapiens today, not with our family members in the relatively near past and not with our closest evolutionary cousins.

The answer (of where blame may come to rest) apparently has to do with the pace of evolution and the changes in our diet.  “In effect, our diet is changing too fast for our teeth and jaws to keep up.”  (p. 116)  The adoption of a carbohydrate-rich diet (blame the development of agriculture) has wrecked havoc with our teeth, and it’s all compounded now with the inclusion in our diet of high levels of sugar.  This is largely a cavity issue.

As for our occlusion problems, there’s a fascinating debate over their source, but it’s possible that the actual issue is that our jaws are too small for the size and array of teeth we try to cram in, and we’ve contributed to this situation.
And, indeed, human jaws have become shorter since the Early Stone Age.  Our jaws are most likely underdeveloped because soft, highly processed foods don’t provide the strain from heavy chewing needed to stimulate normal growth of the jaw during childhood.  (p. 120)
So, as I slip off my lower, partial bridge at night and wedge a mouth guard onto my upper teeth (and reverse the procedure in the morning), I guess it wouldn’t be fair to curse, even faintly, my very distant, initial vertebrate ancestor, or my somewhat less distant, first mammalian ancestor.  The blame, it would seem, lies closer to home.
 
Nature Blog Network