The Trouble with Teeth…
…is that we seem to have too many of them. Hands up if you’ve had your wisdom teeth removed? Obviously this ad hoc survey is flawed since I cannot see whether your hand shot up, but mine did, so at least one of us has been under the dentist’s knife.
I bring this up for three reasons: I spotted a recent review in Science that talked about a controversial new hypothesis for the influence of diet on early hominin development; I also saw an article in Wired about teeth crowding; and finally I thought this would make a good topic for the NESCent blog contest to maybe get me some dollars to go to Scio12 (wink).
You are what you eat
Our knowledge of what our ancestors ate is almost entirely based on the fossil record. The earliest hominins (the group of apes with direct ancestry to Homo sapiens) likely appeared on Earth about 7 million years ago. Of these, perhaps the most famous are the Australopithicines, whose recovered remains date back over 4 million years. From these upright walking primates our own genus, Homo, arose a mere 2.5 million years ago. All of these hominins lived in eastern Africa, their two-legged stance perfectly adapted for hunting and gathering the fauna and flora ever-expanding savanna that covered the continent.
But what exactly did they hunt and/or gather, and how did they eat what they foraged? As the African landscape shifted from thick forest to open grasslands, our ancestor’s diets likely shifted from sugary tree-born fruits to harder, more brittle nuts and seeds. This idea is based on various fossilized teeth that show gradually thickening enamel and a steady increase in size. Homo teeth were also subsequently adapted to tear through meat, and, thanks to their habit of making and using tools, hard foods no longer had to be consumed without some initial processing.
An additional feature of these ancient teeth, called dental microwear, allows for a more detailed analysis of a tooth’s surface. Small pits in the enamel provide evidence of hard object chewing, whereas long stripes and groves suggest shearing of leafy foods. A combination of pits and striations would suggest a varied diet, whereas seeing only one of the two patterns would imply the tooth-owner was not an adventurous eater. Moreover, these data give us a snapshot of the food ingested in the last weeks before death, kind of a “last supper” picture of the tooth’s activity. Conventional theories would suggest that the earliest Australopithecus species ate a lot of hard objects, and would therefore be expected to have highly pitted teeth. This however turns out not to be the case, with much lower levels of pitting actually being observed. Perhaps our earliest ancestors did in fact have the tools to grind their food before they ate it, or they simply ate different foods than we predicted.
In a complementary technique, paleobiologists are able to learn more about the type of food our ancestors ate by analyzing elemental isotopes deposited in their teeth and bones. You literally are what you eat; the composition of these hard tissues is a direct reflection of what you consume. In particular, carbon isotope deposition tells us a lot about the quantity of plant matter in a given diet. It seems from these data that Australopithecus enjoyed a far more varied diet that just nuts and seeds, perhaps incorporating meat and tuberous vegetables.
The rise of the farmer
All of the changes in jaw and teeth morphology that have been observed during early hominin evolution happened over millions of years allowing for gradual adaptation to a changing environment. However, around 14,000 years ago modern humans (Homo sapiens sapiens) started to rely on agriculture as a means of survival. They cleared natural landscapes to plant crops that would yield enough food initially for their family and later for entire cities. They also developed animal husbandry methods, breeding docile, meaty animals for consumption, and more intensive food processing.
NB: There is quite a large difference between 7 million and 14,000 years.
It is clear from various studies that our jaws are shortening and thus leaving us with less space for all the teeth we house there. What is not so clear is exactly why this is happening. A recent study published in PNAS suggests that the shift from hunting and gathering to modern agricultural practices may indeed be to blame. Noreen von Cramon-Taubadel, an anthropologist at the University of Kent, decided to look at the jaw length of 11 different human populations. What she found was that the jaw-lines of hunter/gatherer populations are indeed longer than those of agricultural civilizations.
But what this correlation does not tell us is why this jaw-shortening predominates in humans who rely on farming to survive. Is it because we adapted to our softer, more processed foods, or is there a difference in the nutritional value of the food we eat as infants? Previous studies have shown that young animals fed soft foods develop shorter jaws than those fed hard foods, and given the relatively short period of time that we as a species have been farmers, natural selection for shorter jaws remains a controversial hypothesis. And even though dentistry has really only been perfected within the last century, it seems extremely unlikely someone would die of an impacted wisdom tooth before producing offspring.
Vexed by your vestigials?
And so we find ourselves, in 2011, paying our dentist a hefty fee to remove those teeth that no longer fit in our dainty little jaws. I had my wisdom teeth out over 10 years ago, and after reading all the materials needed to write this essay, I have decided to blame my Dad for the discomfort. He’s a farmer.
Ungar, P., & Sponheimer, M. (2011). The Diets of Early Hominins Science, 334 (6053), 190-193 DOI: 10.1126/science.1207701
von Cramon-Taubadel, N. (2011). Global human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1113050108