The pattern and regularity of mammalian teeth are not genetically inherited, per se, but reveal instead an evolutionary tool kit for their formation. Illustration by Alison Schroeer
I want you to contemplate your teeth for a moment. I'm sure they're very familiar to you, to the point that you take them for granted (except, perhaps, on visits to the dentist), but take a moment to run your tongue over them one by one. They're beautiful! You have, lined up in an arc in your mouth, rows of exquisite abstract sculptures, each one an enameled column, with whorls and bumps and edges in stately and regular array, all matching and meshing to chew and cut. Almost all of us have imperfections in our teeth, of course, and yet there is something exquisite in their predictable patterns and characteristic shapes.
Teeth are wonderfully diagnostic; a skilled comparative anatomist can identify your genus, tell you about your diet, and summarize your evolutionary lineage by examining the shape of a single tooth. A whole mouthful can serve as a detailed biological road map to your ancestry.
Regularity is the hallmark of the arrangement of our teeth. Humans have a standard pattern: On each side, top and bottom, we have from front to back, two incisors, a single pointed canine, two premolars, and three molars. (The third molar is the wisdom tooth, which may or may not have erupted, depending on your age, or may have been extracted.) Each of these teeth has its own characteristic size and shape and a standard pattern of cusps or bumps, a pattern that is heritable and specified to a surprising degree.
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Check it out: Feel one of the molars on the left side of your mouth, then the corresponding molar on the right side, and you'll find the same bumpy surface on both.
How does that happen? What is the nature of the set of instructions to the tooth sculptor that leads to this strongly determined morphology?
The first answer that might come to mind is that it is genetic (and it is true that the pattern is heritable), that there is some kind of template or blueprint in your genome that says, for instance, to make precisely three molars and two premolars and, further, lays out the arrangement of cusps. However, this is not the case. No such map of the jaw has been found in the genome, nor is it even clear how such a thing would be represented in an assortment of genes. Instead, the details of the architecture of the teeth are specified epigenetically. What could that possibly mean?
It means there is no simple one-to-one mapping of a gene to a phenotype—in this case the shape and size of a tooth. The genome, instead, contains instructions for interacting proteins—for instance, an activator molecule that can tell a tissue to grow a tooth or an inhibitor that can suppress a tooth—and the pattern of their activity is generated within the jaw itself, with interactions between the components leading to the emergence of specific features. You won't find the architecture of a tooth in the genome—you have to let the tools in the genomic toolbox play out in the context of the embryonic environment to generate the pattern. Further, only a few, relatively simple genetic elements generate greater epigenetic complexity. The work of University of Helsinki biologist Kathryn Kavanagh and others on the underlying rules of tooth development is revealing that simplicity.