Teeth development may hold evolution clues

Examining the fossil record through the lens of evolutionary developmental biology may help scientists reassess the the evolutionary history of humans and other hominins, suggests a new report in the journal Science Advances.

Although the human body has 206 bones, the most durable and long-lasting part of the human skeleton is the teeth. Because of this, they are the most common item found in the hominoid fossil record and are thus a vital resource for paleoanthropology.

Molars, those large teeth at the back of the mouth used to grind food, are widely used to determine the species of human and other hominoid remains. This is because the crowns of molars have cusps, rises and protuberances on the surface in patterns that can serve as a kind of species fingerprint.

However, not much is known about how these molar cusp landscapes develop in an organism, and hence how they change from species to species along evolutionary lineages has been unclear.

Now Alejandra Ortiz, a post-doctoral researcher at Arizona State University in Tempe, US, has worked with an international team of researchers to apply an evolutionary developmental biology approach to molar morphology.

Affectionately known as ‘evo-devo’, this approach seeks to understand how evolution (changes in species over long timescales) interacts with developmental processes (how individual organisms develop from embryos).

Since developmental processes produce variations among organisms, and natural selection then operates on these variations, the evo-devo view sees development at the heart of the evolutionary process.

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The authors suggest that a single developmental ‘program’ previously known in other mammals such as seals, called the patterning cascade model (PCM), might be responsible for the variation among hominoid molars. PCM postulates that a blueprint for the shape of a mature molar is built from interactions between signalling centres called ‘enamel knots’ during development.

The team analysed 763 molars from six hominoid genera, both living and extinct, and discovered that most of the diversity of the molar cusp landscape can be explained by the PCM. Because of this they argue that they “have provided a developmental explanation for … long-standing patterns of molar crown configuration observed throughout human evolution.”

“These results,” the authors conclude, “have implications for the accurate interpretation of dental phenotypic variation in hominoid systematics.”