Looking to fossils to predict tooth evolution in rodents

Fifty million years ago, all rodents had short, stubby molars--teeth similar to those found in the back of the human mouth, used for grinding food. Over time, rodent teeth progressively evolved to become taller, and some rodent species even evolved continuously growing molar teeth. A new study publishing April 23 in the journal Cell Reports predicts that most rodent species will have ever-growing molars in the far distant future.

Skull of a Laotian rock rat. Over evolutionary time, rodent molars 
have become taller [Credit: Vagan Tapaltsyan and Ophir Klein]

"Our analyses and simulations point towards a gradual evolution of taller teeth, and in our future studies we will explore whether tinkering with the genetic mechanisms of tooth formation in lab mice--which have short molar teeth--will replicate the evolution of taller teeth," says co-senior author Ophir Klein, an associate professor at the University of California San Francisco School of Dentistry.

For their research, Dr. Klein and his colleagues used fossil data from thousands of extinct rodent species to study the evolution of dental stem cells, which are required for continuous tooth growth. They found evidence that most of the species possess the potential for acquiring dental stem cells, and that the final developmental step on the path toward continuously growing teeth may be quite small. "Just studying how molars become taller should tell us about the first steps in the arrival of stem cells," Klein says.

This is a diagram of the crown:root ratios of rodent teeth 
[Credit: Vagan Tapaltsyan and Ophir Klein]

The team's computer simulations predict that rodents with continuously growing teeth and active stem cell reserves will eventually outcompete all other rodent species, whose teeth have a finite length. This won't likely apply to people, however.

"As we humans have short teeth, evolutionarily speaking we would have to go through multiple steps that would take millions of years before we could acquire continuously growing teeth. Obviously, this is not something that would happen as long as we cook our food and don't wear down our teeth," says co-senior author Jukka Jernvall, an evolutionary biologist at the University of Helsinki, in Finland. "However, regarding rodents, it will be interesting to resolve the regional and taxonomic details of the 50 million year trend."

Source: Cell Press [April 23, 2015]