Sometimes, answers to biology’s most important questions can be found in the most unexpected of places.

As reported in the most recent issue of the journal Cell Reports, researchers at the University of California, San Francisco (UCSF) and the University of Helsinki describe how studying fossilized rodent teeth has helped them inch closer to grasping the origins of a particular type of stem cell.

Rodents’ ever-growing teeth hold clues to the evolution of stem cells, according to a new study.

Understanding the microenvironment that surrounds each stem cell, known as a stem cell niche, is key to grasping the key mechanisms that drive stem cell growth. But as UCSF scientist Ophir Klein explained, many aspects remain a mystery.

“Despite significant recent strides in the field of stem cell biology, the evolutionary mechanisms that give rise to novel stem cell niches remain essentially unexplored,” said Klein, who served as the study’s senior author. “In this study, we have addressed this central question in the fields of evolutionary and developmental biology.”

In this study, Klein and his team focused on the teeth of extinct rodent species. Why? Because many species of rodent—both extinct species and those alive today—have what’s called ‘ever-growing teeth.’

Unlike most mammals, including we humans, the teeth of some rodent species continue to grow as adults—with the help of stem cell ‘reservoir’ hidden inside the root.

And by analyzing the fossilized teeth of extinct rodent species, the researchers could gain some initial insight into how these reservoirs—which were essentially a type of stem cell niche—evolved.

Most stem cell niche studies take cell samples from hair, blood or other live tissue. Teeth, as it turns out, are the only stem cell niches that can be found in fossil form.

In fact, teeth are “the only proxy…for stem cell behavior in the fossil record,” says Klein.

After analyzing more than 3,000 North American rodent fossils that varied in age between 2 and 50 million years ago, the researchers began to notice a trend. The earlier fossils showed short molar teeth. But over the next few million years, the molars began to increase in length. Interestingly, this coincided with the cooling of the climate during the Cenozoic Period. The types of food available in this cooler, drier climate likely became tougher and more abrasive—leading to evolutionary pressures that selected for longer teeth. By 5 million years ago, three-quarters of all species studied had developed the capability for ever-growing teeth.

The team’s models suggest that this trend has little chance of slowing down, and predicts that more than 80% of rodents will adopt the trait of ever-growing teeth.

The next step, says Klein, is to understand the genetic mechanism that is behind the evolutionary change. He and his team, including the study’s first author Vagan Tapaltsyan, will study mice to test the link between the genetics of tooth height and the appearance of stem cell reservoirs.