Budding. Centers of cell signaling (yellow) shift in the developing mouse molar (left) to yield even rows of cusps, while the vole (right) retains a zigzag pattern.

The Roots of Tooth Evolution

After digging up fossils, paleontologists try to figure out how the extinct animals are related to each other by comparing the shapes of bones and teeth. But how meaningful are these shapes? Now biologists have contrasted the way molars grow in two types of modern rodents. According to research published in the 19 December Proceedings of the National Academy of Sciences, relatively minor differences during early development can easily alter tooth shape--a warning to paleontologists that the similarities of some fossil teeth may be coincidental.

Many living and fossil mammals have molars with distinctive patterns of bumps called cusps. Recently, biologists have begun to understand how these cusps take shape in the developing tooth bud. Jukka Jernvall, an evolutionary biologist at the University of Helsinki in Finland, and others have found that the cusps originate as clusters of growth-stimulating cells called enamel knots. All mammals studied so far have these knots. So what makes the molar in one species differ from that of another?

To find out, Jernvall and his colleagues compared the teeth of mice and voles as they formed. The researchers measured expression patterns of four genes active in the enamel knots. They discovered that the location of the enamel knots shifted in mice before the cusps began to appear. This explained how mice end up with a square pattern of cusps, unlike the zigzag pattern of ancestral rodents. The tooth buds in the vole, meanwhile, elongated much faster than in the mouse. This created more rows of cusps than in mice and ancestral rodents, even though the cusps retain the zigzag pattern. Because just two relatively simple developmental differences can create these distinct cusp patterns, Jernvall says, some features of teeth may not be reliable for sorting out fossil relationships.

Paleontologists are excited by the possibility of learning from developmental biology. "The really nice thing about this paper is that it's comparative development," says paleontologist David Polly of Queen Mary & Westfield College in London, United Kingdom, who says that differences in development will be particularly informative for studying the evolutionary significance of fossil teeth.

Related sites
The PNAS paper
Home page of Jernvall's colleague, Irma Thesleff
A database of gene expression in teeth