A palaeontologist ponders how genes and fossils can illuminate mammalian evolution.

Mammals are defined by what they eat with: dental features are characteristic of different species and the evolution of teeth is well recorded by abundant dental fossils. If mammalian diversity and evolution were a language, then a cornucopia of tooth features, including cusps, crests, basins and grooves, would form a large part of its alphabet.

Fossil muroids of the rodent group show specific evolutionary patterns of dental cusps and crests. The laboratory mouse, Mus musculus, is a muroid species, and the morphological development of its dental crests and cusps is influenced by a host of genes, including Fgf3.

Dental evolution in mice has now been replayed, in reverse, in a clever experiment by Ophir Klein at the University of California, San Francisco, Laurent Viriot at the University of Lyon in France and their colleagues (C. Charles et al. Proc. Natl Acad. Sci. USA 106, 22364–22368; 2009). They compared tooth characteristics in mice with two copies of the Fgf3 gene, just one copy, or none.

The authors found that with decreased Fgf3 dosage, tooth morphogenesis shifted from modern-day patterns of crests and cusps to more primitive forms seen in fossil muroids from 14 million years ago. Humans with defects in FGF3 also have a cusp–crest pattern similar to that of fossil anthropoid relatives. Increased Fgf3 dosage seems to correlate with the evolution of derived dental features in both muroid and anthropoid mammals.

Gene patterning of morphogenesis can provide insight into evolution if the corresponding morphological traits are reflected in the fossil record. Fossils can also suggest when in evolution a developmental process occurred. And, when compared with fossils, the dental cusps and crests of mice with the mutant Fgf3 gene show how development and evolution can shed light on one another.

View the archive at http://blogs.nature.com/nature/journalclub