Developmental biologists have historically used mice to study the development of mammalian embryos. In work published online January 29 in Development, Stephen Frankenberg and his colleagues from the University of Melbourne, showed that embryos of the tammar wallaby, a marsupial, develop somewhat differently from mouse embryos. Some early embryonic genes seem to play similar roles in tammar and mouse, Frankenberg and colleagues showed. The protein products of other genes, however, appear in different cellular locations in tammar embryos than in mouse embryos, which suggests that these proteins may function differently in the two species and in other mammals. This work challenges established ideas and demonstrates the possibility of wider evolutionary variability in mammalian development than previously expected.