Why do scientists think the platypus is an important clue in history of evolution?

The Platypus (Ornithorhynchus anatinus) is one of the few mammalian species in the world that lays eggs; a group known as the monotremes. Monotremes are extremely important in the study of mammalian evolution because of the adaptive transition from egg laying to live birth. By studying the genes and reproductive physiology of monotremes in comparison to marsupials and placental mammals biologists can determine how the reproductive strategy of live birth branched from precursory traits.