The iconic Australian egg-laying mammals have revolutionised our understanding of the evolution of mammalian sex chromosomes. Non-homology and differentiation of sex chromosomes in monotremes and therian mammals revealed the independent evolution of those sex chromosomes approximately 180 million years ago. The parallel evolution of two sex chromosome systems in mammals provides a unique opportunity to investigate fundamental aspects of sex chromosomes differentiation and regulation, including dosage compensation and meiotic sex chromosome inactivation. In addition, analysis of the monotreme autosomes that share homology to the therian X chromosome could allow insight into sex chromosomes in early mammals. Here I will discuss our research into the monotreme sex chromosome complex and share recent work that show sex chromosome like characteristics of the autosomal homolog to the therian X in platypus, which may provide a first glimpse into the sex chromosome system in early mammals.