Abstract: My group’s research centres on understanding the genetic and developmental bases of organ evolution. In the first part of my talk, I will describe our work on the development and evolution of sex differences. Sexually dimorphic traits are widespread among mammals and emerge during development through sex-specific gene expression programs. We know little about these programs, including the genes, regulatory networks and cell types that underlie them. It is also unclear when differences between the sexes emerge during organ development and how fast these differences evolve across mammals. We have answered these questions using bulk and single-cell gene expression time-series data from six species (human, mouse, rat, rabbit, opossum, and chicken) covering the development of five organs. In the second part of my talk, I will describe my group’s strategy for determining how morphological novelties - new cell types, new tissues, and new organs - arise during evolution. We focus on an organ that has originated multiple times independently and exhibits an extraordinary phenotypic diversity: the placenta. I will describe our work on the mammalian placenta, which I will show is a unique model to study how new cell types arise. I will also discuss our work in a family of fish (Poeciliidae) where complex placentas have evolved independently multiple times, making it a unique system to study how new complex organs arise during evolution.