Former Colorado congresswoman Pat Schroeder once quipped: "I have a brain and a uterus and I use both." The former is what separates humans from other mammals; the latter is what separates mammals from everything else.
Although mammals number only some 4,300 species (not much in the grand scheme of things), we've done pretty well for ourselves. We might not be as numerous as other animals, but without getting too self-congratulatory, we're by most measures the most adaptable and certainly the dominant class of animal on the planet.
Arguably, we owe it all to the uterus. It's our defining characteristic. Other animals have to lay eggs, but we mammals nurture our offspring internally until they are well developed. It's perhaps the most intimate of structures produced by natural selection. But how did it evolve?