Ed Yong in Not Exactly Rocket Science:
Then, around eight years later, Grutzner got back in touch. His team had collected tissues from a platypus that had been killed by someone’s dog. They had some uterus. Did Lynch still want some?
“Hell yes!”
The platypus was the final critical part of a project that Lynch, now at the University of Chicago, had longed to do since he was a graduate student. He wanted to study the evolution of pregnancy in mammals, and specifically the genetic changes that transformed egg-laying creatures (like platypuses) into those that give birth to live young (like us).
The platypus enjoys a short pregnancy. Its embryo sits in the uterus for just 2-3 weeks, surrounded by a thin eggshell, and nourished by a primitive placenta. It then emerges as an egg. Marsupials, like kangaroos and koalas, also have short pregnancies. But mothers give birth to live young, which live in a pouch until they’re big enough. Other mammals—the placentals, or eutherians—keep their babies in the uterus for as long as possible, nourishing them through a complex placenta. Their pregnancies can be marathons—up to two years in an elephant.
The move from egg-laying to live-bearing was huge. Mammals had to go from holding a shell-covered embryo for weeks to nourishing one for months. To understand how they made the leap, Lynch compared 13 different animals, including egg-layers like the platypus, marsupials like the short-tailed opossum, and eutherians like the dog, cow, and armadillo.
More here.