There has been a lot of buzz in the news about a genome project decoding the genes of the Platypus. (There's been a lot of really bad science writing too, horrendously characterizing platypus genes as part reptile, part bird, and part mammal.)
There are many interesting things that we can learn from the platypus about our own history. Most importantly, the platypus genome provides conclusive genetic proof that the three subclasses of mammals (the Prototheria, Metatheria, and Eutheria) are the product of evolution and not special creation. Animals that first lay hard shelled eggs (amniotes) developed genes that permit the production of vitellogenin, the principle protein of egg yolk. This is critical for the development of young inside a shelled egg - if the young are to grow inside a closed system, that system must have food. In mammals, yolk became less and less important as first milk evolved, and then among marsupials and placentals, live birth.
In chickens all the vitellogenin producing genes (three of them) are active. In the platypus, which captures for us a point in time in our own evolution about 170 million years ago when we last shared a common ancestor, only one of these genes remains active, with the other two residing dormant and broken as pseudogenes. Not surprisingly, we have no working vitellogenin producing genes - but we do have them, in the form of pseudogenes, "junk DNA" that no longer works or does anything, but which still closely resembles its functional ancestor. What is a bit surprising is that this gene appears, in our lineage, to have only stopped working about 70 million years ago. Which means that once upon a time, a mammal mother's belly was filled with yolk!
By the way, if you've ever wondered what the Jurassic Park version of a platypus looked like, here are some cool pictures of its early Cretaceous ancestor!