I love all your podcasts!
I especially love the intro music this time!: )

RE: Koalas
I don't think Marsupials are less evolved than their Placental cousins. They might be considered lactation specialists, while the placentals are gestation specialists. Marsupials produce different kinds of milk depending on the developmental stage of their offspring. So, I would say... not so much primative as having followed a different evolutionary strategy.

PS - a platypus is a monotreme, and so is the echidna.