I heard this story on NPR yesterday, and in it they were saying that they could use the Platypus to prove or disprove theories about evolution of creatures that devolped 150 million years ago, which made me wonder - why has the Platypus not evolved at all in 150 million years? Is it perfect???
Whose to say it hasnt?
This is a common misconception regarding evolution; that predecessor species all evolve at the same time. If this was the case, everything would be human, which isn't the case.
Mutations that can be caused by evolution don't happen to all species at the same time, so you still have the predecessors around.
Monotremes are an evolutionary beginning step between reptiles and other mammals, such as Metatheria (marsupials) and Eutheria (Placental mammals).
The egg that reptiles and monotremes lay needs to be attended, and is very vulnerable (as most reptiles found out when mammals ate them.) so animals that kept their developing young with them evolved.
Notice we still have reptiles laying eggs? They aren;t perfect either, but the ones that stuck around were good enough to survive, even with their faults.