P.Z. Myers says that the platypus is a fine animal--it has just been pushed around by different evolutionary pressures over the past 160 million years than have modern mammals, reptiles, and birds. I say that the platypus is an otter gone horribly wrong:
Pharyngula: The platypus genome: Every organism is going to be a mix of conserved, primitive characters and evolutionary novelties — a mouse is just as "weird" as a platypus from an evolutionary perspective, since each is the product of processes that promote divergence from a common ancestor, and each are equidistant from that ancestor.... [M]odern echidnas, elephants, and emus are all products of different evolutionary trajectories through history, and no one by itself is a representative of the ancestral condition. We derive the ancestral state by comparison of multiple lineages... [this] adds another lineage to the [genomic] data set, one that diverged from ours over 160 million years ago. It is a lens that helps us see what novelties arose in that 160 million year window....
So what are the details that we've learned from the platypus? One important message is the unity of life. The platypus has about 18,000 genes; humans have 18-20,000 genes. Roughly 82% of the platypus genes are shared between monotremes, marsupials, eutherians, birds, and reptiles....
An interesting specialization in the platypus is the evolution of venoms. The platypus has small, sharp spurs on its hindlimbs that it uses to inject defensive poisons into predators, a very unusual feature not found in other mammals. Where did these venoms come from? As it turns out, by duplication of genes that have other functions, with subsequent divergence, and many of these genes also come from the innate immune system. In particular, there are a set of proteins called the β-defensins... the bullets of the immune system; they can bind to viral coat proteins, they can punch holes in bacterial membranes.... The platypus has repurposed these genes, making copies that have been selected for more effective toxicity when injected into other animals.
One very cool observation is that these are also the same proteins used in venomous reptiles... two distant relatives, the lepidosaurs and the monotremes, all use β-defensin derived venoms. Does this imply that their last common ancestor also used these venoms? No, and this is where the details are important. Venomous snakes and the platypus have different duplications of the β-defensin genes... these are independently derived features, not primitive at all... convergent evolution....
One virtue of the platypus is that it provides a relatively closely related outgroup to help tie together, and give perspective on, the various mammalian genome projects. It's all part of the big picture in defining what a mammal is...
