One Cool Animal

The duck-billed platypus is cool. It's always been one of my favorite animals. How could it not be? It loudly insists on it's individuality, refusing to be like other mammals. The platypus has been in the news lately, because it's genome has just been sequenced. This is an important genome, because the platypus has a common ancestor with other mammals dating to over 150 million years ago, more distant than any other mammal. You have to go back about 300 million years to find the next ancestor we share with other living animals, when the other amniote groups (which includes snakes, lizards, birds, crocodiles, and turtles) branched off.

The platypus has always caused confusion with non biologists, and I think the confusion is instructive about how people think about evolution and classification. A biologist recognizes the platypus as being a mixture of primitive, derived, and analogous traits. But without an understanding of those terms and how they relate to classification, it seems to be an arbitrary mix of reptile, bird, and mammal.

A platypus has several primitive traits, especially in its reproductive system. By primitive, we don't mean they are more simple or "lower" in any since. We simply mean this form came first, and has been altered in other groups, or clades. So a platypus lays eggs, like the ancestor of all amniotes did. It's mammary gland isn't well developed and doesn't have a nipple. Other mammals have altered this mammary gland into the form we are more familiar with. The platypus also has several primitive, reptile-like, skeletal features.

These things make it easy to think of the platypus as a remnant of the past, lower in the chain of mammals. But evolution is not a chain. Because we share a more distant ancestor with the platypus than other mammals, there are derived traits that we have that it does not share. But there are also derived traits it has that we do not. The two most obvious are that it has venom-delivering spurs on it's hind legs, and the duck bill. The bill is the other cause of confusion. We call the animal the duck billed platypus because it has a bill that resembles the bill of a duck. However, the bill of a platypus is only superficially like that of a duck. This similarity is analogous only. (I should also point out that the eggs of the platypus are not avian features--the eggs are leathery, the primitive form, like a reptile. Birds have calcified eggs, which is a derived trait unique to their clade)

The bill of a platypus is one of the most amazing organs I know, another reason the platypus is cool. It is made of keratin--modified fur. It's developmental and anatomical origin is completely different form that of a duck. And it is not just an organ for eating. It is actually a sense organ. A platypus scientist would not speak of the five senses--they literally have a sixth sense. The bill is a very sensitive eclectic sense organ. If can "see" nerve impulses in other organisms, especially the small invertebrates that the platypus calls food.

This is a derived trait, and more highly developed than any similar structure in other mammals. The platypus is not primitive in all features. It is primitive in features that we consider important, from our biased view. Since we have placentas and nipples and big brains, we think of a platypus as somehow less developed than us. We are just paying attention to the things that happened to change in our line. But if a platypus was doing the biology, they would note that humans are very primitive in their electrical senses, although they have developed some odd specializations in their brains. They have changed as much as us, just with different features.

There is more confusion because people want to classify things according to a list of traits and don't think in terms of nested groups. They have been told that mammals have live young, and get confused by this egg laying creature. But mammals have many other traits as well, such as three bones in the middle ear, hair, milk teeth, etc. Evolutionary, each of these traits was added independently. In the past, there perhaps would have been creatures with hair yet without the bones in the ear (since fur doesn't fossilize, we don't know if it arose early or late). There could have been creatures with hair but no mammary gland at all. Mammals are a clade--a group of organisms with a common ancestor. A subclade within that clade are the placental mammals, which share a more recent common ancestor. We are related to the platypus, but more closely related to a cow.

Another example of this problem would be dolphins, which don't have hair. Does this mean they are not a mammal? It does not. Actually, whales do have fur during development, and lose it. Is that why we can still call them mammals? No. If whales or any other group of mammals completely lost any hair in any stage of its development, it would still be a mammal, because it shares a common ancestor with other mammals. In fact, if some group were to lose their mamary glands, the trait from which mammals get their name, they would still be mammals.

As I write this, it has occurred to me that the platypus is a perfect test case for understanding evolution, and perhaps I should use it as a model in teaching evolution. It seems that if a student can understand why a platypus is a mammal, how it relates to other mammals, and how it is not some chimera but rather an expected mixture of primitive and derived traits, they understand some of the most important ideas in evolution. There is a tendency to think in terms of a ladder rather than a bush and in terms of concrete categories and traits instead of a nested addition of traits. If they understand the platypus, they should also understand what a transitional form is, and that it is traits that are transitional, rather than organisms.

Plus, the platypus is just cool