One of the things evolution rules out is chimeras – animals that are mixes of two or more different animals. Evolution is a theory of descent with modification, and since animals can only mate if they are the same species, it means the offspring will have the genome of its (nearly identical) parents plus a few mutational changes (i.e. the modification). There are plenty of chimeras in mythology though: mermaids, centaurs, minotaur, pegasus, and the greek lion/dragon/goat chimera.
I’ve heard a number of creationists counter this by claiming that the platypus is a chimera – it has hair like a mammal, lays eggs like a reptile, and a bill like a duck.
For example, AnswersInGenesis says:
The platypus is an animal with a bill like a duck, and a beaver-like tail, it has hair like a bear, webbed feet like an otter, claws like a reptile, lays eggs like a turtle, and has spurs like a rooster, and poison like a snake. You can see why scientists first thought it was a fraud. But, the platypus is a real problem for the evolutionists. You see, they believe animals have evolved into other animals over millions of years. So, the question is: now which animal did the platypus evolve from? It would have to be just about everything. I think that every time an evolutionist looks at the platypus, I believe God must smile. Maybe He created it just for them. (Link)
Monotremes are a scientific puzzle if you are an evolutionist. They are clearly mammals because they have milk glands, hair, a large brain, and a complete diaphragm. Yet they also resemble reptiles and birds in that they lay eggs, their blood temperature is influenced to some extent by their surroundings (as is reptiles’), and the platypus’s bill is like a duck’s. (Link)
Kent Hovind also makes the platypus argument here.
(1) The platypus’ bill is not like a duck’s bill. It “resembles a duck’s bill but is actually an elongated snout covered with soft, moist, leathery skin and sensitive nerve endings.” (Link) Further, the platypus bill is covered with electroreceptors, enabling them to detect prey hidden in the mud (something ducks are not known to do).
(2) The platypus has poison spurs, but that doesn’t mean its venom is anything like snake venom. Lots of animals – including spiders, scorpions – have venom, but that doesn’t mean they all gained their venom from a common ancestor. Rather, they all evolved it independently, which is why, biochemically, the platypus venom does not resemble venom in any other animals: “platypus venom is a cocktail of toxins, most of which is a mixture of proteins which resemble no other to date.” (Link) On the other hand, genetic studies of snake venom shows that their venom is just modified versions of an original venom that appeared millions of years ago. (And, interestingly, they share this venom with a few legged reptiles – revealing that venom first evolved while snakes’ ancestors still had legs.)
(3) The Platypus lays eggs, “like a reptile”. Actually, the platypus is part of a family of animals called monotremes. The only other living animals which belong to this group are spiny anteaters. Monotremes existed before placental mammals or marsupials. Those branches of mammals appeared later, and became the dominant mammals. A diagram of their relationship through time would look like this:
Giving birth to live young is something that placentals and marsupials do, but the laying of eggs is something that mammalian ancestors (i.e. reptiles) did. What’s interesting, then, is that monotremes resemble the patterns of earlier, reptilian ancestors. They have retained the traits we would expect of an early mammal. This isn’t the only similarity monotremes have with reptiles:
(4) Monotremes’ urinary, defecatory, and reproductive systems all open into a single duct (the cloaca). This is similar to reptiles, but in placental and marsupial mammals, these ducts have separate openings. (Pharyngula also has an article touching on the monotreme / placental / marsupial reproductive system variations, which also shows the early branching of monotremes from placentals and marsupials – and the monotreme female anatomy resembles the reptilian version.)
(5) Monotremes have their limbs on the sides of their bodies, which is similar to reptiles (think of turtles, alligators, and lizards). Mammals typically have their limbs directly below their bodies.
(6) Monotremes produce milk for their young, but unlike placentals and marsupials, they don’t have nipples. Instead, they simply have patches of skin that produce milk. Again, this resembles what we might expect of an early mammal.
(7) The platypus maintains a lower average body temperature (32′C) than marsupials (35′C) and placentals (38′C). The spiny anteater can even turn off it’s temperature regulation in cold weather. Reptiles are cold-blooded, so their body temperature varies.
So, the platypus seems like a mixture of reptilian and mammalian traits because it branched from the mammalian tree very early, but retained many of the ‘early mammal’ traits. Other features – like the venom, bill, electrolocation, and flat tail were evolved after the branching occurred, and this is backed up by the fact that their physical and genetic structure is very different from similar structures in snakes (venom) and ducks (bill).
Here’s how the diagram might look:
Now, you might be thinking – gee, that looks like the nested hierarchy one would expect from an evolutionary process. You’re right. Not only does it show the gradual accumulation of “mammalian” traits, but it is similar to the tree produced in Talk Origin’s 29 Evidences for Macroevolution. Creationists, like AnswersInGenesis and Kent Hovind get all the facts wrong and play on people’s ignorance by claiming that the platypus is somehow a problem for evolution. It isn’t. It fits with evolutionary theory.