Monotremes are fascinating animals. They are egg-laying mammals. There are only two of them left: the duck-billed platypus, and the spiny anteater. There is but one species of platypus, while there are two genera and four species of spiny anteater (from wikipedia, verified by a U. Cal. Berkeley site).
Picture furry little creatures that lay eggs. If that's not unusual enough, the duck-billed platypus has a duck bill, a beaver tail, and a poisonous spur on its hind legs. All monotreme species nurse their young, but they don't actually have nipples, just openings in the skin for the mammary glands.
Monotremes, Geographical Distribution, and Evidence for Evolution
Monotremes are found only in Australia and New Guinea. Most marsupials (mammals with pouches) are found there as well, though many species of opossum still exist in the Americas and the monito del monte in South America.
There's a pattern to the development of mammals. Monotremes lay eggs like reptiles, marsupials give birth to live young, who then delevop in a pouch, and placental mammals—like ourselves—develop their babies internally in a womb.
In the fossil record, marsupials replaced monotremes where they competed, and placental mammals have replaced marsupials. In Australia, for example, there used to be even more marsupial species, but the introduction of placental mammals by man led to the extinction of some.
There is also a clear connection between monotreme and marsupial species in Australia and South America. These continents were once connected by Antarctica in the southern super-continent of Gondwana.
Monotremes and Physiological Evidence for Evolution
Monotremes, bearing closer resemblance to reptiles in their reproduction, also bear closer resemblance to reptiles in one of the more fascinating reptile-mammal differences: their hearing.
Reptiles have three bones in their jaw, and they use their jaw to help with hearing, laying it on the ground to pick up vibrations. In mammals, those bones have migrated into the ear, becoming our unique middle ear bones, the anvil, hammer, and stirrup.
Monotremes have the middle ear bones and only one jaw bone typical of mammals, but their ear opening is at the bottom of the jaw. Interesting, isn't it?
Monotremes also have legs that come from the side of the body as in reptiles, rather than below the body, like later mammals.
You can also see a progression in body temperature. Duck-billed platypuses average only 90o fahrenheit for a body temperature, while marsupials average 97o, and placentals are around 99o. It's been suggested that the lower temperature—and thus lower metabalic rate, requiring less calories—helped them outcompete placental mammals in Australia. (Though I personally believe the explanation I give at the end makes more sense, since placental mammals had no problem out-competing marsupials, even driving the tasmanian wolf to extinction, when introduced by man.)
Geography and Evolution: Where Should God Put Mammals?
If one rejects evolution, is there some reason that marsupial mammals should be only in Australia and the Americas? Is there some reason that monotremes should be only in Australia but also have fossils in South America?
Evolution explains those things this way. Monotremes evolved from very early mammals in Australia. Marsupials evolved about 25 million years ago from the same widespread early mammals, but in China. Marsupials spread all the way across Asia, Europe, North and South America, and into Australia through Antarctica.
When they got to Australia, they displaced all the monotremes except the aquatic platypus, which later evolved into the spiny anteater as well.
Behind the marsupials, in the northern continents, placental mammals evolved and replaced all the marsupial species except in Australia. The reason for this would be that Australia had separated from Antarctica by then, and marsupials were preserved there by separation from the rest of the world some 60 million years ago.
This distribution makes sense from an evolutionary standpoint. If we reject evolution as the means by which God created the earth, then why the geographical spread of these creatures? Why would most marsupial species in North and South America and all Eurasian marsupials be extinct? God made them, preserved them in the ark, and then let them be driven to extinction by placental mammals, but only in the northern continents?
I'm not saying God didn't create things, but I am saying that God's greatest concern has never been physical. His greatest concern has always been spiritual, and it was fine for creation to happen over long periods of time by evolution. Time is almost meaningless, anyway, being relative to the observer.