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Question: Why does the platypus and echinda lay eggs rather than giving birth like other mammals?
These animals are called monotremes, which are mammals as they have fur, provide milk for their babies, and have teeth that are differentiated into incisors, premolars and so on. But, like most reptiles they lay eggs.
This situation, of having some characteristics of one group of animals and some of another suggested the kinds of thinking that led to the concept of biological evolution. So, if you arrange animals in a series from simpler to more complicated, you may find some kinds to be intermediate between different major kinds, as with the monotremes. You might then place these in one group or another based on most of their traits, but somehow it's hard to forget the question it raises. Why should any kind be intermediate between other kinds?
The discovery of fossils and our ability to date them, showed that there was a world of reptiles before mammals appeared in the fossil record, and reptiles became many fewer after they appeared. So, mammals could reasonably be thought to have derived from reptiles. Then, if we think that this would likely have taken quite a long time to happen, and might also proceed by steps, it becomes possible to think that some aspects of animals might evolve more slowly than others. If so, that would give you intermediate kinds of animals, which have actually been found in the fossil record. Then we can conclude that these Australian monotremes must also be such intermediate kinds.
But why should their family have been slower to evolve in the direction of mammals than so many other mammal-like reptiles? Well, maybe it was just better for these kinds to keep their old reptile way of making babies, given their way of life. That raises the question of why other mammal-like reptiles and early mammals went on to evolve live birth. This is a general question on its own because, curiously, live birth has evolved in several families of fishes, amphibians and reptiles, even though these did not become mammals. So there seems to have been some benefit to be gained by live birth for several kinds of animals. To this question there doesn't seem to be any one explanation. As with many questions about nature, we cannot yet find definitive answers to all of them, and some, like this one, may require a separate story for each one.
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