Evolution of Mammal Traits

Sometimes I am just fascinated by how mammals became, well, mammals. It is wild to think that the incredible variety of mammals we see today, from the smallest mouse to the biggest whale, all came from just a handful of ancient species. And a lot of what makes us mammals is not what people typically image.

Take our hearing, for instance. Most people do not realize its unique composition. We have three tiny bones in our inner ear, the malleus, incus, and stapes, that work together to give us our amazing hearing. The first two actually evolved from bones that other animals use for their jaws! This is a huge difference from most other animals. And, of course, there is the one thing everyone knows: we feed our young milk. This is such a complex process that it is no wonder some people find it easier to believe we were just "made" this way. But the evidence for evolution is out there, and it is not all in the fossil record.

A Living Look at Evolution

The platypus is probably the most primitive mammal alive today, and it shows us a lot. It still lays eggs, a very reptilian trait. But, it also feeds its young a kind of milk, though not from nipples. Instead, it secretes a milk-like substance from a hairy patch on its belly. This secretion does not just feed the babies; it also has antibacterial properties that might help protect the egg and the young after they hatch.

From there, we can look at the echidna. This little guy represents a step toward marsupials. It lays eggs too, but it places them into a pouch on its body. This must be what a primitive marsupial looked like before egg-laying was replaced with embryo birth.

Marsupials have their own unique advantages. Because their young leave the womb so early, there are fewer complications from the mother's immune system attacking the embryo.

But then you have us, placental mammals. The placenta is a complete game-changer. It allows the fetus to develop for much longer and in a more protected environment inside the mother. The fact that placental mammals are now the most dominant form of mammals shows this is an advantageous trait.

Even with all our advanced development, mammals have not been around for very long. We have far fewer varieties of species than reptiles, for example. Given how far we have come, I am excited to see what future classes of animals will evolve on our planet.