Sometimes I'm fascinated at how mammals become mammals. Most mammals are so similar to one another in our number of traits that it amazes me to think that the diversity of mammals today came from a very small number of species in the past. One trait the most people don't even know about is our acute hearing. I imagine most people never think about how we mammals hear differently than other species. We have three bones in our inner ear, two of which are unique to us. (Most animals use parts of their jaw bones to hear.) Another unique adaption is the fact that we feed our young with milk from the mother. This is such as complex function, I can imagine why some people find it easier to accept we are just "made" this way. However, there is evidence that shows the path of evolution, and it isn't in the form of fossils.
The platypus is likely the most primitive mammal alive today. It lays eggs. It does feed it's young with a milk-like secretion, but not with nipples. Instead, it has a hairy patch its underside. The secretions not only serve to feed their young, but apparently, it also adds an additional layer of protection to the egg shell before the hatchlings emerge.
Then, the path to marsupials is given to us in the form of the second most primitive mammal, the echidna. This is a group of a few species that also lay eggs. However, when it lays its eggs, it places them into a pouch. This likely represents a primitive marsupial, before egg laying was replaced by embryo birth.
Marsupials in turn do have one advantage to placental mammals (that's us). There are less complications from having to protect the embryo from the mothers immune system since the young leave the womb at such an early stage.
Of course, the advantage of having a placenta during pregnancy has its own advantages, which can be seen by the fact that placental mammals are now the dominate form of mammals.
Even given our high level of development, one can see that mammals have not dominated the Earth for very long. For example, there are far fewer varieties of mammals than reptiles. Given the advancements of mammals, it will be interesting to see what future classes of species evolve on our world.