Seals and penguins are distantly related but have comparable body forms for life in water. Observing these patterns we want to know why things are the way they are and how things came to be that way. Obtaining scientifically sound explanations for these observations is very difficult and is the task of evolutionary biology as a discipline. Darwin and Wallace's theory provided a mechanism by which this descent with modification might operate. We must then draw on material from many disciplines, from astronomy and geology through biochemistry and genetics to ecology and behavior, to really account for the staggering diversity of life on earth.
Humans are notable in that by the nature of their very actions are altering their evolutionary fate.
Many organisms do this, but humans have the intellectual abilities to do this by design. And that is both fascinating and frightening at the same time.