Until recently, most people who accept that evolution takes place at all have tended to view it as a process of increasing diversity, with the human race perched at the pinnacle of creation. The purpose of evolution is to produce us. This attitude is largely subconscious, but it shows up again and again, as Stephen Jay Gould argues in his book Wonderful Life. There he describes the bizarre fossils of the Burgess shale, which between them encompass more biological diversity than exists anywhere on Earth today. He concludes that if evolution were to be run again, the chance of any intelligent lifeform developing is minute.
Evolution tends to attain similar ends by different means.
The marsupials of Australia bear remarkable resemblances to placental mammals elsewhere. Senses such as vision or hearing are other examples. And so, perhaps, are brains. In evolution, the constraints include Darwinian 'survival of the fittest', which eliminates vast numbers of imaginable structures, and the laws of physics and chemistry, which preclude even vaster numbers of structures from being possible even in principle. Against a background of random variation, the process takes place within some body of constraints and stabilising influences.