Let me gift this a try.
There are many species that have existed for a long, long time. Some, many in fact, have existed pretty much unchanged since the time of the dinosaur. They are among the evolutionary success stories and IMHO, play a very important part in proving the theory of evolution.
Some examples that come to mind are the crocs and turtles. There were some changes, size for example, but overall, a crocodile that most likely preyed on dinosaur would be immediately reconizable in both form and function from contempory bretheren.
But there are other examples where the record shows that there has been much change. Of greatest interest to us are the Primates.
This difference in rates of change between two groups IMHO, can only be explained by evolution. Change is random. It happens. When it happens, it may allow expansion into some new and less competitive ecological nitch.
There is no design, no plan, no purpose.
Aslan is not a Tame Lion