Really, the opposite is true. Given the amount of change we can see through both natural and artificial selection in historical timescales, and given the environmental instability evident in the last several hundred million years, it would be very surprising if we did NOT see adaptations accumulate to the point where creatures were "totally different".<quoted text>It is not the same process. We have observed small scale change, but there is no reason to think that it will lead to a totally different creature. It is even laughable to me to even think that it could. You are assuming it can and science is not about assumptions.
But then what does "totally different" mean anyway? You are not totally different from a chimpanzee, you are by any measure only slightly different. You are only slightly more different from a cat. Same basic body plan, organs, functions. You are a bit more different from a crab, but that is in the body plan. At the cellular level you are amazingly similar.
There is no "totally different" life on Earth when you look closely. Its all degrees of similarity.
Assumptions? Well, initially, it all might have looked like an assumption. But then scientists went out and said, now if this assumption is TRUE, what would we expect? And if it is FALSE, what would we expect.
And what they found is that the evidence was aligned to the "assumption" being true, turning this hypothesis into an accepted scientific theory.