Who said it didn't happen.
The same species lived for millions of years before it extinct.
For example Homo erectus remained as one kind for 2 millions of years before it go extinct, where is the tiny gradual changes while we can see with evidence that the same kind have remained for so long without significant changes, while we can see a new species came to existence while the previous one went extinct.
Because without mutations which is random, evolution would never happen.
We don't have evidence for who did it, for me it is ID, for you it is Darwinian theory.
The first mammals were tiny.
Do you know why it were tiny while reptiles were great in size ?
Do you know what happened millions of years ago that made such an amazing shifting in size.
The First Mammals (Triassic, Jurassic, and Cretaceous)
How did you know, you did see already how reptiles were huge where mammals were very tiny and completely different kinds ?
I think your guess doesn't make sense here.
So how did we know that the CHLCA exist without finding fossils ?
DNA similarities isn't the answer, we have 88% similarity with the mouse.
And that what i call guessing, you can't bring it as a fact that the kinds of species have been shifted to a new kind because of climbing, using hands,trying to fly ..etc
I don't have problem if you want to believe and to imagine such things happened but don't bring it as a mere fact.
I have my reasons to believe so, first of it logic and the 2nd thing you won't know it.