Because marine crabs & land crabs evolved from a common crablike ancestor hardly counts as proof that crabs, humans, amebas, and trees all evolved from a common ancestor!!
Given that the only difference between crabs, humans, ameobas, and trees is the content of their genes, then yes, it does constitute evidence that all those evolved from a common ancestor.
What would it take to go from that ancestor to a human or a tree? Genetic change. Well, we've got a mechanism that will provide that change.
If you think there's a barrier in the way, that's fine, but you have to show us what and where it is, and how it works. It's not enough - it's not even coherent - to simply say "I say there's a barrier and you have to prove me wrong."