If Americans descended from Europeans, why are there still Europeans?
Not even close to the same thing as humans descending from apes. Just because people moved across the water, wouldn’t mean their genetics would change.
Evolution doesn't mean that the genetics of every individual in a species change in lockstep. When members of a species become geographically isolated from others, one group can have genetic changes that the other doesn't.
Fairly recently in human history, the groups that moved to Europe developed white skin and (sometimes) blue eyes, while the groups that remained in Africa did not.
The same process, albeit over a longer time, led to the proto-apes that lived in African forests developing into modern apes, while the proto-apes who moved to African savannas developed into bipedal proto-humans.