To be fair, though, the common ancestor of all vertebrates was a fish - it would have had fins and gills. It just wasn't any fish that is alive today. Likewise humans did evolve from monkeys; our ancestors were simians with tails. It seems to me there is a growing tendency to abstract ancestors completely out of the picture, like the featureless circles in your diagram, but we can say more about them than that.
Anyway, my answer to the question would be "why, how well do you think you would do living in the treetops?" Except in practice, I don't usually engage much with people who prefer gotcha questions to understanding something.