the nature of phylogenetic branching?

In evolution, we all know that it is not a linear path such as:

single cell -> fish -> reptile -> monkey -> human

It is rather a kind of branching tree of life to show common ancestor such as A evolved B and C, then B evolved D and E etc..

However, this kind of tree of life is still nothing more than a multiple linear process, we can still identify a single path from the tree, e.g. A -> B -> D or A -> B ->E in the above example.

So, my question is what exactly the difference between evolutionary tree of life and linear path BY THEIR VERY NATURES?

Thanks