The modern theory of evolution is easiest illustrated by the tree of life. This is a metaphoric visual representation off all past and present species (as far as we know them).
The root or origin of the tree represents the origin of life on earth. This is still a debated subject, but for the tree of life, it is assumed that it was a single celld organism.
This single cell, over time, evolved into the species that now live on Earth. Each species has its place on the tree's branches, which illustrate the evolutionary relationships and evolution on all life on earth. The closer the species are on the branches, the closer related they are.
Thus, according to the theory of evolution, at some point in time a fruit fly and a fruit bat <em>did share a common ancestor</em>. However, due to the fact that fruit flies are insects and bats mammals, they are part of different phylum (insects - Euarthropoda and mammals - Chordata).
Both species are far apart on the tree of life and which makes their common ancestor to have existed a long, long time ago, well over 525 million years (when the first vertebrates evolved).
