Until relatively recently, species and their ancestry was determined mostly by things such as physical appearance and function, i.e. two species look similar, so they must be closely related. Funnily enough, after DNA technology took off and it was possible to determine how closely related species are to one another, a lot of these associations and assumptions that were wrong could be rectified. That similarly looking species may not actually have a recent common ancestor, and different looking members of the same species share most of their genetic make-up. Look for yourself.
Flying squirrels on different continents developed their flying wings in parallel with each other, and don't in fact have a relatively recent common ancestor.
DNA is key to determining evolution in this sense, much more so than simple physical similarity that we as humans are likely to be biased in.