Are you not familiar with cladistics?
a method of classification of animals and plants according to the proportion of measurable characteristics that they have in common. It is assumed that the higher the proportion of characteristics that two organisms share, the more recently they diverged from a common ancestor.
The one based on an assumption, in other words a “big leap of faith?”
Do you think an assumption requires a "big leap of faith"? I assume the sun will come up tomorrow and I'm pretty sure I'm not alone in that.
But let's look at a counter-example:
How is it that both of these creatures are birds?