For example, both you and the spider have legs, therefore either you descended from the spider or from some other creature which the spider also evolved from or there is the possibility the spider descended from you. Yes, there are similarities. But, because of the DNA code, evolution cannot happen. That code is a barrier stopping one species from changing into another. So similarities can only point to the fact that each species was made by a single Master Designer who had immense intelligence, power, and ability.
Similarities also reveal that the Designer used the best method for making many different creatures.
So most land creatures have legs of one type or another. It is simplistic to suggest that legs prove evolution, when we all know that the extremely complicated DNA coding for each true species is quite different. Yet such a concept violates a number of solid biological facts. Animals not only have similar structures, they have different ones also. Considering these differences, the idea of common ancestry fades out.