How do you define the word Evolution?

In the case of two objects being dropped towards the earth's surface, the mass of the earth is significantly greater than the mass of either object, so they're effectively the same mass, 1, and the gravitational force working on both objects would be the same.

You don't seem to understand the relationship between historical contingencies and theory. Let's use gravity as an example. The theory of gravity explains the orbit of Mercury. However, the theory of gravity does not require the existence of Mercury in order to be correct. In the same way, the theory of evolution does not require a universal common ancestor. If there were multiple origins of life, and life evolved from those multiple ancestors, then that would be the theory. However, the theory does explain how the evidence points to a universal common ancestor with the evidence we do have.

What we are saying is that the theory does not require a universal common ancestor. However, since the evidence does point to a universal common ancestor then it is part of the theory.

To put it another way, if there were multiple origins of life then the theory would change to reflect that reality while still proposing the same mechanisms of change.