Are you and I related by a common ancestor?

The races themselves had a common ancestor. A more accurate analogy is apes and humans.

According to Alpha Memory, they diverged during the Time of Awakening:

http://memory-alpha.org/en/wiki/Time_of_Awakening

Around the year 300 A.D. That lends the Romulans nearly 1,000 years (ending when the crew first encounter them in TOS) to adapt to their new enviroment and evolve. It is conceivable that the differences became physiological in that frame of time.

Though, admittedly, not enough for them to be distinguishable at face value.