Back in the day, all humans had eyes that were a shade of brown. Recently it was discovered that a genetic mutation 6,000-10,000 years ago is responsible for all the blue eyes we see today. A single genetic mutation in the OCA2 gene literally acts like a light switch for melanin production in the eyes. The mutation makes it so those with the gene cannot produce the required melanin in the eye to get a brown color. They are left with an ugly blue that they must carry with them for the rest of their lives. Because of this single genetic mutation it is determined that all blue eyed humans on the planet are direct ancestors of this one individual.
This goes to show the grand scale that evolutionary time plays in the augmentation of a population. A single person just 6,000-10,000 years ago is now directly responsible for the lives of millions of individuals. Think of where we will be 10,000 years from now with our population of 7 billion....