I think better with visual aids. Thus, while reading recent discussions on the intertubes regarding Adam and Eve (see here for a summary), I had to wrap my head around the concept of one person in a relatively large ancient population being a common ancestor to everyone living today. So, I drew up the graphic below.
Here we have a relatively simple case of four non-overlapping generations, each with seven individuals (A through F). Each individual has two parents, as we would expect (i.e., each individual has exactly two lines emanating from the top of its circle toward the previous generation). The heavy black lines indicate that every individual alive today (generation 4) could, in principle, have a common ancestor, in this case individual C from generation 1, even though the size of generation 1 is the same as that of the current generation.