If you counted your ancestors all the way back 40 generations, you'd count more humans than there are stars in the Milky Way. That means that there was definitely some inbreeding—which means that you almost definitely share an ancestor with almost everyone you meet. In fact, scientists believe that all the humans on Earth can probably trace their lineage back to a single human ancestor. Think about that the next time someone cuts you off in traffic! Explore this concept in the video below.