1. Genetic Diversity. Human children inherit 3 billion base pairs of DNA from each parent, but they are not an exact duplicate. The rate of change has been measured precisely to an average of 70 bases (out of our 6 billion total) per generation. So as we go back on the family tree, there are more and more genetic differences between us and our ancestors. For example, there would be about 140 differences between your DNA and that of your four grandparents, and 210 differences between you and your eight great-grandparents, and so on. That enables us to make a prediction from the amount of genetic diversity between two species about the time since their common ancestor population lived. Using non-genetic evidence, the common ancestor between humans and chimpanzees was estimated to have lived about 6 million years ago. The calculation from genetic differences gives a figure remarkably close to the estimated value.