[@Aisling] "Let me give you an example," the father said. "At the beginning, there's only one kind of bacteria. The bacteria of this kind replicate constantly, but those replications don't always work well: A part of the 'baby bacterium's' DNA is different from its parents' DNA. This difference gives the baby bacterium an advantage over its parents, let's say in getting food. That causes the baby bacterium to live longer and thus have more 'children' than its parents. The type of bacteria the parents were will eventually die out; the bacterium has evolved."