Being the only child is a unique position in a family. Without any siblings to compete with, the only child monopolizes his parents' attention and resources, not just for a short period of time like a firstborn, but forever.
Like gravity and plate tectonics, evolution is a scientific theory. In science, a theory is the most logical explanation for how a natural phenomenon works. It is well tested and supported by abundant evidence. It means quite the opposite from our informal use of the word theory, which implies an untested opinion or guess. As a scientific theory, evolution enables scientists to make predictions and drives investigations that lead to new kinds of observable evidence.
1. Evolution is only a theory. It is not a fact or a scientific law. Many people learned in elementary school that a theory falls in the middle of a hierarchy of certainty--above a mere hypothesis but below a law.Scientists do not use the terms that way, however. colleague gave me a great summation recently: In science, a theory is an explanation for something that is seen in nature. By that definition, evolution is, indeed, a theory because it explains the mechanisms (the how, the why) that lead to the changes that we see in animals (us included).