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.
Noun. 1. theory-a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena; theories can incorporate facts and laws and tested hypotheses; true in fact and theory.