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.
In everyday speech, the word theory is used as a best guess. In modern science, a scientific theory is a tested and expanded hypothesis that explains many experiments. It fits ideas together in a framework. If anyone finds a case where all or part of a scientific theory is false, then that theory is either changed or thrown out.