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.
Definition of theory for English Language Learners. : 1  an idea or set of ideas that is intended to explain facts or events. : 2  an idea that is suggested or presented as possibly true but that is not known or proven to be true. : 3  the general principles or ideas that relate to a particular subject.