Leonard never really believed in evolution. He had studied it, knew it well, and even referenced Darwin as a great naturalist, but he never accepted it as a concrete scientific explanation for the invent of the world. It had shocked him to witness evolution before his eyes.
Howard and Raj, of course, had ignored it for the most part; Howard because his mind was in his pants and Raj because he was busy following Howard in the pursuit of women. Leonard lived there and he witnessed it happen before his eyes. It was just amazing to him, this microevolution of a single mammal becoming human. He would sit on his couch, composition notebook and freshly sharpened number two pencil in hand, trying his best to work on his hypothesis, when all he could write were notes about the dynamic changes occurring in his specimen. His margins were peppered with snippets of conversations it held, little glances, and changing habits. He really didn't notice it all until it was almost too late and the changes were going to be reversed. When the evolution was complete, and the specimen changed to resemble his new species and adapted to his new environment, Leonard stored that composition notebook under his bed. The title: The Evolution of being Human.