Where did radicals come from?
This term was created by Xu Shen.
Xu Shen lived during Eastern Han dynasty, and he spent 20 years completing the first Chinese dictionary: Shuo Wen Jie Zi.
Xu Shen did not know of oracle bone scripts; he wrote the Shuo Wen Jie Zi based on simple seal scripts.
Xu Shen used his research into character form and meaning to describe six principles of character formation, called 'Liushu'.
The Liushu explained four methods of creating characters: pictographic,indicative, ideograph, and meaning-sound; plus two ways of expanding the messages of characters: synonym and 'borrowed' characters.
For convenience, he divided nine thousand characters into 540 groups.
Each character in a group shares a common component, known as the 'radical'.