The surname ‘Tomlin’ is itself one of the numerous variants of the name ‘Thomas’ and is generally considered to be of patronymic form. i.e. a name derived from a father.
A pet form of Thomas is Tom and, with the double diminutive suffixes “el” and “in” added, Tom-el-in eventually became Tomlin meaning ‘Little Tom’.
The name Thomas was originally an Aramaic name meaning ‘a twin’. The Aramaic language was the international trade language of the Middle East between 800BCE and 100CE.