He was the first to claim that there was a common ancestral Indo-European language.
(Source: Wikimedia Commons)

(1612 - 1653) A Dutch philologist who taught at the University of Leiden, he was the first to notice similarities in what eventually came to be called the Indo-European languages (e.g., Dutch, Greek, Latin, Persian, and German).

He also postulated the existence of a common ancestral language, which he called Scythian. The name Scythia was antiquity's name for an area north of the Black and Caspian seas.
Note: According to modern comparative linguistics, he pretty much got it right.