Final answer:
William Jones was the first linguist to propose a single Indo-European family of languages based on his comparison of Greek, Sanskrit, Latin, and Persian.
Step-by-step explanation:
William Jones was the first linguist to propose a single Indo-European family of languages in 1786 based on his comparison of Greek, Sanskrit, Latin, and Persian.
He noticed the similarities between these languages and concluded that they must have a common origin. This idea laid the foundation for the study of historical linguistics.