1) In the late 18th century, Sir William Jones suggested that languages from different geographical areas may have a common ancestor, launching the field of linguistic investigation into language families. 2) The Proto-Indo-European language is reconstructed as the common ancestor of languages like Sanskrit, Latin, and Greek, based on comparative analysis of cognates using principles like the majority principle. 3) English history is usually divided into Old English (7th-11th century), Middle English (1100-1500), and Modern English (1500-present), with each period showing changes in sounds, syntax, borrowing, and meanings due to various influences.