system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: It is understandable that English is related to German, Dutch, Spanish, French and so on, so they fall in the same language family, Indo-European. How did Russian, Polish, Hindi, Persian and so on fall in the same group? Is it because of geographic location or something else?
gpt
All Indo-European languages are descended from a single language which linguists have named Proto-Indo-European (PIE) which was spoken 4500-6500 years ago. Linguists first came to the realization that these languages were related in the 18th century, when Sir William Jones noticed that Sanskrit (and ancient Indian language and the ancestor of modern Hindi) bore a strong similarity to Latin and Greek. It is now widely accepted that a group of people (today called Proto-Indo-Europeans) migrated from the Caucuses outward to Europe and into Persia and eventually India. The same group of people also brought their religious beliefs (Hinduism/Greco-Roman religion/Norse religion/etc. all being derived from the same proto-religion)

The Indo-European languages all share similar grammatical features, as well as many cognates.