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Proto-Indo-European (PIE) is the common ancestor of hundreds of related languages and dialects spoken in Europe, northern Indian subcontinent, Iranian plateau and most of Central Asia. Though its existence is accepted by most linguists, they couldn't decide on exactly when PIE was spoken (some said in the Early to Middle Bronze Age whereas others placed it as early as the Late Stone Age).
This neat "language tree," illustrates the evolution of modern languages from PIE. What's even neater is that PIE is supposed to be a just one branch in an even larger language tree.