One European language has no known relatives anywhere on Earth

Basque, spoken in the mountains straddling Spain and France, is a language isolate — it isn't part of the Indo-European family that includes nearly every other European language, and no relative, living or dead, has ever been convincingly identified. Its grammar predates the arrival of Indo-European speakers in Europe, making it a linguistic relic whose origins remain genuinely unresolved.

— R. L. Trask, The History of Basque — Routledge, 1997
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