Another theory proposed that the family of languages closest to Basque was the american Na-Dene… (both, accordin to this hypothesis, would have originated somewhere in Asia. I believe in Siberia. People who went east became the Iroquois and people who went west became the Basques, or somesuch…)
I don’t remember who proposed this hypothesis, but it’s one of these linguists who are trying to establish super-families of languages in a way which doesn’t appear convincing to most of their peers.