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In particular, he looked at Yeniseian and Na-Dene verbs, since languages in both groups have a template of fixed positions before and after the verb for specifying various attributes.

If Yeniseian represents a return migration from Beringia, the question of the source population in Siberia of Native Americans is thrust back into obscurity.

But in 2008, Edward Vajda, a linguist at Western Washington University, said he had documented a relationship between Yeniseian, a group of mostly extinct languages spoken along the Yenisei River in central Siberia, and Na-Dene.

Several Yeniseian languages are known only from czarist fur tax records.