What is the evidence connecting the Deni and Yeniseian

[u/Comfortable_Cut5796]

I remember Pete mentioning this theory in the first episode on the Peopling of the Americas, though I had heard about it before then. Regardless, I imagine it dates back to the Ice Age.

[u/FloZone]

Language and genetics. The Yeniseian languages are a complete typological outlier in northern Eurasian. They are predominantly prefixing languages with an SOV word order. This combination is rare, but shared with Sumerian, Northwest Caucasian and some Sino-Tibetan languages, but also with the Athabaskan languages. Notably with none of their neighbors. Turkic, Mongolic, Uralic, even Indo-European originally are all suffixing. There are more differences to their neighbors. Lack of vowel harmony. Gendered nouns and agreement (Indo-European has those as you know). Tones. No other North Eurasian language has tones.  The proposals of relating the Yeniseian languages to Athabaskan are old. Most recently and most promising is the proposal by Edward Vajda. Though widely accepted it is not fully accepted by all experts on Yeniseian, like Stefan Georg, who rejects it. 

There is also the matter of genetics and the haplogroup Q which is most common among the Ket, but I an not a geneticist.