Athabaskans are a gigantic language group,some dialect of this language is spoken from Alaska all the way across the continent to Hudson Bay...And south as far as Mexico...(also of late the scientists decided that Yenisei Kets are also Athabaskan...so all the way to Western Siberia in the past at least).
Yukon river is a huge east-west valley(nearly quarter-million sq. miles)between two very high mountain ranges,and it has never become glaciated.During assorted ice ages the ice sheets piled up these huge hills,but never came tumbling down into the valley itself.
So it became this huge refuge for the last of the mega-fauna,mammoths,giant sloths,huge cats and bears...And people to be sure.
The Bering Straight,the sea separating Alaska from Asia was then a plateau,a giant plain...(it's still very shallow today,with nary a deep-water port anywhere...).So the scientists propose that people have slowly migrated East,and eventually South,through the Yukon valley,and eventually down to South West.
The very ancient people that were there already there like the Hopi have it freshly(you may say)in their lore,when these warlike,restless Athabaskans first showed up(and what a pain in the ass they were to deal with).Carrying nothing but their weapons,and an ability to learn Anything new they ever came across.Eventually they became expert horsemen,and sheepherders,and weavers and potters and silversmiths...All these things they picked right up.
So the theory goes,that the scientists hold as most likely, to date.Athabaskans here,however,don't think that much of scientists
...Fortunately i'm not a scientist,i'm just a stray ne'er-do-well,who lived here over 30 years...so they're much kinder to me...(i'm friends with quite a few scientists though,and get a bit of flack for that...but in a more or less gentle way,people here are ridiculously kind and decent...).