I'm guessing that a majority of this area speak Yakut(3) as their first language. The bare majority of the republic is Russian,(4) and they are concentrated near the upper course of the Lena and its tributaries.(5) By arithmetic then, the north must not have a Russian majority; Yakuts--one third of the republic's people--are at least a plurality, and probably a majority. Russians are presumably a substantial minority.
Almost everyone is Russian Orthodox Christian, or non-observant.
There are no cities in this area, nor is any settlement predominant.
Conversions to Christianity resulted from 19th century Russian missionaires. Before that the Yakuts followed a shamsit religion.
It is speculated that the Yakuts, a Turkic people, moved into the Lena Basin from the south as a result of Mongol expansion in the 13th century. Some Yakut speakers' ancestors spoke other languages and adopted the new comers tongue.
There are remnants of a miscellany of people who were probably in the area for a long time. These are the Even(6), the Yukaghir and Chukchi, and they are not linguistically related.(7) these groups also practiced shamanist religions; a few still do. The Yukaghir kept bones of deceased shamans as amulets. The Even's clan shaman's tent was a microcosm; and ritual acts there were thought to work correspondingly in the macrocosm. The Chuckchi have grades of shaman, from one serving just his family, to itinerant professionals.
(1) Also called Jakutija or Yakutia (tansliterated from Russian).
(2) Strictly translitered from Russian as Rossija or Rossiya.
(3) Sakha is their name for themselves. It means people of the edge.
(4) Orttung, the Republics and Regions of the Russian Republic, 2000. Other sources give higher proportions to non-Russians.
(5) Map in Dalby, Dictionary of Languages, 1998.
(6) Also called Lamut. It is a Tungusic language.
(7) Yukagir is an isolate. Chukchi is in the Chukotko-Kamchatkan group.