That revealed that the part of their society that'd written the archive (and later on lost the art of writing) had actually left Yakutia hundreds of years earlier, moved to Nepal, conquered Northern India, produced Buddha, and then, under pressure from resurgent Hinduism, moved back North to Yakutia.
That was about 200 AD.
350 years later (as the Dark Ages started) they mounted up, moved East, and invaded (and conquered) Korea, and then Japan.
The Ko-Jiki reflects what happened next. Today this group still identifiably makes up the original Turkic language group that invaded in the 500s. They became the Daimyo and the Royal Family, and later on incorporated a substantial part of the original native population (the Emeshi) into their ruling elite (as samurai warriors).
When you do a DNA or MtDNA test of Japanese, you have a fair to middling chance of encountering a Yakutian ancestor, or even an Indian ancestor (from that sojourn in Nepal).
Without knowing of this relatively recent Yakutian insurgency it was actually quite difficult to figure out the place of the Yayoi in all this. Note: The Yayoi are clearly of Chinese origin ~ roughly 500 BC+ ~ and share many MtDNA and dental traits with the Yakut people, but they are also obviously not Yakut. The Han Chinese have their cultural identity affirmed about 300 BC with the first (Qin) emperor, but the Yakut are too far North to be of concern to the Han. Most analysts who've taken a good look at the Yakut have long recognized that they were "mysteriously" technologically the equal of the Han Chinese society, but at the same time seemed to prefer to live in the colder North where they herded horses, goats, sheep, yaks AND, lo and behold, reindeer!
The Yayoi arise out of the turmoil in China in that critical 500 BC to 200 AD period. They are simply Chinese who founded colonies in Japan and introduced "organized" rice culture. With Japonica rice being grown throughout the region, it's not likely the Yayoi introduced just Japonica rice ~ maybe Indicum rice ~ but the Japonica was already there. Traces have been found in the North on an Emeshi (Jomon) archaeological site in just the last couple of years.
Now this is all recent stuff. In more ancient times there was a constant flow of people from the Asian mainland to Japan, and from Japan to the Asian mainland. Although the Jomon/Ainu/Emeshi can be tracked through their teeth (they have extra roots on some molars), the Yakuts are not so easily traced.
Going back further ~ to the 5000BC period, you can find Yakut traces along an arc stretching from Yakutia to Eastern Siberia to Alaska to the Americas all the way to Tierra del Fuego.
At that point in time (going backward here) we are dealing with people who are culturally and technologically Yakut, and probably have a sign language, who herd animals ~ including by this time "tame reindeer" who've been developed to help deliver access to the major herds present all across Siberia.
These people move North during warmer climates, and South during colder climates. Moving North they achieve access to the broad arctic slope which allows them to move East and West vast distances ~ from Scandinavia to North America. Moving South they achieve access to the warmer lands South of the Himalaya mountains, as well as the technological developments that've taken place there.
Their counterparts in Europe are the Sa'ami. Detailed DNA examination reveals that the Sa'ami and the Yakuts did achieve total contact on at least one occasion! (Probably a bridal gift arrived with some tame reindeer in exchange for some really great furs ~ a "bridal gift" in those days actually consisting of "the bride").
It is presumed that similar movements occurred during the period of maximum glaciation ~ and this is always referred to as "ice age contact".
One of the reasons for classing early Yakut as "turkic" is that they appear to have been the first East Asian people to have horses. Odds are good that the horse moved East in China the same way it moved North from Spanish colonies to the American Great Plains.