When I found out that the Udmurts are among the populations with the highest frequencies of red hair, I had to look more into them. Red hair is unusually frequent among Rh negatives, so when looking at any group of people sharing trait frequencies with Rh negatives, the chance of common ancestry answers coming to light cannot be ignored.
Yamnaya ancestry immediately comes to mind:
Interestingly, the Yamnaya-specific component peaks in Udmurts, who live close to where the Yamnaya samples were collected. This can hardly be a coincidence.
https://eurogenes.blogspot.com/2015/03/yamnaya-related-ancestry-proportions-in.html
The Yamnaya culture is preceded by the Samara Culture.
A November 2015 study published in Nature included an analysis of a male hunter-gatherer from Lebyanzhinka, Samara Oblast who lived ca. 5650-5540 BC. He was found to be carrying haplogroup R1b1a1a and U5a1d.