it's referenced early on in the book The Apocalypse of Yajnavalkya and is based on genetic material and genome information and not just human because it was triggered by a cosmic ray blast from a nearby supernova

i didn't actually follow up all the footnotes in that book but the first few chapters repeatedly mention genetic markers that are a key part of their thesis so i suppose the references are probably legit, but you'd have to check

i personally see it this way, if milk is what babies are fed, then part of what determines how a baby's body is functioning is related to the optimality of the food source, and this is also why mammals produce milk, it is essential to enable development of brains that can process social information

thus, it seems logical that a mutation that turns off the post-weaning lactose intolerance might also lead to longer lifespans due to higher vitality and that leads to more experience which leads to less mistakes that cause mortality

i personally switched over to a mostly milk diet recently because it was so cheap compared to everything else, nearly half the cost of grain based foods and 1/3 the cost of a carnivore diet

my milk supply isn't ideal at this point but i think the visible improvements alone speak something