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Several things wrong with this. Number one, immediately stating it show Darwinism at work is, in fact more like Lamarck than Darwin. Second, stating that a ‘gene’ against kuru is “ a gene against kuru” is suspect, as is saying there is “no other gene like it seen in any other person on the world”. Broad sweeping statements that get published but do not bear up under scrutiny. It is entirely possible that they have identified a gene that exists in “survivors” that would exist in any number of other tribespeople. Since the practice of eating the brains of the dead is assumed to continue at the same rate, when it may not be (and, indeed how does one verify— yeah, so you ate your Dad’s brains- with any accuracy) this makes most of the claims here “pop science” and outcome being fitted to science theory. Not very compelling. By the way, if they are claiming a “mutation”, mutations under Darwin were always assumed to be helpful— in research they have generally been show to be harmful and species reacting to a mutation by dying out.

Under rather stable environment, where selection pressure is more or less negligible, most of gene mutation would turn out to be useless or harmful. Those with potential benefit could be slowly discarded because they do not confer any decisive advantages. Only when there is some major change in the environment (such as new widespread plague,) selection pressure jumps and useful genes can be picked up relatively fast.

There are always certain level of mutation going on, whether it is needed or not. It only make noticeable contribution when 'acute' selection pressure is applied on the population. New plague, sudden habitat change are among them.