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While all humans are remarkably similar at a genetic level — on average any two individuals' genomes are 99.9% identical — those differences that do exist manifest themselves in the dazzling spectrum of human diversity. From a lithe tribesperson of the Kalahari, to a freckled redhead from Ireland, to a sleek-haired, ruddy-cheeked inhabitant of the Tibetan plateau, Homo sapiens come in many different shapes, sizes, hues, and appearances. While we tend to notice differences that are easy to spot, other "stealth" variations in human populations are not necessarily observable from physical appearance alone. For example, many people of African descent are resistant to infection from malaria, while that trait is much rarer among those who hail from other parts of the world. Now new research into Arctic-dwelling Inuit populations points to a surprising origin for one such "stealth" trait — the ability to tolerate frigid temperatures.
Where's the evolution?
Just as other organisms have evolved adaptations that aid survival and reproduction in their unique environments — finches evolving beaks shaped to eat the seeds that are plentiful, moths evolving wings that blend in with the bark of the trees where they sit — so too have different human populations experienced natural selection shaping them in response to their environments. Random mutations occur, some of these mutations affect the traits of the organism that carries it, and sometimes the organism lives in an environment where that altered trait aids survival and reproduction. When that happens, the frequency of the mutation increases, generation by generation, as the descendants of the original carriers multiply in the population. That is the essence of evolution by natural selection. And that is how, for example, malaria-resistant African populations came about: a random mutation that happened to confer resistance to its carrier occurred, and when that mutation wound up in an individual living where malaria was common, that person left more offspring (who were also likely to carry the mutation) than did others. Additional traits adapting humans to different local environmental conditions are shown in the figure below.