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The really interesting and enlightening case is Coralville's link to the Smallpox/HIV study conducted by UC Berkeley researchers. Thanks to the strong evolutionary pressure of smallpox (up to 30% child mortality), the gene allele confering resistance has spread over Europe and grown to 10% of the overall population in just 700 years since the initial mutation. That is an amazing pace, at least 60 million copies of a single mutated gene in about 30 generations.

Light skin colour is a little more complicated, since there are many degrees which cannot be explained by a single mutation. But apart from the slight and gradual differences between people at different latitudes, there are two large populations which have evolved an extremely light, almost transparent skin colour: northern Europeans and northern Chinese/east Asians. The additional factor which distinguishes these two groups from native North Americans and from the circumpolar populations of Asia and europe which are all significantly darker, is the reliance on agriculture, ie. a year-round mostly vegetable diet. With a mixed diet in summer and a largely animal diet (containing significant vitamin D) in winter, populations relying on hunting/gathering or fishing can live even in the northernmost parts of Scandinavia, Siberia, Alaska, Canada and Greenland without being excessively hit by rickets (rachitis). The huge benefit of agriculture is of course that it allows a much higher population density.

Is rickets really such a deadly disease? It hits children and causes skeleton deformities which may make it unlikely that the patients grow up to be successful parents. Maybe most significantly, severe rickets causing pelvic distortion in women may preclude vaginal delivery(*), which historically means death at childbirth or a childless life.

Apparently, the 6,000-10,000 years since agriculture became widespread in northern climates has been long enough to cause a strong prevalence of extremely light skin in the northern parts of Europe and east Asia. Interestingly, in Europe light skin is correlated to blond/reddish hair, whereas in East Asia it is not. Consequently, some of the mutations involved must have been separate.

How then can we tell that the introduction of almost-transparent skin 7,000 years ago and smallpox-resistance 700 years ago were mutations and not some of a whole series of beneficial gene modifications made by the creator and intelligent designer of life in general and humans in particular? The answer is Occam's razor: We know that mutations are taking place all the time, caused by cosmic radiation (plus sometimes toxins or viruses), at a largely constant rate under the climatic conditions prevailing before the first hominid saw daylight and until this day. No more is necessary to ensure that some of the possible beneficiary mutations do take place, and in cases where the benefit is huge, selection will take care of propagation of the new and better alleles, and there is no need to assume supernatural ingerence.



(*) http://www.emedicine.com/ped/topic2014.htm
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