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According to the article Dark skin and blue eyes: How Europeans once looked:

It is widely accepted that Man's oldest common forefather was dark skinned, and that people became more pale as they moved further north out of Africa into colder climates with less sunlight.

I thought that humans' oldest common ancestor was light-skinned, because

  1. Dark-skinned people have white palms and soles, but light-skinned people have more consistent skin tones. This suggests that light skin is more ancestral and dark skin evolved to protect skin from the sun.
  2. Bonobo chimpanzees, our closest primate cousins, look like they have light skin under black hair.

I understand we all originate in Africa, but I thought that the environmental conditions in Africa were different from today. How do we know that the first humans were dark-skinned?

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According to wikipedia, "comparisons between known skin pigmentation genes in chimpanzees and modern Africans show that dark skin evolved along with the loss of body hair about 1.2 million years ago and is the ancestral state of all humans." This is several million years after after the time estimated for the last common human-chimpanzee ancestor, but at least 1 million years before the emergence of Homo Sapiens. This suggests that the earliest species in Homo may have had lighter skin, but that the advantageous genes for dark skin were universal by the time of the first true humans.

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The first modern humans evolved about 200.000 years ago in Africa. When they lost their body hair (or at least most of it), they needed some other protection of their skin from the sun - otherwise they are prone to develop melanoma. Melanin is such a protection, and the rate of melanoma is much lower in dark skinned people. There is also a nice correlation between latitude and skin color - the more to the north (and to some degree to the south as well) you get, the lighter the skin color of the population gets. The reason for this is likely the better ability to synthesize Vitamin D (for which you need sunlight on the skin):

The lightening of the skin occurred after humans moved to Europe:

And we even know, that this occured more than once, since the genetic reasons for lighter skins are different between europeans and light skinned asians. A lot of this we know via sequence comparisions of genes involved in pigmentation, there you can trace mutations and see, where they show up first and how they migrate over time. One very good example is the melanocortin-1-receptor (MC1R), which is mutated pretty often in europeans (mutations in this gene are generally associated with lighter skin and hair color; people with red hair carry certain mutations in this gene) but its under selection pressure to not mutate in africa (there are almost no known mutations in MC1R for africans). Another example would be a mutation in the SLC24A5 gene, which is exclusively found in europeans, but not in asians and which accounts also for lighter skin. I stop here for the moment, if there are more specific question, go ahead.

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Nicely referenced answer. –  fileunderwater Jan 27 at 9:36
    
As per the linked article, new research suggests that Europeans evolved light skin because of diet, not latitude. –  anomal Jan 27 at 23:48
    
+1 for mention of the european 'innovation' of being pale! –  shigeta Jan 28 at 5:46
    
@anomal This is discussed, yes. –  Chris Jan 28 at 7:58
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