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If you are at the equator and start moving north, the further you travel, the lighter the skin of the indigenous peoples. Considering that we live on a ball, why do we not find the same traveling south from the equator? This relationship does not hold for individuals as you move south from the equator. At the very least, one could say there are no blond hair blue eyed peoples south of the equator.

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  • $\begingroup$ Welcome to BiologySE...interesting question... if you could cite some examples online to back up your statements in the question it would make the question much better $\endgroup$ – Vance L Albaugh Apr 1 '16 at 1:50
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    $\begingroup$ Because there are no significant land masses that far south of the equator that have been occupied by humans long enough for selection to have had an effect? Africa doesn't extend that far south (and the southern tip is relatively small), and has plenty of opportunity for admixing/invasion by peoples to the north (see e.g. Zulu history), Australia doesn't extend far south, South America was only settled ~10K years ago, and like Africa, the southerly portion is small. $\endgroup$ – jamesqf Apr 1 '16 at 4:43
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I rather would say that the lack of North/South Symmetry in pigmentation is that we forget how quickly human beings have spread. In prehistory, people have come to populate every continent over perhaps the last 60,000 years.

While in that time its clear that several mutations have popped up to influence skin color, they are pretty rare compared to the speed with which we migrated over the globe. The dark color of our earliest ancestors has been lost and regained more than once, but not so often that the selection pressure of latitude have made indigenous in the tropics of south america or south east asia uniformly as dark as africans. Or for that matter have lightened the skin of pre-colonial South Africans.

One might guess that clothing and shelter have reduced the selection pressure on pigmentation further over the past few thousand years.

Individual cases should be studied - this is just a sketch - but the general thought here is that the pigmentation of human beings have more to do with where they have been and how quickly they have recently come from in their migrations.

Summarizing the comments below: There is a correlation between pigmentation and latitude. The answer is really a combination of latitude, how long a people has been there, and the speed at which pigmentation mutants show up. It doesn't change so quickly that all people are equally dark at the same latitude or so slowly that you find completely pale people at the equator.

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    $\begingroup$ The dark color of our earliest ancestors has been lost and regained more than once That looks interesting, can you please add some references? $\endgroup$ – A.L Apr 2 '16 at 1:16
  • $\begingroup$ Just remembering some of the south pacific and australian aborigines being so dark sometimes.... no strong reference. I did find this reference that pigmentation is correlated to latitude. To the extent that it is symmetric or not depends on how perfect you expect the symmetry to be... sciencedirect.com/science/article/pii/S0047248400904032 $\endgroup$ – shigeta Apr 7 '16 at 12:34
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    $\begingroup$ If the dark color of our earliest ancestors has been lost and regained more than once and pigmentation is correlated to latitude, does it mean that our ancestors traveled several times between the equator and poles? $\endgroup$ – A.L Apr 7 '16 at 12:51
  • $\begingroup$ It means that the answer is really a combination of latitude, how long a people has been there, and the speed at which pigmentation mutants show up. It doesn't change so quickly that all people are equally dark at the same latitude or so slowly that you find completely pale people at the equator. $\endgroup$ – shigeta Apr 9 '16 at 16:41
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    $\begingroup$ @shigeta: But as I noted above, the mass of Australia isn't that far south. Excluding Tasmania, the southernmost point is only 39°08' S latitude, and the mass of the continent is quite a bit closer to the equator. It's also a mostly desert climate with lots of sun, unlike most of Europe north of 40° or so. $\endgroup$ – jamesqf Apr 9 '16 at 17:53

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