Wednesday, June 17, 2009

Why do Europeans, North Asians and Native Americans have pale skin?

There is significant scientific evidence to suggest that Europeans, North Asians and North Americans are all descended from Africa. The Out Of Africa model suggests that homo sapiens arose in Africa and then migrated to other areas of the world to replace other hominid species such as homo erectus, and the Multiregional Continuity model suggests that hominid species that left Africa then evolved into homo sapiens all around the world. Whichever view is taken, it remains that homo sapiens descended from groups that lived in Africa. These groups almost certainly had dark skin to protect themselves from the strong tropical sun in Africa, and so there poses a question, why now do Europeans, North Asians and North Americans have pale skin, a clear differential from the dark skin of their ancestors.

There is one main hypothesis as to why this differentiation occurs. The basis of this answer lies within vitamin D. Vitamin D is needed for effective bone mineralisation and when it is deficient, bone softening diseases such as rickets and osteomalacia in adults occur, and possibly osteoporosis. These diseases cause the legs to be deformed, making walking harder. The thoracic cage also becomes deformed, causing more effort to be exerted in order to breathe. However, the most important problem with this disease in terms of evolution is pelvic deformations, where the pelvic bones collapse together, so childbirth and therefore reproduction is prevented from occurring. In countries more northern than Africa, the protective dark skin useful in Africa will not let enough of the weaker sun in, and so deficiencies could occur. The gene for skin colour has been identified in Europeans and Africans as SLC24A5, with one allele being present in 98% Europeans and the other allele being present in most Africans and East Asians. The gene for skin colour in North Asians is still being investigated.

Recent evidence has uncovered that in fact human skin stayed dark for 30 000 years after the migration out of Africa, when it was originally thought light skin occurred when they first moved. Therefore, this theory about vitamin D is flawed. There are theories as to why there was such a long gap. One of them is that at first humans were hunter-gatherers, herders and fishers, and so their diet included enough vitamin D not to cause any problems. However, at around the same time as the skin colour change occurred, farming became a new way of life and expanded, which did not provide enough vitamin D in the diet. Farming allowed for much larger population sizes to live together, and because mutations are rare, larger populations give a higher chance of a mutation occurring, and that that mutation is actually useful. A mutation is also more likely to occur in one of these populations because they are living in a challenging environment (not enough UV) that causes selection pressures. These new challenging environments and a growing population cause accelerated human evolution, and in this example it would be a lightening of skin colour. Another factor in this could be cultural for example heavier clothing being worn, and so decreased the area available for absorption of UV.

There are other theories however, as many people think that pale skin is not an advantage in most environments in the higher latitudes, due to the vulnerability to sunburn and malignant melanomas, and so do not think that pale skin evolved due to natural selection. The two presented here are sexual selection and parental selection.

Another theory about how lighter skin has come about is that there has been a common selective force that acted on three characteristics – skin, hair and eye colour - in the northern latitudes. This force has been suggested to be sexual selection, and could have acted on an existing sexual dimorphism of men generally being browner and women being fairer. The selection is seen in all upper classes around the world of having a preference for fairer skinned females to marry. Because of this, upper class men may choose lighter females and so over generations the class lightens. Some believe that this sexual selection for light skin counteracts natural selection for dark skin. The difference in skin colour geographically may be due to a balance between natural and sexual selection, so suggesting that it is a combination of both factors. This balance differs according to the latitude, so at low latitudes dark skin is prevalent as natural selection overrides the male preference for light skin. But at higher latitudes where natural selection ceases to act, sexual selection becomes more important, and causes light-skinned populations with the females lighter than the males.

Another factor that is argued by Judith Harris is parental selection, mainly maternal selection. In many cultures a form of birth control was infanticide, where the child was killed after birth. The most recent example of a culture in which this takes place is the !Kung. This way of living gave mothers the power to exert an influence on evolution by deciding to keep or abandon a new born. A decision may be made before birth to abandon the child due to bad timing as the previous child may not have been weaned, or food was scarce, or there was no partner to provide for the baby. However, once the baby is born, this decision may be overturned depending on the reaction to the newborn itself. The newborn may have been sickly, weak, the wrong sex, or have a congenital malformation. However the decision may go the other way if the baby is especially beautiful, conforming to the mother’s standards of beauty. Although this decision may only be made in a small percentage, this all adds up and Harris argues that it is big enough to exert an influence on the characteristics of our species. Harris proposes that hairlessness and pale skin colour seen in northern countries is partly the result of parental selection. She accepts that sexual selection also played a part but that it was parental selection that speeded the evolutionary process up, producing very noticeable changes in a relatively short space of time. She combines the concept of hairlessness and pale skinned because a hairless pale human would be very exposed to the dangers of the sun, and so she believes there must be some other reason why so much of the world’s population is not adapted as best as it can to the environment. The concept of beauty that may sway a mother to keep her baby is of a pale skinned baby. This may be linked to sexual selection, as parents would want a pale skinned daughter so that she can have more choice of husband, and so passing on desirable characteristics that the parents will have as grandchildren.

It appears that there is a difference in opinion regarding how pale skin came about. Unfortunately, evidence to prove either theory is hard if not impossible to come by, and so there is a possibility we will never know. However, in my opinion, I believe that it could be a combination of all three, but with the main influencing factor being natural selection. This is because i don’t believe that such a huge change could come across just through sexual or parental selection alone. Along with this, there is not concrete evidence to support the sexual selection claim and one study has shown that there is no evidence for a correlation between increasing distance from the equator and increased sexual dimorphism.

Skin colour evolution in Europeans and social skin colour vs disease in Puerto Ricans, Dienekes Anthropology Blog
Sexual selection as a cause of human skin colour variation: Darwin's hypothesis revisited by Aoki K, Pubmed.
Human skin colour dimorphism, Kelly and Madrigal, Department of Anthropology, University of South Florida
Judith Harris, Parental Selection: A Third Selection Process in the Evolution of Human Hairlessness and Skin Color

1 comment:

  1. A good overview. Note Linnean taxonomy requires genus and species to be separate words, both in italics and with a capital letter for genus: Homo sapiens. What is the evidence that Europeans retained light skins for 30,000 years? Is it not also worth considering the idea that there is no positive selection for light skin, merely a relaxation of selection against, coupled with genetic drift?