[Marxism] Humans have evolved from a common genetic pool

[Greg McDonald]

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December 16, 2005  http://www.sciam.com/print_version.cfm? 
articleID=0002E7CA-F27B-13A1-AFAA83414B7FFE9F

		Researchers Identify Human Skin Color Gene

Ten years ago researchers embarked on a study of zebrafish--a quick  
breeding aquarium pet. While searching for cancer causing genes they  
ended up isolating the gene that makes European skin white, thanks to  
the golden variant of the fish.

The genetic basis for human skin color has eluded scientists for  
years, with previous studies pointing to more than 100 different  
genes involved in the production of melanin--the pigment responsible  
for skin color and a natural sunblock. Cancer geneticist Keith Cheng  
at Pennsylvania State University and his team determined that the  
golden zebrafish--a lighter version of its "wild" cousin--has a  
genetic mutation that cuts short a protein critical to the production  
of melanin.

Simply adding the normal length protein to the golden zebrafish  
returned it to a darker color. More significantly, adding the human  
gene SLC24A5, which is responsible for production of that protein in  
people, to zebrafish embryos also restored the darker coloration.  
Cheng then turned to Penn State anthropologist Mark Shriver, who had  
been studying the evolutionary genetics of human skin color, to  
determine whether the gene played a similar pigmentation role in humans.

Using the human genome database, the so-called HapMap, the  
researchers found that SLC24A5 has just two variations. Nearly all  
humans of European descent have a version of the gene with one type  
of amino acid, threonine; nearly everyone else has another, alanine.  
This suggests that a so-called "selective sweep" for the gene,  
wherein a gene variant confers a benefit and is thus selected for,  
took place among European ancestors.

The researchers then measured the effect of this gene in 308  
individuals of mixed European and African heritage and determined  
that those who predominantly carried the threonine variant of SLC24A5  
were the lightest, whereas those who predominantly carried the  
alanine type were the darkest. Those subjects who possessed both  
versions of the gene fell somewhere in between, reflecting the broad  
array of skin hues in the human palette. Using a relative  
pigmentation scale, the researchers concluded in their paper,  
published today in Science, that variation in SLC24A5 accounts for  
between 25 and 38 percent of the skin color
difference between Europeans and Africans.

Uncovering this gene, however, does nothing to solve the question of  
why Europeans developed lighter skin in the first place--though it is  
believed to represent an effort to boost production of vitamin D in  
sun-deprived latitudes. Neither does the work reveal the genetic  
basis for the lighter skin tone of some Asians. The finding does  
promise, however, to yield new insights into potential skin cancer  
treatments and other skin-related diseases.

"We know so little about the genetic and evolutionary architecture of  
human traits," Shriver notes. "We cannot expect to use human genetics  
to understand complex diseases most effectively without first working  
out how fundamental characteristics, such as eye, hair and skin  
color, are determined."

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http://evolution.berkeley.edu/evolibrary/news/060201_zebrafish


But how did different ethnic groups wind up with these different  
versions of the pigmentation gene? Research suggests that more than  
100,000 years ago, the earliest humans lived in Africa and carried  
the "G" allele, which causes lots of melanin to be present in skin  
cells and hence, dark skin. Melanin absorbs UV rays from the sun and  
controls the amount of UV radiation that penetrates our skin. Our  
bodies need some UV radiation (to build the essential vitamin,  
vitamin D) but not too much (because UV radiation can damage the skin  
and destroy another essential vitamin, folate). In the sun-drenched  
environs of Africa, dark skin was advantageous, preventing UV rays  
from doing too much damage, while allowing in enough UV to synthesize  
vitamin D. In that environment, individuals born with a mutant  
version of the gene associated with less melanin and lighter skin  
would probably have had poor health and low reproductive success.



Many biologists hypothesize that between 55,000 and 85,000 years ago,  
humans began to migrate out of Africa. Some of them wound up living  
in the colder, darker climes of Europe. There, too much UV radiation  
was not a problem, but too little UV to synthesize vitamin D probably  
was. At some point either before or after the migration out of  
Africa, a mutation occurred in one of the ancestors of modern  
Europeans. This mutation was tiny, changing just a single base, but  
it caused much less melanin to be present in the skin of those who  
carried the mutation. This was the "A" allele. Among the new  
Europeans, this allele likely had an advantage over the "G" allele.  
Individuals carrying the "A" allele had less melanin, which allowed  
more UV light to penetrate their skin, which could have allowed them  
to synthesize vitamin D better than those carrying only the "G"  
allele. These "A"-carrying individuals had increased reproductive  
success in their sun-poor environment, and via natural selection, the  
"A" allele spread throughout the European population. Meanwhile,  
among Africans, the "G" allele continued to be advantageous and to  
maintain its majority there.The discovery of this pigmentation gene  
has helped us piece together a more complete picture of the  
evolutionary changes that underlie skin color differences between  
human ethnic groups. It suggests that a great deal of the skin color  
difference that has delineated the boundaries of social tensions for  
much of recorded history can be traced back to a tiny genetic change  
that allowed humans to better survive and reproduce in particular  
environments.

This discovery also highlights the fundamental similarity of all  
humans. Skin color, it turns out, really is skin deep; at a genetic  
level, the skin color difference between a European descendent and an  
African descendent might be largely influenced by a single base pair  
difference in a genome composed of three billion base pairs. However,  
neither does this single gene tell the whole story. For example, many  
East Asians carry the "G" allele and yet have light-colored skin.  
Why? We don?t know, but it is probably due to many other undiscovered  
genes. And though, on average, people who carry only the "A" allele  
have lighter skin than those who carry only the "G" allele, there is  
a huge overlap in skin color between the groups ? so knowing which  
alleles an individual carries will not let you predict with much  
certainty the shade of his or her skin. It is clear that the concept  
of "race" is more a sociological construct than a biological one, and  
that many genes and environmental factors influence skin color. As we  
learn more about these genes, we will learn more about our own  
evolutionary history as a species.


Finally, methods used to discover this pigmentation gene reveal the  
deep connections between our own species and others that have lived  
over the approximately 3.5 billion years of life's history. Studying  
the zebrafish, for example, led to discoveries about our own  
evolutionary history and genetic variation. And because of the common  
ancestry of all life on Earth, other species (as different from us as  
they may seem) represent our evolutionary cousins and can teach us  
about ourselves and our history.