22 juillet 2006

Film : Race - The power of an illusion

This documentary by the Public Broadcast Service can be found at http://www.pbs.org/race/. The first of the three episodes – “The difference between us” – does a good job of making a few facts clear about skin colour and genetics:

The measured amount of genetic variation in the human population is extremely small. Genetically, we aren’t very different. In fact, we are among the most similar of all species:

- only 1 out of every 1000 nucleotypes that make up our genetic code differs from one individual to another;

- penguins have twice our amount of genetic difference, yet they all look alike to us;

- and fruit flies, ten times; hence any two fruit fruit flies may be as different as a human is from a chimpanzee.

We have a long history of searching for racial differences, and attributing performance and behavior to them. It is frightening to see how many “scientific” studies were made on the subject. This was so true in the United States that 28 states passed laws forbidding intermarriage to safeguard white racial purity. The nazi propaganda machine pointed out that their eugenic policies were entirely consistent, and in fact derived from, ideas of american race scientists. Then, as we remember, Jesse Owens won four gold medals at the 1936 Berlin Olympics…

Why is skin colour differentiated according to the lattitude? A plausible hypothesis is that sunlight is essential to have active vitamin D. Therefore, in higher lattitudes, children who have lighter melanin might be healthier thanks to their capacity to absorb more light. Along those lines, it soon becomes clear that the best explanation for genetic differences is not rather, but rather geography.

In addition, environment along with opportunity combine with genes to explain individual performance. In the 1930’s, jewish teams dominated the american basketball. For a long time afterwards, as opportunities changed, basketball teams were almost african american. But today, 20% of NBA’s players are foreign-born. Who is the NBA’s top pick? Chinese!

Mitochondrial DNA does not code for any trait, and is inherited only from the mother. Therefore, it tells us something about one of our distant ancestors: our mother’s mother’s mother’s mother’s mother’s etc. An easy test proves that mitochondrial DNA differences are not correlated with skin colour. As for other genetic differences, Richard Lewontin did pioneering work to find out just how much variations occurred within, and across, the groups which were called races. He was using gel electrophoresis, and his findings were a milestone. For proteins, blood groups, as well as DNA sequencing, it alwas comes out the same: 85% of the variation between human beings is between any two individuals within the same local population. Of the small amount of variation in our genes, there is therefore likely to be almost the same average difference between two african american persons as between an african and a caucasian american.

Still, of the small remaining genetic variation, we know that some genes are found more often in some populations. For instance, the sickle cell disease, which can alter the shape of red blood cells, is thought to be an african american disease. In fact, this trait is not uncommon in people from the mediterranean region (up to 30% in Greece), and this is due to the relative resistance it confers to malaria. Therefore, sickle cell is not a racial trait, but rather the result of having ancestors who lived in malarial regions.

Today, we almost have genetic proof that all preexisting humanoid species died – Neanderthals in Europe, Homo Erectus in Asia. A second migration of the current Homo Sapiens from Africa occurred, and all modern humans are descendants of this second migration which is probably less than 100,000 years old. Therefore, because everything originated in subsaharan Africa, most of humanity’s genetic variation today can be found in Africa.

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