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Sunday, June 10, 2018

Our white skin

We "whites" seem inordinately proud of our white skins.  We have always considered it to be a sign that we are superior to the darker variety of homo sapian.  So I thought it would be fun to consider where our white skin came from.

We all know that people living in sunny climates which includes much of Africa, where we originated, have dark skins.  Undoubtedly, this included all the species of the genus Homo that preceded us.  Look at our nearest relatives, the Chimpanzies and great apes.  All with black skins despite the fact that they live, for the most part, in jungles.  Early hominids must have spent many hours in the sun and often, due to warm temperatures,  went with limited body covering.  They had to spend considerable time in the open finding their daily crust of bread and they developed dark skins to protect their skin from too much sunshine.
 Image result for image chimpanzees

Note that the babies have white faces while the adult is black.  Why do you think this might be.

Sunshine of course, provides us with vitamin D which is essential for the calcification of strong bones.  A hominid with weak fragile bones hasn't got much of a show in an environment in which his speed agility and strength will often make the difference between eating or being eaten.

But, vitamin D is important for so much more and we keep finding additional functions  of this important vitamin.  Some functions which have so far being discovered include:

*Strengthened immune system
*Enhanced muscle function.
*Improved lung health
*Anti-inflamatory properties
*Reduced blood pressure
*Reduced hardening of the arteries
*Protection from kidney disease
*Suppression of a pathogenic appetite
*Protection agianst Alzheimer

So there is a strong selective pressure for anything that ensures you obtain enough vitamin D

Sunshine is not the only source of vitamin D either.  Other sources include:
*Fresh fatty fish
*Oysters and some other 'shell fish'
*Some livers
*Egg yokes
*Raw milk
*Some mushrooms
*Some fish eggs.

Think of the dilemma of the first hominids that left sunny Africa.  Those that migrated along the coast toward India had little selective pressure to evolve a light colored skin.  Not only did they migrate through sunny climates but if they were able to fish, they would have got their vitamin D that way.

However for those that left the beach and migrated northward it was a different story.  There was a double whammy against them.  First, if they migrated into northern climates, it was more cloudy giving them less exposure to sun.  In addition in these colder climates, they would have had to dress in skins or clothes made of wool which was shed annually by various woolly animals.  They probably had only their face and hands exposed to the sun so there was a strong selective pressure to loose their protective melanin so that they could obtain their vitamin D.

For the males, there was another factor.  We had beards, possibly for the same reason that a male peacock has beautiful feathers so even less of our skin was exposed to the sun.

The interior of continents are very warm in the summer so they probably also evolved the ability to sun tan, if they didn't have this already.

According  to our present archaeological knowledge, Homo erectus  arrived in Eurasia at least 1.6million years ago.  For those that challenged the northern interior of the continent, there would have been a strong selective pressure for the evolution of a white skin.  Erectus sites from 1.6m years ago contain  flint tools including  retouched flakes so he was no dummy and it is not unreasonable to assume he fashioned some sort of covering.

Homo erectus likely evolved into  Homo heidelbergensis and then into
Homo neanderthalensis.  It is unlikely that this was a new migration from Africa since neanderthal genes are found in European people of today but not in modern Africans.  Incidentally another species which may have developed from Homo erectus is Homo sapiens denisova  further East in Asia.  Presumably, Denisovians who lived in northern climates characterized by cold cloudy conditions, they also had pale skins.

But there is something curious.  Bones of Homo sapien have been found in Africa from 315,000 years ago but only appeared in the fossil record in Europe 45,000 years ago.  How come.  Of the various theories presented, my favorite is armament.  It is likely that Neanderthals and our species came into contact much earlier than 45,000 years ago in the bridge from Africa to Europe, namely in the middle East. Think what a conflict that would have been.  The gracile Homo sapien comes up against the powerful, robust Homo neanderthalensis who makes his living stabbing large and small animals at close quarters with his spear.  No prizes for working out who would win that contra-ton.

I suspect the critial development which allowed our species to make inroads into Eurasia was the development of throwing weapons.  This could have included the lance, atyl atyl*, sling and even the bow and arrow

*Trowing stick which allowed a spear of a size between a lance and an arrow to be thrown with much more force that with the hand alone.

Our whole history is one of killing at greater and greater distances*.  It has culminated now with cowardly soldiers sitting safely in America, killing people with gattling guns and hell fire missils from their drones in other peoples countries as if they were in some sort of video game.

*In more modern times, the Battle of Agincourt

Neanderthals were at two disadvantages.  Their culture was one of killing prey close up and with humans the way we are, there was probably a strong sexual selection for the man that did this most effectively.  You bring home the bacon and the girls want you for their mate. So their whole culture favored the man who was a real man and could kill a deer at close quarters. But they had a second disadvantage.

You would think that they would rapidly adapt throwing weapons once they saw how effective they were.  Unfortunately, through evolution, they had evolved a shoulder joint that while it was very powerful, it was not adapted for throwing.  Even if they wanted to adopt the new weapons, they would have been very clumsy at it. Neanderthals lacked a throwing arm.

An effective throwing weapon would have completely changed the balance of power. And again, the way we are, when a tribe of Sapiens defeated a tribe of Neanderthals, they probably killed all the males and took the females for themselves.  Here a little diversion into genetics.

With the difficulty of travel when all you could call upon were your legs, and considering how territorial humans are, the two tribes were what we might consider pure breeds.  In other words, they would have had a high degree of homozygosity* at many more sites on their chromosomes than modern humans who breed with other humans who are distant both in geographical terms and in their genetics.

*Each cell of our bodies has two copies of each chromosome.  The genes on each chromosome at the same locus can be the same (homozygose) or different (hetrozygose).  When closer related individuals mate, this increases homozygosity and of course when a recessive lethal gene or even a disadvantagous gene comes together in an individual, he is either dead or disabled.  Inbreeding over time, to some extent, weeds out deleterious recessive genes from the population.  When two inbred individuals from different genetic lines breed, their offspring are likely to be particularly robust

 We keep chickens for the production of eggs.  We start with a variety called brown shafers who are great layers.  They are a hybrid of at least 4 so called pure breed varieties and express hybrid vigor. However we allow them to breed freely and you should see the varieties of chickens produced.  They not only revert to the original varieties but produce all sorts of assorted mixes.

Imagine the offspring of this first mating of male Sapien with female Neanderthals.  The two populations had been isolated and inbreeding for thousands of years.  The result would have been the equivalent of our Brown Shafers.  They would have all looked very similar and likely would have been very strong with a somewhat better throwing arm than Neanderthals but not a good as Sapien and with an intermediate skin colour.

In order not to make the whole story too complicated, I will assume that a throwing shoulder is determined by one gene and likewise skin color.  Any genetisist will tell you that many genes are involved in the determination of these characteristics and most others.  Mendel lucked on to some characteristics of peas that indeed were determined by one gene and so laid the basis of genetics.  I will also assume that both genes have equal influence.  That is neither is dominant over the other.  We will use W for a white skin and w for a dark skin.  Also T for a throwing shoulder and t for a non throwing shoulder.  This is a huge simplification but will make the situation easier to understand.

In what the genetisists call the F1 generation we have a cross between two quite homozygous populations (pure bred) The offspring will have half their genes from the mom  and half from the dad and those from all the dads will be quite similar to each other and likewise all those from the moms, similar to other moms.  Then the fun starts.

When the children start to reproduce, their offspring will have all sorts of mixes of these genes.  We will have:
WWTT - a white skin and a throwing shoulder
WwTT - a dusky skin and a throwing shoulder
wwTT - a dark skin and a throwing shoulder
and so forth.  Use a Mendelian square to work out all the variations.  On one axis you put WT, Wt, wT and wt and the same on the other axis.  These are the genes in the gametes (eggs and sperm).  Now selection begins.  Since humans are nasty and of great danger to each other, a throwing shoulder will be selected for as will a lighter skin, especially as these hybrid humans move away from the coast into more northern areas.

We have all heard by now that we have a small percent of Neanderthal genes.  This begs the question of what is a species.  The old definition was that two individuals are of the same species if they can produce a viable offspring that itself can breed and produce viable offspring.  Of course the situation on the ground is more complicated than this.  Apparently the various species of hominin in Africa bred back and forth in all sorts of combinations and clearly we bred with Neanderthals.  Otherwise we wouldn't have their genes.  Over the thousands of years since we started to breed with Neanderthals, individuals would be selected with characteristics from both strains of human that had the best survival.  Here we have only considered two.

I suppose we we were lucky to breed with a species that had already adapted to northern climates.  It would have speeded up considerably our rate of adaptation as we kept those characteristics that benefited us rather than having to start with favorable mutations - a much slower process.

We can thank the Neanderthals for our white skin.

Post scriptum
Facts are such a bitch when it comes to a great theory.  It was great fun poking fun at the white supremicists but it turns out that it probably is not true.  Have a look at the youtube videos from this chap.   Apparently it is true that at least some neanderthals were white and interestingly they had red hair.  The whatsit in the woodpile is that the genes that make present day whites, white are different from the ones that made Neanderthals white.  We seem to have evolved out own genes to turn our skin white and hence give us our daily dose of Vitamin D.

Now, of course, this doesn't mean necessarily that all Neanderthals were white.  If there was a population that lived in sunny parts of the world or on the sea shore where they got enough vitamin D from the sea, they could well have been black or brown, just as present populations of humans display all color variations.  Anyway, so much for a fun theory.

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