Tuesday, August 19, 2008

The European pattern of skin, hair, and eye color

The following is an executive summary for one of my book proposals. The book itself will probably take me a year to write and I’m sure I’ll have to update the manuscript continually as new information comes in. Comments are welcome.

Humans look strikingly different in Europe, particularly within a zone centered on the East Baltic and covering the north and the east. Here, skin is unusually white. Hair is not only black but also brown, flaxen, golden, or red. Eyes are not only brown but also blue, gray, hazel, or green.

This pattern also stands out chronologically. It arose very late during the time of modern humans and long after their arrival in Europe some 35,000 years ago. Such is the conclusion now emerging from genetic studies of skin, hair, and eye color.

Europeans owe their light skin to alleles that go back only c. 11,000 years at one gene and 12,000–3,000 years at another. As a Science journalist remarked: “the implication is that our European ancestors were brown-skinned for tens of thousands of years.” They were also uniformly black-haired and brown-eyed. Then, just as recently, their hair and eye color diversified as new alleles began to proliferate at two other genes.

The challenge now will be to narrow the time window. If these changes happened after 7,000 BP, the cause might be northern Europe’s shift from hunting and gathering to cereal agriculture. The change in diet may have reduced the intake of vitamin D, thus favoring the survival of paler Europeans whose skin could synthesize more of this vitamin.

This theory explains how European skin could have turned pale almost at the dawn of history. It leaves unexplained, however, why selection for lighter skin would have multiplied the number and variety of alleles for hair or eye color, especially when so many have little effect on skin color.

If these changes had happened earlier, before 10,000 BP, the cause might involve the last ice age. At that time, the tundra ecozone ran further south in Europe than in Asia, having been pushed down on to the plains of northern and eastern Europe by the Scandinavian icecap. The lower, sunnier latitudes created an unusually bioproductive tundra that could support large herds of game animals and, in turn, a substantial human population—but at the cost of a recurring shortage of male mates. Among present-day hunter-gatherers, similar environments raise the male death rate because the men must cover long distances while hunting migratory herds. The man shortage cannot be offset by more polygyny, since only a very able hunter can provide for a second wife (tundra offers women few opportunities for food gathering, thus reducing their self-reliance in feeding themselves and their children). With fewer men altogether and fewer being polygynous, the sex ratio is skewed toward a female surplus.

In this buyer’s market, men will select those women who look the most feminine. Since human skin color is sexually dimorphic (women are the ‘fair sex’), this sexual selection would eventually whiten the entire population. Where pigmentation has no female-specific form, as with hair and eye color, sexual selection would favor women with color variants that stand out by their novelty, the outcome being an increasingly diverse polymorphism.


Tod said...

The concept of hair colour variation is present, could the (buzz) word "diversity" be used to advantage in explaining it?

That being a blue eyed blonde woman has advantages will come as no surprise to many. Could touching on the Frequency Dependancy of hair colour be a way to get them attentive?

The last paragraph is clearly written but seems to have an awful lot of information packed in it.

Anonymous said...

Not only when, but also where these mutations happened, is important.

Did they pop up randomnly in different populations and locations or did they arise primilarly in a few clans in continental europe and then were spread during migrations ?

For example, if these mutations arose in continental europe after 7000 bc, then it should be explained how they spread in ireland, for example.
According to Bryan Sykes and his study of irish population, Irish people largely descent from the first hunter gatherers who settled into Ireland, around 12000 years ago, still irish are rather white skinned.

Finally, you are touching here with the mechanism of evolution itself. It is safe to say that today's genetic allows evolutionary mechanisms to be far more complex and flexible than the simple darwinian theory of random mutation combined with natural selection.

The importance of RNA in developmental biology is now recognised and is pushing more and more toward a lamarckian mode of evolution.
From a global point a view, it ressorts that networks of genes that are important in development are heavily controled by feedback loop and often by their own transcriptional activity. The abscence of transcript itself, or the presence of inhibitory interfering transcripts in the network can interfere with the DNA genome itself. This can lead to mutational hotspot IN RESPONSE to environmental modifications.

During period of environmental stress, it has been shown that bacterial evolution (or mutation rate) speed up for similar reasons. When in period of relative stability, bacterial DNA is extremely protected against any change or replication error, in effect blocking evolution. During stress, correction mechanisms are inhibited and even DNA strand recognition is released, allowing bacterias to be promiscuous and recombinate with foreign DNA, in effect speeding up evolution.
This lamarckian mode of genetic evolution was proposed in the 90s for bacteria, but the whole picture for humans is that it is not completely impossible to imagine genetic hotspot mutations in the same gene poping up denovo into distant and isolated european populations, without the need to import these mutations from a few clans.


Anonymous said...

That scenario seems to represent some pretty strong selection on males as well.

What might it have produced? More co-operative hunting? Stronger planning skills? Greater future time orientation?

Tod said...

"What might it have produced ?"

The following may be included in what Eals and Silverman 94 call complex mental transformations though it's not specifically mentioned in the hunting post.

One thing that I think it may have a "bearing" on is mental rotation. To take one example ( OK its my best example) when coming back the way you came on a long hunting trip, it would help to be able to visualize how a memory of the landscape ought to look rotated 180 degrees.

Captainchaos said...

Ah, those fair-haired, light-eyed, pale-skinned Euros. Such a delicate phenotype. Any admixture (read: miscegenation) pretty well puts the kibosh on all that. What a bleeding shame. But if people want to engage in 'admixture' (a nice, safe euphemism) then that is their PoMo, 'you go girl', empowered choice. Ain't life grand. I love watching Oprah (no, not really, that was sarcasm).

Well, what if the Sistine Chapel were 'public property'; it belonged to everyone. Everyone decided to chip a little chunk out as a souvenir. "You go girl!" Wouldn't that be a bleeding shame?

If it shouldn't be outlawed at least shouldn't it be stigmatized?

I think so. But who am I? I don't have a PhD. I'm just a working class stiff. (Grumble, grumble.)

Captainchaos said...

I'm a bit of a South Park fan. I especially like it when they do musical numbers.

Here we go (sung by Mr. Mackey):

"Genocide is bad, mmm-kay. Don't throw your genes away. Mmm-kay."

Peter Frost said...

Yes, I tried to pack in as much information as possible with as few words as possible. Most publishing houses impose a word limit on book proposals (in this case, 500 words). You have to make your 'pitch' within a very tiny window of opportunity.

Pigment mutations happen all the time, but in most cases they go nowhere because the disadvantages outweigh the advantages. A good example is albinism. It happens in all populations, but there are only a few (e.g., the Zuni and Navajo of the American southwest) where albinos are valued and have a chance to multiply.

So the constraint was not mutation rate. It was the intensity of selection for mutations whenever they happened to pop up.

Imagine the following situation in ice-age Europe: one out of three women would never marry and have children; the other two would lose considerable reproductive time waiting to marry (or remarry, in the case of young widows). Somewhere, through random mutation, a family had a blonde daughter, and she was immediately snatched up the moment she reached puberty. Some of her daughters would also be blonde and they too would be married off with no loss of reproductive time. Soon, word got around and blond daughters eventually became articles of trade with other clans on the great European tundra. Within only a half-dozen generations, the random mutation had spread throughout Europe.

And the same principle would hold true for other variant hair and eye colors. Or even something like longer head hair. This kind of evolution can operate very fast if the selection is strong enough.

So I don't think it's necessary to postulate a high mutation rate. The main factors were intensity of selection and geographic mobility.

There is much evidence for long-distance trade among Upper Paleolithic Europeans for things like flint, amber, and fossil marine shells. Such trading networks could have facilitated the spread of novel hair and eye colors.

Anonymous said...

yes Peter, Tha's what I had in mind when I asked you to comment on the woman "trade" that occured in south germany 6000 years ago, where 34 squeletons, all only males, were found.

But still, if these mutations are not older than 3000bc, you have a long way to go to whiten all Europe entirely and you are talking about serious trading of the sort mentioned above. From central europe you said ? are we sure of that ?

The other possibility is, as you said, these whitening mutations are 12 or 13000 years old rather than 3000 old.
Why is the datation range so wide by the way ?


Captainchaos said...

Ahhh, stretch, yaaaaawn. Fun facts about the 'playing out of the evolutionary tape'. Gould would never fudge his 'science' to advance his EGI, would he? No, not in a million years. Well, Dr. Kevin MacDonald miiiight disagree with that. MacDonald is a brainy guy, I'll bet his IQ is well past 150.

Thats a far sight more than I got (screw grammar, I'm feeling frisky). Probably goes for others as well?


Ahem, anyways. Scientific edjumucation is fun and all but its what you do with it (yeah, in a political sense) that really makes the rubber meet the road.

Just some random thoughts from a working-class stiff. Ciao.

Anonymous said...

Peter, I'm not an expert in populations but I have another comment about the gene "trade" or exportation theory.
Your explanation sounds solid and is Ok for the blonde genes, since not everybody is blonde in europe, I can conceive that trading genes would be enough to generate 10-40% blonde population.
But what about white skin ? Everybody is white skinned in Europe, so if you start from a population that is not white skinned but their decendants (today) are all white skinned, that would mean that ALL these descendants have exchanged their skin genes in the past. Let's say again we talk about the first, putative brown skinned Irish, is it possible to exchange their entire gene pool if the white skinned mutants are far away, let's say continental Europe, and women can only be exported in small number ?
And how would you define "intense" selection exactly ? like taking the blonde and feeding the white skinned kids and/or killing non-white skinned kids ?
I'm just curious here since these issues are so politically incorrect that we are not supposed to ask, we are not even supposed to know.
But anyway, good text. Thank's for your work and blog.


Peter Frost said...

It wasn't simply long-distance trading networks that drove selection for new hair colors. These networks only increased the availability of new color variants over a wider geographic area. The selection itself was driven by a chronic oversupply of women (or, conversely, a shortage of male mates).

For example, we know that European women were exported to the Muslim world over a long period, so much so that the term 'white slavery' has become a synonym for international prostitution. Yet the effects of this trade have been relatively modest. Hair color is still overwhelmingly black in North Africa and the Middle East. It isn't enough to have a variety of hair colors available. Nor is it enough to have a marked preference for certain hair colors. You must have a situation where large numbers of women lose reproductive time -- something you will never see where polygyny is common.

In the case of hair and eye color, sexual preference seems to be frequency-dependent: the more common a particular color is, the less men will prefer it. This has been shown by Thelen's study and more recently by the studies posted on Gene Expression. So you will never get a population where blond hair has become predominant. Blondness seems to lose its sex appeal once it hits 30-40% of the population.

In contrast, preference for light-skinned women doesn't seem to be frequency-dependent. I think we are looking at a very different mental algorithm here, one that is related to reduction of male aggression and stimulation of male care-taking behavior. For more on this point, click the link to my other website.

Dating is still approximate for these European hair and skin color alleles. Better estimates should become available within the next two years. Ideally, If we could extract DNA from early Europeans, it should be possible to pinpoint the exact time frame when Europeans acquired their present physical appearance. I believe that this transformation happened relatively fast, perhaps within the second half of the last ice age (15,000 to 10,000 years ago).

Tod said...

If white skin stimulates resource provision I can see that the less white skinned wives of S.T. hunters might tend to die off from neglect. The hunters being in demand from other women could be tempted away and take their food resources with them.
Would this lead us to expect a preference for white skin on the part of men? A genetic mechanism such as the Baldwin effect would have to be operating to bring this about; would their be enough time?
If all white skin does is inhibit aggression and induce provisioning men should be indifferent to white skin and be looking at indexes of fertility. If being white skined made females sexually attractive would that not confuse the male assessment of the reproductive quality of females.

Why would aggression be a particular problem in the ST scenario?

Surely being abandoned by a husband with a roving eye would be the biggest problem for a ST wife / mother/expecting. Therefore we ought we to expect the effect of white skin to be to inhibit abandonment not aggression.

I think a possible way white skin females might be able to get and keep an -- in demand -- ST man would be if white skin helps the woman to get the man to fall in love with her. White skin could inhibit impulsive components of sexual desire because it encourages romantic love. Lets face it a man is going to need a powerful emotion to resist abandoning his family if his sex drive (and willing women) are both telling him to do that very thing. Romantic love needs to be take into account as a potentialy invaluable pair bonding mechanism in a scenario where men are in short supply and sole food providers IMO.

Anonymous said...

This is so interesting but with so many unanswered questions.

For example, Todd and Peter, you only mention selection for white skinned females, but what about men ? Do we know for sure that there is no selective advantage for white skinned men, beside vitamin D synthesis.

Then, why would male hunters provide food only to the white baby's mom ? When a male provides to a family, it's not just for the mom, it's also for the baby.
So, maybe romantic bonding is possible to explain sexual selection for females, but that would extend to the babies as well.


Peter Frost said...

There is evidence from the primatological literature that light skin is recognized as an infant cue and thus tends to inhibit aggression and stimulate care-taking behavior. Russell Guthrie was the first to suggest that this might explain the evolution of lighter skin in women:

"I believe the sexual differences in skin color resulted from female whiteness being selected for because it is opposite the threat coloration, although the selection pressures may have been rather mild. Light skin seems to be more paedomorphic, since individuals of all races tend to darken with age. Even in the gorilla, the most heavily pigmented of the hominoids, the young are born with very little pigment. … Thus, a lighter colored individual may present a less threatening, more juvenile image."

R.D. GUTHRIE. 1970. "Evolution of human threat display organs", Evolutionary Biology 4:257-302, 1970.

In a monogamous mating system, the male has closer and more sustained social relations with the female. Among other things, he has to provide her and her infants with food. The female is thus at a higher risk of aggression -- not because monogamous males are more aggressive, but because they are more present.

Anonymous said...

Bottom line is, in absence of any mechanism for females to select males, and you don't provide any, sexual selection is a male-driven phenomena only.
Females don't have any roles in it and, consequently, human race differentiation appear mostly because of the work of men.

Can we extrapolate and say that thousands of years of that repetitive behavior in men, could have encrusted in their innate tendancies ? amplified by cultural habits it would eventually be unfairly qualified of "racism" thousands of years later.

Of all this, it comes that men are no more responsible for their apparent race-awareness behavior, that many people call racism today, than women are not responsible for it.
It is all the consequences of biological necessities that were relevant tenth of thousands of years ago. If women are immune to it, it is not by their own will or moral standards, but because nature spared them the burden to carry this selective behavior.

Accusing men to show race awareness is like accusing women to be indifferent to it. None of us choosed what we are.


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Mills said...

Evolution 101 - The pressures of natural selection ultimately determine what is and isn't 'attractive'.

If a preference for X leads to a genetic advantage, then there are more offspring with a predilection for X and X becomes more attractive.

If paler skin is more attractive then it can only be because paler skin provides a genetic advantage.

You've got a square peg and a round hole!