PDA

View Full Version : new study challenges lighter vs darker skin color story



MikeWhalen
07-08-2014, 06:28 PM
This seems pretty interesting...If I understand it correctly, while skin getting lighter as humans migrated northward and needed extra help with UV light to get enough Vitamin D was still a factor, the new study says dark skins ability to retain moisture in high heat environments became less needed as humans migrated north to cooler area's and thus the body spent less energy making it...

"In human evolution, changes in skin's barrier set northern Europeans apart"
http://www.sciencedaily.com/releases/2014/06/140630140836.htm

The popular idea that Northern Europeans developed light skin to absorb more UV light so they could make more vitamin D – vital for healthy bones and immune function – is questioned by UC San Francisco researchers in a new study published online in the journal Evolutionary Biology.


Ramping up the skin’s capacity to capture UV light to make vitamin D is indeed important, according to a team led by Peter Elias, MD, a UCSF professor of dermatology. However, Elias and colleagues concluded in their study that changes in the skin’s function as a barrier to the elements made a greater contribution than alterations in skin pigment in the ability of Northern Europeans to make vitamin D.
Elias’ team concluded that genetic mutations compromising the skin’s ability to serve as a barrier allowed fair-skinned Northern Europeans to populate latitudes where too little ultraviolet B (UVB) light for vitamin D production penetrates the atmosphere.
Among scientists studying human evolution, it has been almost universally assumed that the need to make more vitamin D at Northern latitudes drove genetic mutations that reduce production of the pigment melanin, the main determinant of skin tone, according to Elias.
“At the higher latitudes of Great Britain, Scandinavia and the Baltic States, as well as Northern Germany and France, very little UVB light reaches the Earth, and it’s the key wavelength required by the skin for vitamin D generation,” Elias said.
“While is seems logical that the loss of the pigment melanin would serve as a compensatory mechanism, allowing for more irradiation of the skin surface and therefore more vitamin D production, this hypothesis is flawed for many reasons,” he continued. “For example, recent studies show that dark-skinned humans make vitamin D after sun exposure as efficiently as lightly-pigmented humans, and osteoporosis – which can be a sign of vitamin D deficiency – is less common, rather than more common, in darkly-pigmented humans.”
Furthermore, evidence for a south to north gradient in the prevalence of melanin mutations is weaker than for this alternative explanation explored by Elias and colleagues.
In earlier research, Elias began studying the role of skin as a barrier to water loss. He recently has focused on a specific skin-barrier protein called filaggrin, which is broken down into a molecule called urocanic acid – the most potent absorber of UVB light in the skin, according to Elias. “It’s certainly more important than melanin in lightly-pigmented skin,” he said.
In their new study, the researchers identified a strikingly higher prevalence of inborn mutations in the filaggrin gene among Northern European populations. Up to 10 percent of normal individuals carried mutations in the filaggrin gene in these northern nations, in contrast to much lower mutation rates in southern European, Asian and African populations.
Moreover, higher filaggrin mutation rates, which result in a loss of urocanic acid, correlated with higher vitamin D levels in the blood. Latitude-dependent variations in melanin genes are not similarly associated with vitamin D levels, according to Elias. This evidence suggests that changes in the skin barrier played a role in Northern European’s evolutionary adaptation to Northern latitudes, the study concluded.
Yet, there was an evolutionary tradeoff for these barrier-weakening filaggrin mutations, Elias said. Mutation bearers have a tendency for very dry skin, and are vulnerable to atopic dermatitis, asthma and food allergies. But these diseases have appeared only recently, and did not become a problem until humans began to live in densely populated urban environments, Elias said.
The Elias lab has shown that pigmented skin provides a better skin barrier, which he says was critically important for protection against dehydration and infections among ancestral humans living in sub-Saharan Africa. But the need for pigment to provide this extra protection waned as modern human populations migrated northward over the past 60,000 years or so, Elias said, while the need to absorb UVB light became greater, particularly for those humans who migrated to the far North behind retreating glaciers less than 10,000 years ago.
The data from the new study do not explain why Northern Europeans lost melanin. If the need to make more vitamin D did not drive pigment loss, what did? Elias speculates that, “Once human populations migrated northward, away from the tropical onslaught of UVB, pigment was gradually lost in service of metabolic conservation. The body will not waste precious energy and proteins to make proteins that it no longer needs.”
For the Evolutionary Biology study, labeled a “synthesis paper” by the journal, Elias and co-author Jacob P. Thyssen, MD, a professor at the University of Copenhagen, mapped the mutation data and measured the correlations with blood levels of vitamin D. Labs throughout the world identified the mutations. Daniel Bikle, MD, PhD, a UCSF professor of medicine, provided expertise on vitamin D metabolism.


What do you guys think?

M

Jean M
07-08-2014, 07:14 PM
What do you guys think?

I think I need to do a wholesale re-write of my paragraphs on this topic! I'd had a bit of a clue a few months ago from something in a TV programme. This takes things a lot further.

Stellaritic
07-08-2014, 07:26 PM
This doesn't explain why Both the Denisova and Mesolithic hunter-gatherers were dark skinned .

lgmayka
07-08-2014, 10:08 PM
“For example, recent studies show that dark-skinned humans make vitamin D after sun exposure as efficiently as lightly-pigmented humans, and osteoporosis – which can be a sign of vitamin D deficiency – is less common, rather than more common, in darkly-pigmented humans.”
...
Elias speculates that, “Once human populations migrated northward, away from the tropical onslaught of UVB, pigment was gradually lost in service of metabolic conservation. The body will not waste precious energy and proteins to make proteins that it no longer needs.”
The cited studies are very valuable contributions to the debate, but Elias' alternative explanation is extremely weak.

Erik
07-12-2014, 01:31 AM
I once read another theory that European light skin has to do with diet.

Jean M
07-12-2014, 09:55 AM
This doesn't explain why Both the Denisova and Mesolithic hunter-gatherers were dark skinned .

Dark skin is just the standard model for Homo, presumably since our ancestors lost their fur. If a creature does not have fur to protect its skin from ultra-violet light, it needs melanin to do the job. What geneticists are aiming to explain is the gradual change to paler skin in regions further from the equator. A crucial word is 'gradual'. Natural selection works slowly, over thousands of years.

lgmayka
07-12-2014, 10:52 AM
What geneticists are aiming to explain is the gradual change to paler skin in regions further from the equator. A crucial word is 'gradual'. Natural selection works slowly, over thousands of years.
As we have discussed earlier, "natural" (biological) selection operates over tens of thousands of years. It cannot possibly explain the rapid change in European skin color that we now know occurred within the span of a few thousand years at most. Here is a typical popular science article on the issue (http://www.livescience.com/42838-european-hunter-gatherer-genome-sequenced.html):
---
The analysis of the man, who lived in modern-day Spain only about 7,000 years ago, shows light-skin genes in Europeans evolved much more recently than previously thought.
...
Many scientists have believed that lighter skin gradually arose in Europeans starting around 40,000 years ago, soon after people left tropical Africa for Europe's higher latitudes. The hunter-gatherer's dark skin pushes this date forward to only 7,000 years ago, suggesting that at least some humans lived considerably longer than thought in Europe before losing the dark pigmentation that evolved under Africa's sun.
...
But the new discovery shows that latitude alone didn't drive the evolution of Europeans' light skin. If it had, light skin would have become widespread in Europeans millennia earlier, Lalueza-Fox said.
...
The finding implies that for most of their evolutionary history, Europeans were not what many people today would call 'Caucasian', said Guido Barbujani, president of the Associazione Genetica Italiana in Ferrara, Italy, who was not involved in the study.
---

In contrast, a Neanderthal introgression may play a role in the skin color of some East Asians, particularly Taiwanese aborigines (http://mbe.oxfordjournals.org/content/early/2014/06/02/molbev.msu180.abstract):
---
We further discovered that all of the putative Neanderthal introgressive haplotypes carry the Val92Met variant, a loss-of-function variant in MC1R that is associated with multiple dermatological traits including skin color and photoaging. Frequency of this Neanderthal introgression is low in Europeans (~5%), moderate in continental East Asians (~30%), and high in Taiwanese aborigines (60-70%).
---

GailT
07-12-2014, 02:27 PM
As we have discussed earlier, "natural" (biological) selection operates over tens of thousands of years. It cannot possibly explain the rapid change in European skin color that we now know occurred within the span of a few thousand years at most.

A highly beneficial gene could sweep through a population much more quickly, right? Sexual/cultural selection could also cause a more rapid population transformation than natural selection.

There is also quite a bit of variation in skin colour within Africa. Is there an explanation for lighter skin color among Khosians? The melanin/vitamin D trade-off seems like a possible evolutionary mechanism. Skin moisture does not seem like a plausible explanation for Khosians.

lgmayka
07-12-2014, 06:27 PM
A highly beneficial gene could sweep through a population much more quickly, right?
In extreme cases, yes:

- In a shrinking population (i.e., during a bottleneck period), an allele with a very high survival rate may effectively sweep through the population rapidly. For example, an allele that confers immunity to an otherwise common and often fatal illness.

- In a growing population, an allele with a very high reproduction rate may effectively sweep through the population rapidly. For example, an allele that extends fertility an extra couple decades.

But otherwise, humans are really not analogous to fruit flies. Their generations are so long, and their survival and reproduction is subject to so many different factors, that it is difficult to imagine a single allele sweeping through a human population rapidly by biological means alone.


Sexual/cultural selection could also cause a more rapid population transformation than natural selection.
Yes, exactly. That was my unspoken point. In general, human survival and reproduction is more strongly influenced by nonbiological factors such as power, wealth, status, and (perceived) beauty. Any of these can drive a swift selective sweep. Even if an allele has some biological value, its sweep through a human population is likely to be primarily social (if the biological value is observable).

Jean M
07-12-2014, 06:54 PM
Many scientists have believed that lighter skin gradually arose in Europeans starting around 40,000 years ago, soon after people left tropical Africa for Europe's higher latitudes. The hunter-gatherer's dark skin pushes this date forward to only 7,000 years ago ....


Not as far as I know. Pigmentation is influenced by more than one gene. The KITLG mutation seems to the earliest outside Africa, as it is shared by Western Eurasians and east Asians. One estimate puts the KITLG change at 30,000 years ago.

The Western Eurasian-type alleles at TYRP1, SLC24A5, and SLC45A2 appear to have arisen much later - all within the last 11,000-19,000 years. Given that the incoming farmers were SLC24A5 derived, that particular mutation could have arisen in the Near East 10,000 years ago or more, and been subject to selection to the point that it was fixed before arrival in Europe.

I haven't had time to dredge through the aDNA papers to check exactly which SNPs were tested in each case, though I made a note in some cases. http://www.ancestraljourneys.org/autosomaladna.shtml. I suppose we should really find out about KITLG.

GailT
07-12-2014, 07:16 PM
Given that the incoming farmers were SLC24A5 derived, that particular mutation could have arisen in the Near East 10,000 years ago or more, and been subject to selection to the point that it was fixed before arrival in Europe.

We have modern day examples of how this might have happened, with fair skin being considered more desirable than dark skin in some parts of the world. Before the industrial revolution most labourers worked outdoors and fair skin was a mark of status in Europe. Neolithic labourers who spent time working outdoors would have had darker skin due to sun exposure. If Neolithic elites spent less time in the sun, this could have been the first time that fair skin was perceived as a symbol of status.

GailT
07-12-2014, 07:35 PM
Interesting paper on filaggrin: One remarkable molecule: Filaggrin (link (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3378480/)).

Quoting from the paper:

Another unanswered question is why filaggrin mutations are so prevalent in various human populations? It has been suggested that filaggrin deficiency may confer some heterozygote advantage and that “natural vaccination” against microbial pathogens via the skin barrier may have driven natural selection for these sequence variants during pandemics in our ancient past (Irvine & McLean, 2006 (http://www.nature.com/jid/journal/v126/n6/full/5700365a.html)).

Jean M
07-12-2014, 07:41 PM
If Neolithic elites spent less time in the sun, this could have been the first time that fair skin was perceived as a symbol of status.

Neolithic elites? They scarcely existed. The early farmers were all farmers. People all worked outside. We first see signs of significant social differentiation in the Bronze [correction - Copper] Age with urban living, etc.

Fair skin was certainly seen as desirable in a number of societies within historic times, and part of that was to do with the 'I don't work in the fields' signal. Then things changed when money = holidays in the sun, and a tan was the status symbol. Now that is going into reverse a teeny bit because of the recognition that heavy sun exposure ages the skin and there's the skin cancer worry on top. Things keep changing. That's the only thing we can be certain of.

We always see the past through the prism of the present. So it may be hard to believe that communities ever existed who had different values and a different social structure from our own. But they did. :)

GailT
07-12-2014, 07:51 PM
Neolithic elites? They scarcely existed. The early farmers were all farmers. People all worked outside. We first see signs of significant social differentiation in the Bronze Age with urban living, etc.

So perhaps not a true leadership elite in the early Neolithic, but there might have been special forms of skilled labour that had more status than farming:

Quoting from this link (http://history-world.org/neolithic1.htm):

But in the Neolithic period the specialized production of stone tools, weapons, and perhaps pottery was a more important consequence of the development of agriculture than the formation of elites. Originally, each household crafted the tools and weapons it required, just as it wove its own baskets and produced its own clothing. Over time, however, families or individuals who proved particularly skilled in these tasks began to manufacture implements beyond their own needs and exchange them for grain, milk, or meat.


This level of social differentiation might have occurred as early as the first towns, around 7000 BC.

Jean M
07-12-2014, 08:03 PM
there might have been special forms of skilled labour that had more status than farming

Not in the Neolithic. The link you gave points to a page headed "World Civilizations: The Origins Of Civilizations: The Agrarian Revolution And The Birth Of Civilization". These two major changes are often discussed together, because the one did lead to the other, after about 5,000 years. Civilizations i.e. urban life began in the Copper Age. (Sorry I said Bronze before. More haste, less speed.)

We can tell for example when pottery ceased to be made in the home and became a craft, because there were specialised workshops. Indeed the potter's wheel came into use. We have wonderful Egyptian tomb paintings showing us a pottery "production line" so to speak (though not automated!) That was a long, long time after the first farmers.

DebbieK
07-12-2014, 08:17 PM
Anne Buchanan has done a nice blog post about this paper which might be of interest:

http://ecodevoevo.blogspot.co.uk/2014/07/skin-color-and-vitamin-d-beautiful.html

jeanL
07-12-2014, 08:21 PM
The Western Eurasian-type alleles at TYRP1, SLC24A5, and SLC45A2 appear to have arisen much later - all within the last 11,000-19,000 years. Given that the incoming farmers were SLC24A5 derived, that particular mutation could have arisen in the Near East 10,000 years ago or more, and been subject to selection to the point that it was fixed before arrival in Europe.


Seems like a month is all it takes for people to either forget the evidence, or choose to forget it.

http://www.anthrogenica.com/showthread.php?1088-Who-are-Europeans-descended-from&p=42294&viewfull=1#post42294

http://www.anthrogenica.com/showthread.php?1088-Who-are-Europeans-descended-from&p=42301&viewfull=1#post42301

http://www.anthrogenica.com/showthread.php?1088-Who-are-Europeans-descended-from&p=42424&viewfull=1#post42424

http://i1133.photobucket.com/albums/m582/jeanlohizun/Lazaridisetal2013-TableS81_zps2ce91029.jpg (http://s1133.photobucket.com/user/jeanlohizun/media/Lazaridisetal2013-TableS81_zps2ce91029.jpg.html)

http://i1133.photobucket.com/albums/m582/jeanlohizun/Skoglundetal2014-TableS6_zps09eb679f.jpg (http://s1133.photobucket.com/user/jeanlohizun/media/Skoglundetal2014-TableS6_zps09eb679f.jpg.html)

http://i1133.photobucket.com/albums/m582/jeanlohizun/Duffy_et_al_2010-Table-3_zps7b289e75.jpg (http://s1133.photobucket.com/user/jeanlohizun/media/Duffy_et_al_2010-Table-3_zps7b289e75.jpg.html)

Native European Hunter Gatherers like Motala12 were SLC24A5 derived, so that particular mutation was likely acquired by incoming farmers when they mixed with Hunter Gatherers. Motala12 was also derived for r28777 being A/A with a coverage of 4x, and as I showed before this SNP is strongly(OR 4.31) linked to pale skin in Europeans. So go ahead JeanM keep repeating the same thing about farmers, I will simply repeat the evidence with Motala12(rs1426654,rs28777) and StoraFörvar11(rs16891982) both carrying derived mutations.

Jean M
07-12-2014, 08:30 PM
So go ahead JeanM keep repeating the same thing about farmers, I will simply repeat the evidence with Motala12 and StoraFörvar11 both carrying derived mutations.

Sorry JeanL. To be honest I don't usually read your posts, as they tend to be too lengthy for a person with limited time to spare, like myself at the moment. I see that I don't have those two in my table on the autosomal page. I'll have a look to see why.

[Added] I can't find this data. Lazarides 2013 supplement 3 says "We assessed the Loschbour forager and Stuttgart farmer individuals for their genotype at pigmentation SNPs". Where are you getting the SNPs for Motala12 and StoraFörvar11?

[Added] Oh I see - they must have updated.

jeanL
07-12-2014, 08:45 PM
Sorry JeanL. To be honest I don't usually read your posts, as they tend to be too lengthy for a person with limited time to spare, like myself at the moment. I see that I don't have those two in my table on the autosomal page. I'll have a look to see why.

[Added] I can't find this data. Lazarides 2013 supplement 3 says "We assessed the Loschbour forager and Stuttgart farmer individuals for their genotype at pigmentation SNPs". Where are you getting the SNPs for Motala12 and StoraFörvar11?

Perhaps you missed this, but the paper came up with a second edition:

Lazaridis.et.al.2013 (http://arxiv.org/ftp/arxiv/papers/1312/1312.6639.pdf)

Page-85 reads:



The Motala12 forager, like the Stuttgart farmer, carries at least one copy of the derived rs1426654 pigmentation-lightening allele, and may thus have had lighter skin pigmentation than the Loschbour forager. We typed the three ancient modern humans at 7 SNPs forming three short haplotypes associated with eye color in present-day worldwide populations (Table S8.3)26. The observed reads in the Motala12 forager, like the Loschbour forager match the blue-eye-associated allele at all 7 SNPs. However, this includes two SNPs (rs7495174 and rs1291382) at only 1× coverage. Motala12 carries the blue-eye haplotype at the two BEH3 SNPs, which are in LD with the causal SNP, rs1291382, in present-day Europeans (but not outside of Europe)26. However, the Stuttgart farmer is also homozygous for the two blue-associated BEH3 SNPs, despite being homozygous for the ancestral allele at rs1291382. Stronger support for the inference of non-brown eyes for Motala12 is the observation of the derived allele at rs1129038, a site at almost complete LD with rs1291382 in present-day populations26.
[...]

Examining the SLC24A5 region, we find that the Stuttgart farmer is homozygous for the C11 haplotype found in 97% of all modern carriers of the derived rs1426654 pigmentation-lightening allele27. The A111T mutation is estimated to have arisen at ~22-28 kya28, with the selective sweep favoring its rise beginning ~19kya (under a dominant model) or ~11kya (under an additive model)29. The Loschbour forager does not carry the derived rs1426654 allele. The Motala12 forager, like the Stuttgart farmer, is homozygous for the C11 haplotype. Although three of the SNPs defining C11 were genotyped at 1× coverage in the Motala12 sample, C11 is the only haplotype matching the possible patterns of variation.

The first table is Table S8.1 from Lazaridis.et.al.2013 (http://arxiv.org/ftp/arxiv/papers/1312/1312.6639.pdf). The other table is from Skoglund.et.al.2014 Table-S6. (http://www.sciencemag.org/content/suppl/2014/04/23/science.1253448.DC1/Skoglund.SM.pdf)

Jean M
07-12-2014, 08:49 PM
Perhaps you missed this, but the paper came up with a second edition:

Yes I realised after I posted. I must have edited my post as you were posting. Glad this came up. There is a lot more data in their supplement 3 now.

nuadha
07-14-2014, 08:11 AM
This seems pretty interesting...If I understand it correctly, while skin getting lighter as humans migrated northward and needed extra help with UV light to get enough Vitamin D was still a factor, the new study says dark skins ability to retain moisture in high heat environments became less needed as humans migrated north to cooler area's and thus the body spent less energy making it...

"In human evolution, changes in skin's barrier set northern Europeans apart"
http://www.sciencedaily.com/releases/2014/06/140630140836.htm

Elias speculates that, “Once human populations migrated northward, away from the tropical onslaught of UVB, pigment was gradually lost in service of metabolic conservation. The body will not waste precious energy and proteins to make proteins that it no longer needs.”
For the Evolutionary Biology study, labeled a “synthesis paper” by the journal, Elias and co-author Jacob P. Thyssen, MD, a professor at the University of Copenhagen, mapped the mutation data and measured the correlations with blood levels of vitamin D. Labs throughout the world identified the mutations. Daniel Bikle, MD, PhD, a UCSF professor of medicine, provided expertise on vitamin D metabolism.


What do you guys think?

M

I dont buy his speculation that light skin evolved to conserve energy and proteins in an environment that had less demand for melanin. There seems to have been too much selective pressure in favor of light skin to just come down to the elimination of a very inexpensive process. I wonder if he is even aware of La Brana and his skin color.

Jean M
07-14-2014, 02:30 PM
Yes I realised after I posted. I must have edited my post as you were posting. Glad this came up. There is a lot more data in their supplement 3 now.

Finally had a few minutes to update my page with the pigmentation table. I have added the two samples mentioned above, plus a new column to show the three SNPs that people seem mainly interested in, though obviously there are a lot of others to consider for those who want the full picture. Plus I have made the table easier to find, with a link at the top. You can get straight there with http://www.ancestraljourneys.org/autosomaladna.shtml#pigmentation .

Baltimore1937
07-14-2014, 08:37 PM
Birds have black wingtips (it is said) to strengthen those flight feathers; storks, pelicans, Whooping Cranes, et al.