Why blue eyes?

Factual Questions
61

It’s hard to imagine a population that can’t use milk directly but discovers hard cheese. Maybe they fed milk to their kids? But why preserve something that’s not already valuable?

As for the bottleneck theory – if the people who survived the bubonic plague tended to have blue eyes (perhaps entirely randomly) the gene could have become much more common. That’s the only significant population bottleneck I can think of over the relevant time frame. But people do look at faces, and blue eyes can be visually striking, and it seems entirely plausible to me that blue eyes were favored, and thus blue-eyed people (of both sexes) were able to “breed up” and obtain better resources for their kids.

62

Blue eyes were common way before the plague, and survived several immigration waves of people with dark eyes.

Blue is recessive, so if 50% of the population had the mutation only 25% would have the trait. So you would have to know about genetics to break free of the normal selection of advantageous recessive traits.

If you have a recessive, advantageous trait it can take ~400 generations for it to reach a high level of expression. 400 generations * 20 years per generation would be about 8000 years with simple wag math.

If you have multiple neutral alleles or with no effects on fitness, genetic drift can result in fixation of one of the alleles in a population and the loss of other alleles within just 10-50 generations depending on chance. As the Beaker culture arrived in Britain 4,500 and makes up ~90% of the DNA today but would have been slightly darker skinned (but lighter than the existing population), dark eyed peoples the timing fits with genetic drift more than blue eyes being selected for.

Natural selection does change allele frequencies, but other factors besides selection can make faster changes and even counter advantageous alleles. Note that introduction of only a few migrants per generation tends to prevent the fixation of harmful alleles, but has less impact on neutral alleles.

Selection, drift, and gene flow have complex interactions and rarely work in isolation which makes this difficult without a large amount of data. But the math makes it far more likely it was genetic drift.

Note that in the case of dominant traits like lactose tolerance, selection happens easily within 50 generations, which is a better argument for that being selected for.

63

To the best of my knowledge, the current state of knowledge is that blue eyes were present and possibly fixed in the population known as Western Hunter-Gatherers. A population that spread across western Europe after the end of the most recent ice age. They very probably had black skin, Africans style rather than dark Mediterranean color.

White skin and blonde hair dates back to 14 000 years ago or more in Siberia, and were present in the population known as Eastern Hunter-Gatherers. They had genes for brown eyes though.

The two populations met and mixed in Scandinavia around the time of Cheddar Man, just under 10 000 years ago. The Western Hunter-Gatherers migrated in from the continent through Denmark, whereas the Eastern Hunter-Gatherers came in from Siberia and down the coast of Norway. The mix is where we find genes for Blue eyes together with light skin, which became practically fixed. The population is known as Scandinavian Hunter-Gatherers Although there has been turnover in the gene pool since, the combination has remained fixed.

There was another lot of Hunter-Gatherers in the Caucasus Mountains that mixed the WHG, EHG and other populations in different proportions.

I wouldn’t treat mathematical models as accurate predictors for most recent ancestors, they make a lot of assumptions that make them unusable as real-world predictors.

64

As Siberia was only populated about 10,000 years ago can you provide a cite? Are you thinking of the ~7700 year old dig in Motala? Because just this year they figured out SLC24A5 and SLC45A2 was from Africa.

Both Motala-12 and Loschbour are closely related and Motala-12 only had SLC24A5 and not SLC45A2. Note that Loschbour is WHG and Motala is in the SHG "Scandinavian Hunter-Gatherer” cluster.

As 5% of Western Africans have SLC24A5 I would say that is much older, but also not uniquely Eurasian like HERC2 for eyes. Note Loschbour did not have any of the three light skin mutations but did have HERC2.

The 8,000 BP Motala SHG are fairly distinct from the 7,000 BP EHG from Karelia and NE Europe. Although things get complex.

65

To clarify, multi-colored eyes probably emerged after the westward expansion in Europe but way before agriculture. Light skin seems to be much more recent as being a large portion of the population but the mutations probably came from the population that left Africa.

66

Does anyone know of cites that link skin and eye color? Lil bro and I have red hair and fair skin because the gene mutation we inherited from mom causes both, but as far as anyone can tell it doesn’t affect eye color. Makes sense: he has blue eyes like mom, I have blue-green hazel like our other grandfather did, and we’ve both have met natural redheads with brown eyes too.

67

Yes, they fed milk to the kids, specially as a supplement to their mother’s own milk. And while some people’s reactions to the many things which happen as milk gets old would be “throw it off”, others would say “huh, wonder what that tastes like now?” and “wonder what happens if I leave it here for a longer time?”

68

Eh, what, now?

69

:dubious:
Errr, what? We have artistic description of a near contemperory, Princess Nofret is show with light skin and green eyes.

70

As Mr. Dibble points out, we have a number of samples from Siberia, some quite old. It was something of a happening place during the Ice Age, and the climate is good for DNA preservation. You may be assuming that it was all glaciated, but that wasn’t so. It appears to have been populated by anatomically modern humans, Neanderthals and Denisovans through the last Ice Age and into the previous interglacial (very probably no anatomically modern humans at that point though)

Yes, several gene variants that cause light skin are quite old and predate the latest out of Africa episode. But they were present in very low amounts at that stage. Skin pigmentation is determined by a number of genes and at that point it was probably some individuals with a single skin-lightening gene among many darkening ones.

We first see these genes cluster in the EHG individuals. Its not like we have a lot of samples though, so we could easily find older individuals with a concentration, but 14 - 15 000 years ago in Siberia is where we are at the moment.

I believe that SLC24A5 was more geographically widespread than SLC45A2.

Cite

71

Actually, section 8 of this paper covers a lot of it well. Quite interesting for the entire topic of the thread and reads pretty easily.

72

All I know was that my favorite girlfriends were:

Paula - blond with brown eyes
Diane - redhead with green eyes
Karen - brunette with blue eyes

:smiley:

73

Selkup people of Siberia live in the taiga forest zone and are as light-complexioned as Europeans, along with Asiatic features, brown eyes, and brown hair.

DNA from Rakhigarhi, the biggest known city of the Indus Valley Civilization, was found to have none of the input from Indo-European steppe origins. This shows that the IVC existed before the incursion of Aryans originating from the Pontic-Caspian steppe. That was the place of Proto-Indo-European origins as well as blue or green eyes. It wasn’t Alexander’s army. Blue or green eyes had been extended by Aryan migrations across the steppes into South Asia since prehistoric times.

74

Egypt isn’t Europe, and green eyes aren’t blue. And that’s leaving aside that Egyptian art wasn’t necessarily hyper-realistic when it came to skintones.

75

Just read this article and it brought to mind your post.

Ancient, unknown strain of plague found in 5,000-year-old tomb in Sweden

76

It’s the same gene IIRC. Also, this is early Old Kingdom, when Egyptian Art was more realistic, unlike later. In this particular piece, the artist has shown the fact that the Princess is wearing a wig, by showing her real hair over her forehead and the Prince has a frown. Probably more true to life then most.
The men “tan” and women “ pale” convention is not just Egyptian, it’s all over the Med. The toke must have existed, you finds few representations of people who are violet skinned.

77

With evidence light skinned people? Remember there was an ice age that pushed people south after Ust’-Ishim, the Last Glacial Maximum.

The low depth coverage of MA-1 does show there was East to West gene flow in the Paleolithic, but Afontova Gora and the much earlier Ishim man mostly show that there isn’t much difference between the peoples, and none of the individuals you listed above were light skinned. Afontova Gora 3 is closer to people in the Americas than Europe as an example but yes is related to MA-1.

~8000 years is when those light skin features start to show up in Europe.

78

You mean the same set of genes.

Not in skin tone, though. That’s only really a thing for the Amarna Period. Both before and after, stylized colourings were very much the thing.

I know. I linked to a book that discusses this for both Egypt and Greece, after all.

What I’m saying is, you can’t make inferences about someone’s actual colouring from Egyptian art.

And much more importantly, you can’t make any inferences for European genetics based on contemporary Egyptians. Especially for the Egyptian period before there was significant European contact. Egypt, we know, had quite strong Asiatic ties/trade links. There’s not a lot of evidence for that in Europeans.

ETA: By Europeans, I mean Western/Central Europeans, though.

79

That’s not a qualification you made.

80

There is a lot of information here, but we are talking about lighter skin, which was after the LGM.

Feel free to replace:

As Siberia was only populated

With

As Siberia was only **re-**populated

But note that while I also tried to avoid low coverage claims, Motala 1 suggests there was no bottle neck during the LGM. The oldest known pioneer settlements of central and northern Scandinavia 11,500 BP, and as I mentioned before the eye color probably originated way before then but we don’t know. The diversity of the limited coverage we have on some individuals from Motala, where we do have light skin demonstrated simply counters the bottleneck claim if one wants to choose to accept small percentages of small percentages of DNA.

Ust’-Ishim has amazing high coverage, an despite him not being considered a European ancestor I have 800.5 cM > 1 cM with the largest of 4.9 cM. Which sounds exciting until you realize that the DNA they have for him is very comprehensive compared to even newer samples and my autosomal test probably chose some markers to look at based on his DNA.

The reality is that humans are extremely closely related, so outside of tracking novel combinations from inbreeding populations we can’t make

Novel SNPs are challenging due to low coverage to make absolute claims, and part of the reason these connections seem to change options is that, due to the lack of evidence researchers have to make assumptions or may be subject to biases.

You will notice if you search for “coverage” in this above cited paper and look at the text you will see that they are using ~5million base pairs with differing coverage and this is about 17% of the total genome.

Unfortunately the speculative nature of this isn’t captured in pop-science stories. Trying to describe functions of selection and drift it would require an entire set of volumes to cover all bits of data and so I do have to simplify, especially as today speculative claims are the best we have and are considered more concrete than they would be under normal scientific rigor.

What we don’t have is evidence of Paleolithic samples with light skin in Europe yet, and we think it wasn’t common or a majority until probably the iron age. The challenges and grow when someone wants to claim it was a selected for trait because the archeological evidence doesn’t really support that with any firm stance so one has to layer on top of challenging DNA data.

The reality is that selection, drift and migrations don’t happen in a vacuum in the actual world, so it is also almost certainly a mix of all three. Science is trying to find patterns in fuzzy lines with limited data and this always results in changing and conflicting theories.

The story that is arising is far more movements and that lactase persistence did arrive well after the rise of farming, and that cheese was used and would have provided Vitamin D for adults even if there was a slight advantage for adults being able to directly drink milk.

While not perfect blubber and eggs (char) would have been more useful in Sweden in post-glacial times for Vitamin D. Remember Stockholm is at almost 60 degrees and it would have been cold and they would have been wearing clothes. Without evidence of light skinned individuals to the south at the same time period the theory has issues because the populations most in need of it, more southern farmers, didn’t show a large portion of the population having the trait until later.

Obviously I am putting more faith in the math models but that is due to the limited archeological and genetic evidence. For light skin this tends to indicate that genetic drift is a better fit for the primary driver for fixation.