The Origin and Purpose of Blue Eyes

Why do some people have blue eyes? What is the evolutionary advantage of having blue eyes? Where did blue eyes evolve? Blue eyes have piqued curiosity for centuries, standing out as a mesmerising and rare trait among humans. Discover the underlying science behind blue eyes as we explore various theories and genetic factors that contribute to their existence. They were mainly associated with Mesolithic Western Hunter Gatherers like Cheddar man but the origin and story of this phenotype is complicated. A new study from Liverpool may have the answer as to what benefit this trait has - better ability to see in low light.This channel depends on your support:
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Sources:
https://www.biorxiv.org/content/10.1101/2024.01.17.576074v1
https://www.sciencedaily.com/releases/2008/01/080130170343.htm

Episode Transcript

(00:00):
What is the point of blue eyes? Why do some people have blue eyes?
Why did this trait evolve? Anew preprint to a study in England
may have the answers find out Todayon survivor Jive, blue eyes are frequently

(00:24):
a topic of discussion. Historically,people have found them quite attractive. I
think that's generally true, but thesedays they've also been the subject of a
popular meme which implies that blue eyesare sort of creepy or intimidating in some
way because they have a kind ofpiercing effect, as though they look directly

(00:44):
into your soul. Today, blueeyes are found all over Europe and also
in places where European people have migrated, such as America and Australia, but
they're most common in northeastern Europe aroundthe Baltic Sea, in country like Finland,
Sweden, Lithuania, Estonia. Insome regions of that part of the

(01:06):
world, up to eighty percent ofpeople have blue eyes, but in the
United States only about thirty three percentof the white population have blue eyes,
and this percentage has been decreasing.Globally speaking, blue eyes are very rare,
and that's not surprising when you considerthat that it's a recessive trait generally
limited to Europeans. Why do blueeyes exist, Well, let's start with

(01:30):
where blue eyes come from. Backin two thousand and eight, there was
a study from Denmark and the scientistsdetermined that they thought it was likely that
all blue eyed people had a commonancestor either one person or a single population,
and they speculated that that person orpopulation would have lived in the Neolithic

(01:52):
around the Black Sea. People oftenrepeat this referring to that study, but
in fact, since two thousand andeight, genetic science has come a long
way, and we've got lots andlots and lots of ancient samples, and
the way that they sample ancient skeletonshas become much more efficient and accurate,
and now we know that there werethere are loads and loads of much older

(02:13):
blue eyed samples, skeletons which havethe DNA associated with blue eyes in Europe.
Back in the Stone Age, infact the Stone Age, the Middle
stone Age, which is called theMesolithic, the western part of Europe had
a population that geneticists refer to asWestern hunter gatherers, and pretty much every
single skeleton they found from that periodin that region has blue eyes. So

(02:35):
it was like the entire population fromSpain to Sweden and parts of Greece,
all of Western Europe, that Britain, Ireland, everyone had blue eyes.
And that was way before the Neolithic. So it's very likely that blue eyes
existed long before these Western hunter gatherersdid, but they don't seem to have

(02:58):
been a very common trait in theIce Age before the Mesolithic. I've seen
some people on the internet speculate thatblue eyes were an adaptation for a frozen,
frosty, snowy environment, and theyclaim that blue eyes helped to reduce
glare from the light off of thesnow or ice, and that this will
be advantageous for hunters in the IceAge. But this doesn't really make any

(03:21):
sense because what we see in thegenetic archaeological record is that blue eyes became
common after the last glacial maximum,after all the snow and the glaciers and
everything and the Ice Age started melting. And if you look at modern populations
that live in the Arctic Circle,you know Inuit and Siberian people in northern

(03:42):
Russia, and they all have browneyes. So it doesn't seem like a
necessary prerequisite for efficient hunting in icyconditions that you have blue eyes. In
fact, you know, Greek peopleoften have blue eyes, and Greece is
not a very frozen arctic environment atall, so I don't think that theory
makes any sense at all. However, a new study from Liverpool John Moore's

(04:06):
University may have some answers to thesequestions about the origin and purpose of blue
eyes. The paper's not out yet, but the preprint puts forward a plausible
reason for why blue eyes were selectedfor in Europe. They tested forty individuals
with a simple eye test in increasingluminance to examine if there was a difference

(04:27):
in capacity to see in low lightconditions between blue eyed and brown eyed individuals
after a brief adaptation period adapting tothe light that is, you know,
after this changed, blue eyed individualswere identified to have significantly better ability to
see in lower lighting after a shortadaptation period than brown eyed individuals, making

(04:48):
it likely depigmented irises provide an adaptiveadvantage. Superior ability to see in low
light conditions could be the result ofincreased stray light in pigmented irises, which
in light luminance is disadvantageous, butin low light conditions may provide an advantage.

(05:08):
So the conclusion from that quite smallstudy, or the preprint of it,
is that brown eyes could be betterfor seeing in brighter conditions, whereas
blue eyes are better in low lightconditions. In other words, blue eyed
people can see better in the dark. Is that creepy? Does that creep

(05:35):
you out? Are you afraid ofblue eyed people their nocturnal vision? So
Ice Age Europeans hunted larger game thanMesolithic Europeans did. There were more megafauna
in the Ice Age, and theyhunted them across snowy conditions where we can
imagine that despite the cold weather,the ice and snow created a lot of

(05:57):
glare, and so perhaps that's whyblue eyes were less needed in the Ice
Age. The warmer conditions of theMesolithic may have resulted in an environment with
less light A little bit speculative,but even though it was warmer, they
could have been less light for huntingconditions. I noticed that I've lived in
England and Sweden. Sweden has muchcolder winters than Britain in terms of temperature,

(06:25):
but because of the snow, itfeels brighter. That's my experience.
Another possible explanation is to do withthe fact that during dusk and dawn,
in that twilight period of low lighting, when the sun's coming up or going
down, there's a lot of animalactivity. Birds become active, either they
sing them morning songs or they're lookingto roost. Animals are doing similar things.

(06:47):
They're all extremely busy in that period, and that makes it quite a
good time to go hunting. AndI imagine that Mesolithic hunting gatherer has made
use of the twilight periods for hunting, and also some animals have reduced ability
to see in those conditions, sothat would be an easier time to ambush
them. So it might be thatthe focus on hunting during these twilight periods

(07:14):
resulted in very very strong selection pressuresat the end of the Ice Age in
European populations of hunting gatherers, tothe extent that within a few thousand years
on the entire population of Western Europewas blue eyed. Would have to be
extremely strong selection pressure, And thatexplanation doesn't quite seem to encapsulate it,

(07:35):
because surely not everyone had the exactsame strategy of hunting in the twilight conditions.
But I can't really think of anyother reason why they would have this
huge selective sweep to get rid ofbrown eyes, when brown eyes have been
generally the dominant and selected for phenotypefor all human populations globally in any kind

(07:58):
of condition, whether frozen or tropical. So it's quite a strange thing really
that the whole of western Europe becameblue eyed after the Ice Age. But
yeah, I think it might havesomething to do hunting in low light,
or maybe hunting in forests, becauseone of the things that happened after the
ice melted is that the tree linewent much further north, So that was

(08:20):
a period where much of the broadlyforests of Europe started to appear. Previously
it had been a lot more ofpine forests, and maybe these hunting in
these dark, big forests was alsoa sort of environment that meant it was
adaptive to have better vision in lowlight when you're trying to hunt small game

(08:41):
beneath the foliage. But besides thishuge sweep in the Mesolithic, there's another
thing we have to explain, whichis that after the Mesolithic, Europe experienced
two massive population shifts, the resultof the migrations of two new races into
Europe, both of which were browneyed. The first came in the Neolithic
and brought farming to Europe, andthey came from the Near East, from

(09:03):
Anatolia. And the second came atthe end of the sort of the Enelithic,
at the junction of the Bronze Age, and they dominated Europe in the
Metal Age, and they came fromeastern Europe, sort of from around Ukraine.
And they were also predominantly brown eyed. So you would expect to see
the result being that the population,the mesolated population that was largely replaced,

(09:28):
mean that Europe became brown eyed.But that's not what happened. Instead,
both of these populations, at separateperiods over a thousand years apart from each
other, when they entered Europe mixedwith the previous inhabitants to some extent,
but selected for blue eyes, sothat both of these incoming populations became blue
eyed. Now that is a quitedifficult thing to explain because it's the recessive

(09:52):
trade. But it might not havethe same explanation. These people weren't hunting
gatherers anymore. They did hunt itwill. Of course, hunt has been
part of European diet as late asmedieval times. It's made a substantial part
of a diet. But this stagehave got you know, farming, different
types of agriculture and pastoralism, sothey don't depend on very strongly for the

(10:13):
selection pressures, environmental selection pressures forbeing able to hunt in low light.
There's just not that kind of selectionpressure there. And yet these two populations
both became more blue eyed over timeduring the Bronze Age and the Neolithic.
Now that's quite strange, isn't it. This is in sharp contrast to what
we see in modern history with Europeanpopulations with certain amounts of blue eyes,

(10:35):
such as people from Britain and Scandinaviamoving into non European parts of the world
such as North America and South Africa. Now, the white populations of North
America and South Africa have been gettingprogressively less blue eyed and more brown eyed
over time, and that's not entirelydue to the mixing with other non non

(11:00):
non blue eyed races, because it'sthe same with it's a case with the
white populations I'm talking about. Sowhy is it that in the modern period
blue eyed populations leaving Europe tend tobecome less blue eyed, whereas in the
prehistoric period, brown eye populations enteringEurope became more blue eyed. It certainly
isn't the case that you would expectbrown eye populations moving into Europe now to

(11:26):
become blue eyed. There isn't anyselection pressure to encourage that at all,
and it is, as I saidbefore, a recessive trait. My explanation,
which is purely speculative, please understand, is that if the environmental pressures
that were at play in the Mesolithiccausing an increase in blue eyes in European

(11:46):
people weren't in place anymore during theNeolithic and Bronze Age, then an alternative
explanation could be that, rather thannatural selection, it was a sexual selection
at work, so that the thephenotype of blue eyes was associated with status
or was considered more sexually attractive bythese Neolithic and Bronze A populations to the

(12:09):
extent that the phenotype was not onlypreserved but increased in frequency among the incoming
populations and reaching levels of up toeighty percent in parts of Europe today.
That would be one explanation. Ithink that's more likely than that the pre
existing selective pressures for being able tohunt and see in low light was still

(12:31):
in place so late on. Butyou know, some people maybe did some
farming in the dark. I don'tknow. Maybe it was still selective to
being able to farm in the dark, But I mean for it to have
that big an effect, everyone wouldneed to be farming in the dark,
So I don't know. That doesn'tsound as plausible to me. It seems
more likely to do with sexual selection, and there are plenty of people today

(12:52):
who find blue eyes very attractive.Please be aware this is just an educated
guess. And also the new preprintfor the paper from Liverpool that I've mentioned,
it only had a study size offorty people, which isn't very big,
and this is just a preprint soit hasn't gone through peer review yet.
But I think it's very interesting andit's the most plausible explanation so far

(13:16):
for the emergence of this trade inEuropeans. I think we'll have to wait
to get a conclusive answer on whyblue eyes rose in the Mesolithic and why
it then rose among these incoming populationsin the Neolithic and Bronze Age, but
this is certainly a step towards answeringthese big questions people have about blue eyes.
What do you think of blue eyes? Do you find them creepy.
Do you find them intimidating or doyou think that they're sexy? Let me

(13:39):
know in the comments. And bythe way, this channel can only continue
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(14:01):
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