Nina Jablonski breaks the illusion of skin color

Interestingly, Charles Darwin

was born a very lightly pigmented man,

in a moderately-to-darkly pigmented world.

Over the course of his life,

Darwin had great privilege.

He lived in a fairly wealthy home.

He was raised by very supportive and interested parents.

And when he was in his 20s

he embarked upon a remarkable voyage on the ship the Beagle.

And during the course of that voyage,

he saw remarkable things:

tremendous diversity of plants and animals, and humans.

And the observations that he made

on that epic journey

were to be eventually distilled

into his wonderful book, "On the Origin of Species,"

published 150 years ago.

Now what is so interesting

and to some, the extent, what's a bit infamous

about "The Origin of Species,"

is that there is only one line in it

about human evolution.

"Light will be thrown on the origin of man

and his history."

It wasn't until much longer,

much later,

that Darwin actually spoke

and wrote about humans.

Now in his years of

traveling on the Beagle,

and from listening to the accounts

or explorers and naturalists,

he knew that skin color

was one of the most important ways

in which people varied.

And he was somewhat interested in the pattern of skin color.

He knew that darkly pigmented peoples

were found close to the equator;

lightly pigmented peoples, like himself,

were found closer to the poles.

So what did he make of all this?

Well he didn't write anything about it in The Origin of Species.

But much later, in 1871,

he did have something to say about it.

And it was quite curious. He said,

"Of all the differences between the races of men,

the color of the skin is the most conspicuous

and one of the best marked."

And he went on to say,

"These differences do not coincide

with corresponding differences in climate."

So he had traveled all around.

He had seen people of different colors

living in different places.

And yet he rejected the idea

that human skin pigmentation

was related to the climate.

If only Darwin lived today.

If only Darwin had NASA.

Now, one of the wonderful things that NASA does

is it puts up a variety of satellites

that detect all sort of interesting things about our environment.

And for many decades now

there have been a series of TOMS satellites

that have collected data about the radiation of the Earth's surface.

The TOMS 7 satellite data, shown here,

show the annual average

ultraviolet radiation at the Earth's surface.

Now the really hot pink and red areas

are those parts of the world that receive the highest amounts

of UV during the year.

The incrementally cooler colors --

blues, greens, yellows, and finally grays --

indicate areas of much lower ultraviolet radiation.

What's significant to the story of human skin pigmentation

is just how much of the Northern Hemisphere

is in these cool gray zones.

This has tremendous implications for our understanding

of the evolution of human skin pigmentation.

And what Darwin could not appreciate,

or didn't perhaps want to appreciate at the time,

is that there was a fundamental relationship

between the intensity of ultraviolet radiation

and skin pigmentation.

And that skin pigmentation itself

was a product of evolution.

And so when we look at a map of skin color,

and predicted skin color, as we know it today,

what we see is a beautiful gradient

from the darkest skin pigmentations toward the equator,

and the lightest ones toward the poles.

What's very, very important here

is that the earliest humans

evolved in high-UV environments,

in equatorial Africa.

The earliest members of our lineage,

the genus Homo, were darkly pigmented.

And we all share this incredible heritage

of having originally been

darkly pigmented,

two million to one and half million years ago.

Now what happened in our history?

Let's first look at the relationship

of ultraviolet radiation to the Earth's surface.

In those early days of our evolution,

looking at the equator,

we were bombarded by high levels of ultraviolet radiation.

The UVC, the most energetic type,

was occluded by the Earth's atmosphere.

But UVB and UVA

especially, came in unimpeded.

UVB turns out to be incredibly important.

It's very destructive,

but it also catalyzes the production of vitamin D in the skin,

vitamin D being a molecule that we very much need

for our strong bones, the health of our immune system,

and myriad other important functions in our bodies.

So, living at the equator, we got

lots and lots of ultraviolet radiation

and the melanin --

this wonderful, complex, ancient polymer

compound in our skin --

served as a superb natural sunscreen.

This polymer is amazing

because it's present in so many different organisms.

Melanin, in various forms, has probably been on the Earth

a billion years,

and has been recruited over and over again

by evolution, as often happens.

Why change it if it works?

So melanin was recruited, in our lineage,

and specifically in our earliest ancestors

evolving in Africa,

to be a natural sunscreen.

Where it protected the body

against the degradations of ultraviolet radiation,

the destruction, or damage to DNA,

and the breakdown of a very important molecule called folate,

which helps to fuel cell production,

and reproduction in the body.

So, it's wonderful. We evolved this very protective,

wonderful covering of melanin.

But then we moved.

And humans dispersed -- not once, but twice.

Major moves, outside of our equatorial homeland,

from Africa into other parts of the Old World,

and most recently, into the New World.

When humans dispersed into these latitudes,

what did they face?

Conditions were significantly colder,

but they were also less intense

with respect to the ultraviolet regime.

So if we're somewhere in the Northern Hemisphere,

look at what's happening to the ultraviolet radiation.

We're still getting a dose of UVA.

But all of the UVB,

or nearly all of it,

is dissipated through the thickness of the atmosphere.

In the winter, when you are skiing in the Alps,

you may experience ultraviolet radiation.

But it's all UVA,

and, significantly, that UVA

has no ability to make vitamin D in your skin.

So people inhabiting northern hemispheric environments

were bereft of the potential

to make vitamin D in their skin for most of the year.

This had tremendous consequences

for the evolution of human skin pigmentation.

Because what happened, in order to ensure health and well-being,

these lineages of people

dispersing into the Northern Hemisphere

lost their pigmentation.

There was natural selection

for the evolution of lightly pigmented skin.

Here we begin to see the evolution

of the beautiful sepia rainbow

that now characterizes all of humanity.

Lightly pigmented skin evolved not just once,

not just twice, but probably three times.

Not just in modern humans,

but in one of our distant unrelated ancestors,

the Neanderthals.

A remarkable, remarkable testament

to the power of evolution.

Humans have been on the move for a long time.

And just in the last 5,000 years,

in increasing rates, over increasing distances.

Here are just some of the biggest movements of people,

voluntary movements, in the last 5,000 years.

Look at some of the major latitudinal transgressions:

people from high UV areas

going to low UV and vice versa.

And not all these moves were voluntary.

Between 1520 and 1867,

12 million, 500 people

were moved from high UV

to low UV areas

in the transatlantic slave trade.

Now this had all sorts of invidious social consequences.

But it also had deleterious

health consequences to people.

So what? We've been on the move.

We're so clever we can overcome all of these

seeming biological impediments.

Well, often we're unaware

of the fact that we're living

in environments in which our skin

is inherently poorly adapted.

Some of us with lightly pigmented skin

live in high-UV areas.

Some of us with darkly pigmented skin

live in low-UV areas.

These have tremendous consequences for our health.

We have to, if we're lightly pigmented,

be careful about the problems of skin cancer,

and destruction of folate in our bodies,

by lots of sun.

Epidemiologists and doctors

have been very good about telling us

about protecting our skin.

What they haven't been so good about instructing people

is the problem of darkly pigmented people

living in high latitude areas,

or working inside all the time.

Because the problem there is just as severe,

but it is more sinister,

because vitamin D deficiency,

from a lack of ultraviolet B radiation,

is a major problem.

Vitamin D deficiency creeps up on people,

and causes all sorts of health problems to their bones,

to their gradual decay of their immune systems,

or loss of immune function,

and probably some problems

with their mood and health,

their mental health.

So we have, in skin pigmentation,

one of these wonderful products of evolution

that still has consequences for us today.

And the social consequences,

as we know, are incredibly profound.

We live in a world where we

have lightly and darkly pigmented people

living next to one another,

but often brought into proximity initially

as a result of very invidious social interactions.

So how can we overcome this?

How can we begin to understand it?

Evolution helps us.

200 years after Darwin's birthday,

we have the first moderately pigmented President of the United States.

(Applause)

How wonderful is that?

(Applause)

This man is significant for a whole host of reasons.

But we need to think about how he compares,

in terms of his pigmentation, to other people on Earth.

He, as one of many urban admixed populations,

is very emblematic

of a mixed parentage, of a mixed pigmentation.

And he resembles, very closely,

people with moderate levels of pigmentation

who live in southern Africa, or Southeast Asia.

These people have a tremendous potential

to tan, to develop more pigment in their skin,

as a result of exposure to sun.

They also run the risk of vitamin D deficiency,

if they have desk jobs, like that guy.

So lets all wish for his great health,

and his awareness of his own skin pigmentation.

Now what is wonderful

about the evolution of human skin pigmentation,

and the phenomenon of pigmentation,

is that it is the demonstration,

the evidence, of evolution

by natural selection,

right on your body.

When people ask you, "What is the evidence for evolution?"

You don't have to think about some exotic examples, or fossils.

You just have to look at your skin.

Darwin, I think, would have appreciated this,

even though he eschewed the importance

of climate on the evolution of pigmentation during his own life.

I think, were he able to look

at the evidence we have today, he would understand it.

He would appreciate it.

And most of all, he would teach it.

You, you can teach it.

You can touch it.

You can understand it.

Take it out of this room.

Take your skin color,

and celebrate it.

Spread the word.

You have the evolution

of the history of our species,

part of it, written in your skin.

Understand it. Appreciate it. Celebrate it.

Go out. Isn't it beautiful? Isn't it wonderful?

You are the products of evolution.

Thank you.

(Applause)