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  • 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)