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The Evolution of Skin Color, College

The Evolution of Human Skin Color

This video explains why human skin comes in many shades. It shows how the sun’s UV light, melanin and vitamin D led to different skin colors over many generations. It also covers how people moving around the world changed where skin colors are found.

Video of sunbeams shining through the green leaves of a tree
Apt and aptitude come from Latin aptus, “fastened, fitted.”

Your word of the day is

ad-ap-TAY-shun

ad-to, towardfrom Latin ad-
apt-to fitfrom Latin aptāre
-ationmakes the word a nounfrom Latin -ātiō

A change, produced by natural selection, that makes a living thing better suited to survive and reproduce in its environment.

Satellite map of the whole Earth at night, with city lights showing where people live

Jablonski and Chaplin mapped the world into zones by how well lightly pigmented skin can make vitamin D from casual sun exposure. In the equatorial zone, casual sun gives enough UVB all year.

Before you read the paper, take a guess. What percent of the world’s people live in the equatorial zone? Give your reason in one sentence.

Hint

The equatorial zone covers about 41% of the world’s land.

Hands turning the pages of a textbook full of scientific graphs

Read the paper

Jablonski and Chaplin, “Human skin pigmentation, migration and disease susceptibility,” Philosophical Transactions of the Royal Society B, 2012. The paper is free to read.

Jablonski and Chaplin sorted the world into vitamin D zones for lightly pigmented skin. In the equatorial zone, casual sun exposure gives enough UVB to make vitamin D all year. In the subtropical and warm temperate zone, at least one month has too little UVB, and people rely on stored vitamin D. In the cold temperate and polar zone, UVB averaged over the year falls short, and people need vitamin D-rich foods such as oily fish. Table 1 of the paper counts the people living in each zone, using population estimates from 2000.

ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
A crowd of people crossing a busy city intersection

Table 1 of Jablonski and Chaplin’s 2012 paper counts the people in five vitamin D zones.

The five zones in Jablonski and Chaplin’s Table 1 add up to a world total of ____.

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
The winter sun rising low behind snow-covered buildings

Every zone outside the equatorial zone has at least one month with too little UVB for lightly pigmented skin.

In Jablonski and Chaplin’s Table 1, ____ of the world’s people live in zones with at least one month without enough UVB for lightly pigmented skin.

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
Earth seen from space above the Arctic, with Europe and Asia in view

In Jablonski and Chaplin’s Table 1, which hemisphere holds most of the people living in zones with at least one month without enough UVB?

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
Drawing of the layers of human skin, with a close-up of the cells in the top layer, including melanocytes, the cells that make melanin

Table 1 uses zones drawn for lightly pigmented skin. The authors write that for darkly pigmented skin, the vitamin D safe zones are smaller and shifted toward the equator.

How would the numbers in Jablonski and Chaplin’s Table 1 change for darkly pigmented skin?

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
A NASA TOMS satellite map of ultraviolet light reaching the ground in late August

Which data would you need to make a version of Jablonski and Chaplin’s Table 1 for darkly pigmented skin?

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
Drone video over a green Scottish coastline, with farm fields running down to the sea

People lived year-round in the cold temperate and polar zone long before vitamin supplements. The authors write that early people in Scotland lived mainly on the coast, gathering and hunting vitamin D-rich foods.

How did skin color and diet work together to let people live in the zones with the least UVB?

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
Cod liver oil capsules and a small bottle on a pink background

Many people now living in the Northern Hemisphere’s low-UVB zones have moderately or darkly pigmented skin.

Which vitamin D advice for a public health department in a Northern Hemisphere low-UVB zone fits Jablonski and Chaplin’s paper?

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
A woman in a sun hat holding a pink umbrella on a sunny beach

Which sun-safety advice should a public health department pair with vitamin D advice?

See the table
ZonePeople (millions)Percent of world populationPercent of land area
Southern Hemisphere cold temperate and polar0.363.6
Southern Hemisphere subtropical and warm temperate9516.5
Equatorial3,19840.7
Northern Hemisphere subtropical and warm temperate2,39215.2
Northern Hemisphere cold temperate and polar38624.0
College students with a range of skin tones laughing as they walk together on campus

Table 1 counts the world’s people by vitamin D zone for lightly pigmented skin.

Which summary fits Table 1 of Jablonski and Chaplin’s 2012 paper?

A man working on a laptop in a bright office with large windows

Which question can’t Table 1 of Jablonski and Chaplin’s 2012 paper answer?

A group of friends with a range of skin tones lying on the grass and laughing under a sunny sky

Lesson complete

Today’s word

adaptation

ad-ap-TAY-shun

A heritable change in an organism, resulting from natural selection, that makes the organism better suited to survive and reproduce in a particular environment.

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Sources
  1. Nina Jablonski and George Chaplin, Department of Anthropology, Pennsylvania State University, PNAS, 2010
  2. Nina Jablonski and George Chaplin, Philosophical Transactions of the Royal Society B, 2012
  3. Nina Jablonski and George Chaplin, Philosophical Transactions of the Royal Society B, 2017
  4. Nina Jablonski and George Chaplin, Proceedings of the Royal Society B, 2014
  5. Nicholas Crawford, Sarah Tishkoff and colleagues, University of Pennsylvania, Science, 2017
  6. Ann Webb, Lesley Rhodes and colleagues, University of Manchester, Nutrients, 2018
  7. TEMIS, a satellite UV data service hosted by KNMI in the Netherlands, TEMIS UV station data
  8. The UV numbers average the daily cloud-free UV index at local solar noon for December, 2015 to 2024, from the TEMIS station files for Kuala Lumpur and Tromsø. TEMIS defines the clear-sky UV index for cloud-free conditions at local solar noon.
  9. Vision and Change core concepts and competencies, as summarized in Brownell and others, CBE Life Sciences Education, 2014
  10. Word origins from Dictionary.com: adaptation, adapt, ad-, apt, -ation, aptitude, adapter, adjacent, adrenal, ultraviolet, ultra-, photolysis, folate, cline
  11. Folate’s leaf root from Wiktionary, folic acid. Latin aptāre, Spanish and French words from Wiktionary, adaptō, adaptation, adaptación, apto