There are some mysteries about human evolution that can’t be answered by looking at fossils or our closest living primate relatives. In this episode, you’ll learn how scientists used genetic research to explain how humans came to be the only mammals who continue to drink milk - even after we’re grown.
There are some mysteries about human evolution that can’t be answered by looking at fossils or our closest living primate relatives. In this episode, you’ll learn how scientists used genetic research to explain how humans came to be the only mammals who continue to drink milk - even after we’re grown.
Further reading
About The Leakey Foundation
The Leakey Foundation is a nonprofit organization dedicated to funding human origins research and sharing discoveries. The Foundation was established in 1968 to fund work at the forefront of fossil and primate studies and provide opportunities for a global community of scientists. Learn more at leakeyfoundation.org.
Discovering Us: 50 Great Discoveries in Human Origins
In 50 lively and up-to-the-minute essays illustrated with full-color photographs, Discovering Us: 50 Great Discoveries in Human Origins presents stories of the most exciting and groundbreaking surprises revealed by human origins research.
Prepared in consultation with leading experts and written by Evan Hadingham, senior science editor for NOVA, Discovering Us features stunning photographs, some taken at the actual moment that groundbreaking discoveries were made. The book presents a highly accessible account of the latest scientific insights into the ultimate question of humanity’s origins. Discovering Us was published by Signature Books.
Find Discovering Us at your local library, bookstore, or amazon.com.
Show Credits:
Discovering Us was made possible by generous support from Camilla and George Smith, the Ann and Gordon Getty Foundation, and the Joan and Arnold Travis Education Fund.
Meredith Johnson:
This is Discovering Us from The Leakey Foundation and Signature Books … an audio companion to the book Discovering Us: 50 Great Discoveries in Human Origins. Written by Evan Hadingham and read for you by Ashley Judd.
I’m your host, Meredith Johnson.
There are some mysteries about human evolution that can’t be answered by looking at fossils or our closest living primate relatives.
In this installment of Discovering Us you’ll learn how scientists used genetic research to explain how humans came to be the only mammals who continue to drink milk - even after we’re grown.
Here’s Ashley Judd.
Ashley Judd:
For months on end, biologists Alessia Ranciaro and Sarah Tishkoff traveled hundreds of miles to remote African villages to carry out perhaps the most ambitious genetics study ever performed in Africa.
Their goal was to tackle one of human evolution’s most intriguing puzzles: why some people, but not others, are able to digest milk.
More than a mere curiosity of dietary history, the solution turns out to be one of the most dramatic examples of how cultural and genetic changes are sometimes intertwined and evolve together.
After obtaining ethical consent for their study, Ranciaro and Tishkoff would contact local authorities and tribal chiefs on arriving in a village and convene a public meeting to recruit volunteers for their study.
Participants had to fast overnight, then give a series of blood samples the next morning after swallowing a strange orange-flavored drink.
Ranciaro said, “This was a very challenging test to do in the field in remote regions. We were careful to make sure that people understood why we were doing this study and that they would need to commit to the hour or more of time needed to do the test.”
By the time their research was published in 2014, they had sampled over 800 people from more than 60 different populations in Kenya, Tanzania, Sudan, and South Africa.
The biology behind the milk puzzle is well understood. All humans digest their mother’s milk as infants, but after weaning, many lose the ability to produce the enzyme lactase, which is essential for breaking down lactose, the sugar in milk.
Until cattle were domesticated around 9,000 years ago, we no longer needed to digest milk after weaning.
Today, for nearly two-thirds of the world’s population, drinking cow’s milk will result in an urgent trip to the bathroom.
The puzzle is why some of us developed the ability to continue to produce the enzyme lactase as adults—“lactase persistence”—and so be able to down pints of milk without discomfort.
From the 1940s, the health benefits of drinking milk were increasingly promoted in the U.S. and northern Europe, where lactase persistence is common.
When researchers found that many African Americans and Asian Americans lacked the enzyme, the deficiency was cast in racial terms; a 1968 New York Times headline read: “Most Nonwhite Persons Found to Develop an Intolerance to Milk.”
However, anthropologists soon reported examples of lactose tolerant pastoralists who rely heavily on milk and dairy products in many parts of West, North, and East Africa as well as the Middle East. For the Maasai (MUHs-eye) and other cattle herders in Kenya and Tanzania, consuming milk is central to their lives and celebrated as a daily blessing.
In 2002, a team of Finnish biologists identified a mutation in a gene associated with the production of lactase in adults. Not surprisingly, its distribution closely matches the pattern of milk drinking: it is most common in the U.S. and northern Europe, and gradually tapers off across the rest of Eurasia.
When scientists analyzed the data to look for the likely age of the mutation, they estimated it had first appeared sometime between 2,000— 21,000 years ago. This was broadly consistent with the theory that it was linked to the domestication of cattle and the spread of farming from the Near East across Europe after 9,000 before the common era.
According to the theory, the ability to digest milk opened up a rich new source of nutrition that gave prehistoric dairy farmers a significance survival advantage, perhaps as a fallback in times of famine or drought.
But this explanation had one big problem: the Eurasian mu-tation was absent in nearly everyone in Africa and the Middle East who drank milk.
So, in the early 2000s, Sarah Tishkoff launched her first field expedition to discover if different lactase-regulating mutations existed among rural cattle herders in Africa. By the time her work culminated in the 2014 study, her team had identified no less than four new mutations, each one enabling specific populations in particular regions to digest milk.
Evidently, each mutation had arisen independently of all the others—a striking case of convergent evolution.
More remarkable still, when the team investigated the dis- tinctive stretches of DNA, or haplotypes, associated with each mutation, they found that natural selection had favored the African ones in relatively recent times—between about 3,000 to 7,000 years ago.
These dates correspond roughly to the domestication of cattle in Africa that began around 6,000 years ago.
Moreover, the selection signal is phenomenally strong; of her first African results, Tishkoff said, “It is basically the strongest signal of selection ever observed in any genome, in any study, in any population in the world.”
The implication is that milk drinking offered such a powerful survival edge that nature kept rapidly “inventing” new versions of the lactase-enabling mutations wherever people raised cattle.
The story of milk underlines the fact that, contrary to wide- spread popular belief, our bodies did not stop evolving when we became “modern humans.”
In reality, cultural innovation can reshape our genetic identity in the blink of an eyelid, from an evolutionary perspective.
And in the view of most scientists, the pace of human evolution is accelerating with the explosive growth of migrations and megacities.
Meanwhile, Sarah Tishkoff’s far-ranging DNA studies of African populations are revealing the extraordinary diversity and richness of Africa’s genetic heritage, which scientists have only just begun to examine.
[MUSIC rises and then ducks under closing credits]
Meredith Johnson:
Discovering Us: 50 Great Discoveries in Human Origins was written for The Leakey Foundation by Evan Hadingham. It was published by Signature Books. The stories are read for you by Ashley Judd.
All the episodes of this audio-companion are available to listen to right now! Make sure to subscribe and share this series with a friend.
You can buy a copy of Discovering Us at your local bookstore or wherever you buy books. There’s a link in the shownotes.
The Leakey Foundation is a nonprofit organization dedicated to funding human origins research and sharing discoveries. The science you heard about today was made possible by Leakey Foundation supporters. Visit our website to learn how you can get involved. Go to leakeyfoundation.org. That’s l-e-a-k-e-y foundation dot org.
This project was made possible by generous support from Camilla and George Smith, the Joan and Arnold Travis Education Fund, and the Ann and Gordon Getty Foundation