The Evolution of Lactose Tolerance — HHMI BioInteractive Video

Summary

This script explores the fascinating evolution of lactase persistence in humans, which allows some adults to digest milk unlike most mammals. It delves into the co-evolution of human biology and culture, starting with the domestication of animals and cultivation of crops around 10,000 years ago. The narrative follows the quest to understand lactase persistence, including genetic studies and archaeological evidence of early milk use. It highlights the strong selective advantage this trait provided in pastoral societies, exemplifying gene-culture co-evolution.

Takeaways

• 🧬 Humans evolved as hunter-gatherers for most of our history, but around 10,000 years ago, the domestication of animals and cultivation of crops began, impacting our biology.
• 🍼 Milk is a staple in many diets, but the ability of adults to digest it varies significantly across different populations, which is a fascinating study of human biology and culture co-evolution.
• 👶 All infant mammals can digest lactose, the main sugar in milk, with the help of the lactase enzyme, but most lose this ability as they grow older, becoming lactose intolerant.
• 🧪 Lactase persistence, the ability to digest lactose into adulthood, is linked to genetic mutations that were discovered by comparing DNA from lactase persistent and non-persistent individuals.
• 🌍 Geographic distribution of lactase persistence is uneven, with a majority of people in Europe being able to digest lactose, while in other regions, lactase persistence is less common.
• 🧬 Researchers found different genetic mutations for lactase persistence in European and African populations, indicating that this trait evolved independently in response to similar environmental pressures.
• 🧬 The mutation that enables lactase persistence in Europeans was identified as a T instead of a C at a specific non-coding position in the DNA.
• 🌿 Pastoralists, such as the Maasai and early Europeans, who domesticated animals for food, are more likely to have the lactase persistence trait, suggesting a cultural influence on genetic evolution.
• 🧪 Organic chemist Richard Evershed used chemical signatures to determine that ancient pots from Europe and Africa contained milk, providing archaeological evidence of early dairy use.
• ⏳ The spread of lactase persistence mutations in populations coincides with the archaeological evidence of the start of milk use, suggesting a strong selective advantage for those who could digest lactose.
• 💪 The selective advantage of lactase persistence was estimated to be around 5% to 10%, meaning that individuals with this trait had a significantly higher survival rate, likely due to better nutrition and reduced exposure to pathogens.

Q & A

What was the primary lifestyle of humans before the shift to agriculture about 10,000 years ago?

Humans primarily lived as hunter-gatherers, obtaining their food by hunting animals and gathering plants.

Why is the ability to digest lactose in adulthood considered a fascinating case study in co-evolution?

The ability to digest lactose in adulthood is a result of the interaction between human culture (the domestication of animals for milk) and human biology (the persistence of lactase production), illustrating how cultural practices can drive biological evolution.

What is the main sugar in milk and why is it significant?

The main sugar in milk is lactose. It is significant because most mammals lose the ability to digest it after infancy due to the cessation of lactase production, leading to lactose intolerance in adulthood.

What happens when a lactose intolerant adult consumes milk?

When a lactose intolerant adult consumes milk, the lactose passes undigested into the large intestine, where bacteria ferment it, causing symptoms like cramps, gas, and diarrhea.

What is the term for the minority of adults who can digest lactose and why is it important?

These adults are called 'lactase persistent' because their ability to produce lactase, the enzyme that breaks down lactose, continues beyond childhood and throughout their lives, allowing them to digest milk without adverse effects.

What was the purpose of Spencer Wells' lactose tolerance test?

The lactose tolerance test was conducted to determine whether Spencer Wells' body still produces lactase, which would indicate his ability to digest lactose as an adult.

What did geneticists find when they compared the lactase gene across lactase persistent and non-persistent people?

Geneticists did not find a change in the DNA sequence of the lactase gene itself that distinguished lactase persistent from non-persistent individuals. Instead, they discovered regulatory mutations affecting the lactase gene.

How did researchers identify the mutation responsible for lactase persistence in Europeans?

Researchers identified Finnish families with both lactase persistent and non-persistent members, compared DNA around the lactase gene, and discovered a one-base difference (a T instead of a C) in a non-coding position that causes lactase persistence.

What did geneticist Sarah Tishkoff discover about lactase persistence in African populations?

Sarah Tishkoff found that the European lactase persistence mutation barely existed in African samples. Instead, she discovered a different mutation in the Maasai population, indicating that lactase persistence evolved independently in different populations.

What evidence did Richard Evershed find regarding the use of milk in ancient cultures?

Richard Evershed found chemical signatures of milk fats in ancient pots from both European and African settlements, dating back 7,000 to 9,000 years ago, indicating that dairy use was prevalent in these early cultures.

How did the practice of dairying potentially drive the spread of lactase persistence mutations?

The practice of dairying provided a selective advantage for individuals with lactase persistence mutations, as they could digest milk and benefit from its nutritional content. This advantage could have led to an increase in the frequency of the mutation in populations that relied on milk.

What is the estimated selective advantage of lactase persistence, and how does it manifest?

The selective advantage of lactase persistence is estimated to be around 5% to 10%, meaning that individuals with this trait had a higher survival rate, which could accumulate over generations due to the nutritional benefits of being able to digest milk.

What is the significance of the gene-culture co-evolution in the context of lactase persistence?

The gene-culture co-evolution signifies that human biological evolution is deeply intertwined with cultural practices, such as dairying, which in the case of lactase persistence, drove the evolution of a genetic trait that conferred a survival advantage.