Evolution of Lactase Persistence

Alan R. Rogers

November 22, 2015

Outline

I Lactose, lactase, and lactase persistence

I Evidence of a selective sweep

I Prehistory of Europe

The trouble with fresh milk

I Contains the sugar lactose

I Digesting lactose requires the enzyme lactase

I Most humans don’t produce it after age 5.

I Fresh milk gives them gas and diarrhea.

I 8000 years ago, all humans had this problem.

Lactase persistence

I Some modern humans produce lactase throughout life.

I Digest fresh milk as adults.

I Caused by mutation near lactase gene.

I When and where?

Distribution of lactase persistence (dark blue) Within countries, lactase persistence more common inpopulations that drink milk

Lactase persistence in Europe

I ModernEuro-peans

I Dashes:Funnel-beakerculture

Outline

◦ Lactose, lactase, and lactase persistence

I Evidence of a selective sweep

I Prehistory of Europe

Evidence for a selective sweep

I In Europeans, persistence allele surrounded by a million basesof LD.

I Indicates strong selection.

I Statistical tests reject the drift hypothesis (Bersaglieri et al2004)

I Increasing for ∼10,000 years (Coelho et al 2005).

LD surrounds lactase gene in Europe

Huge block of LD around lactase allele in Europe

(Nathan Harris)

I Rows are differentSTRs

I Lactasepersistence allele:haplotype TA.

I Has reduced SNPvariation,

I Indicates recentorigin.

I Age: 7,450 or12,300 years(depending onassumptions)

Outline

◦ Lactose, lactase, and lactase persistence

◦ Evidence of a selective sweep

I Prehistory of Europe

Lactase persistence in ancient Europeans

T is lactasepersistence allele

Appeared in Europeby 2500 BC

(Kruttli et al 2014)

What washappening there5000 y ago?

Modern Europeans derive from 3 ancestors

EEF: Early EuropeanFarmers

WHG: WesternHunter-Gatherers

ANE: Ancient NorthEurasians

(Lazaridis 2014)

Old Europe

I Expanded into Europe fromMiddle-East and Anatoliabeginning 7000 bp.

I Earliest European farmers.

I Lactose intolerant.

Otzi, the Iceman

I 3300 BC on border betweenItaly and Austria

I Hair had high levels ofcopper and arsenic—a metalsmith.

I Last meals: chamois and reddeer mean, and einkornwheat

I Lactose intolerant.

Long house

Sculpture The Yamna culture: pastoralists

Mare’s milk

I One kg of mare’s milk has 190 Calories of fat and protein 250Calories of lactose.

I With 5 kg per day yield one mare feeds two lactase persistentchildren.

I Less that one non-lactase-persistent child.

I Lactase persistence more than doubles food supply with onesimple mutation.

I This advantage cannot be appropriated by others.

Indo-European expansion

Indo-European languages Tocharian

Modern Uighurs Corded ware culture

2400 BC

Overran Europe

Possibly introducedIndo-Europeanlanguages

And maybe also lactasepresistence

Study of Mathieson et al 2015

I DNA from 83 ancient Europeans.

I Track changes in allele frequencies over time.

History of evolution in Europe

Lactase persistence begins in Europe around 4000 BP.

Summary

I Recent adaptive evolution in lactase persistence.

I Appears in Europe about 5000 y ago.

I May have arrived with Indo-Europeans.