The Neolithic Revolution and the Evolution of Life Expectancy

Oded Galor

“It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change”

Charles Darwin

Objectives

The origin of the rise in life expectancy since the Neolithic Revolution

Natural selection vs. Technological advancements, income, education,…

The role of natural selection in the non-monotonic evolution of life expectancy in the process of development

The effect of differential timing of the Neolithic Revolution across countries and regions on the emergence of sustained differences in life expectancy across countries and ethnic groups

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Mesolithicperiod

Neolithicperiod

Copper age Bronze-Ironage

The evolution of life expectancy17,000 – 1,000 BCE

The evolution of life expectancyin the process of urbanization (England)

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The evolution of life expectancyEngland, 1580-2000

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Theories of Aging

The Mutation Accumulation Theory [Medawar, 1946]

Aging is an inevitable outcome of the declining force of natural selection in older age

Adverse mutations in the post-reproductive age have a smaller negative effect on the survival of the genes

…Theories: Evolution of Life Expectancy

The Antagonistic Pleiotropy Theory [Williams, 1957]

Late-activating harmful genes may be favored by natural selection if they have a beneficial reproductive effects in early stages of life.

(Pleiotropy - multiple effects of a single gene)

…Theories: Evolution of Life Expectancy

The Disposable Soma Theory

[Kirkwood and Holliday, 1979]

Optimal allocation of metabolic resources between reproduction and maintenance.

The Disposable Soma Theory

Evolutionary trade-off between:

Resources channeled toward:

current reproduction

& somatic investment

longevity & future reproduction

…The Disposable Soma Theory

Evidence:

Longevity is correlated with resources devoted to cellular repair and maintenance

Capacity of somatic cells to carry out effective maintenance and repairs (e.g., DNA repairs, accurate gene regulation, tumor suppression, and antioxidants), governs the time taken for damage to accumulate

[Kirkwood (1998), Williams and Day (2003)]

Main Hypotheses

The Neolithic Revolution increased the extrinsic mortality risk

- rise in population density

- domestication of animals

- increase work effort

- poorer nutrition (e.g., reduced meat intake, reduced mineral absorption by the cereal-based diet)

Time Trend in Pre-Columbian Health

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Source: Richard H. Steckel

Health Index

Skeletal infections - inflammatory responses to bacterial invasion

Iron deficiency anemia - nutritional deprivation, low body weight, chronic diarrhea, parasite infections

Dental health -- indicator both of oral and general health

Degenerative joint disease -- mechanical wear and tear on the joints of the skeleton due to physical activity

Enamel hypoplasias - enamel deficiency commonly found in the teeth of people whose early childhood was biologically stressful

Trauma

…Main Hypothesis

This deterioration in the health environment increased the evolutionary optimal allocation of resources towards somatic investment, repairs, and maintenance (e.g., enhanced immune system, DNA repairs, accurate gene regulation, tumor suppression, and antioxidants)

Short-run: (the composition of the population is unchanged) Life expectancy declined

Long-run: (an evolutionary process)Evolutionary advantage to individuals who were genetically pre-disposed towards higher somatic investment

Increased representation of individuals who are genetically pre-disposed towards higher somatic

investment Life Expectancy increases

…Main Hypotheses

The deterioration in the health environment:

triggered the observed improvement in life expectancy since the Agricultural Revolution

was a necessary condition for the dramatic recent improvement in life expectancy

…Main Hypotheses

Differences in the timing of the Neolithic Revolution across regions led to sustained differences in life expectancy across countries.

…Main Hypothesis

The rise in the extrinsic mortality risk

enhanced the genetic potential for longer life expectancy (i.e., life expectancy in a risk-free environment)

enabled the dramatic impact of recent improvements in health infrastructure on the prolongation of life

Rapid Evolutionary Changes in Humans in the past 10,000 years

Lactose Tolerance Variations in the ability to tolerate lactose across regions is inversely

related to the differences in timing of the transition to agriculture across regions and thus in the domestication of dairy animals in the Neolithic revolution

Gluten Tolerance Variations in the ability to tolerate a protein present in wheat, rye,

barley, and some oats, first domesticated in the course of the agricultural revolution, is inversely related to the distance from the Fertile Crescent

Genetic immunity to malaria - Sickle Cell Trait Variations in natural immunity to malaria is related to the engagement

in slash-and-burn agriculture

…Rapid Evolutionary Changes in Humans in the past 10,000 years

700 regions of the human genome have been reshaped by natural selection within the past 5,000 to 15,000 years

Voight et al. (2006)

ASPM (a specific regulator of brain size in the lineage leading to Homo sapiens) arose in humans merely about 5800 years ago and has since swept to high frequency under strong positive selection

Mekel-Bobrov et. al (2005)

Testable prediction

Populations that experienced the Neolithic Revolution earlier have higher life expectancy

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Fig.1 - Life Expectancy and Timing Since Neolithic

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Covariates AdjustedFig.2 - Life Expectancy and Timing Since Neolithic

Evidence (other species):

The effect of a rise in extrinsic mortality risk on life expectancy is ambiguous [Williams and Day (2003)]

Recent study: Guppies that were exposed to higher extrinsic mortality risk had a higher life expectancy in a risk-free environment (i.e., lower intrinsic mortality rate) [Reznick et al. (Science, 2004)]