The Life Span

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Life History Evolution - Eric Charnov
      Trade-offs in mammalian history
      Increased life span increases = Decreased in mortality rate  risk of waiting to reproduce goes down

Grandmother Hypothesis
Grandmothers  passed the reproductive phase. However, most mothers of offspring are at the peak of their
reproductive phase.

Feeding juveniles delay a mother’s next child. Therefore, grandmothers with declining fertility rate are able
to provide support such as nutritional needs that the children can’t provide for themselves.

         Embodied Capital and the Evolutionary
           Economics of the Human Life Span
   -   Question: What forces govern the evolution of an organism’s life span?
   -   Argument: Life spans evolve as part of an integrated life-history program (development and
       reproduction is related to age of death)

Qualitative definition of life span: amount of time between birth and age at which the likelihood of death
becomes high, relative to the likelihoods at younger ages.

Quantitative definition: modal age at death, conditional on reaching adulthood.

Embodied capital and life-history theory
Fundamental tradeoffs in life-history
   -   Section combines basics life history theory developed in biology with analytical approach developed
       in analyzing capital in economics
   -   2 tradeoffs:
            o Current and future reproduction
                     Early reproduction is favoured by natural selection
                            Increase length of reproduction period
                            Increase growth rate if lineage of length of reproductive period is shortened
            o Quality vs. Quantity of offspring
   -   Natural selection expected to optimize the allocation of energy to current and future reproduction
       via investments in growth in maintenance
            o Benefits include:
                     Increase in length of life span by lowering size-dependent mortality
                     Increase efficiency of energy capture allowing for higher rate of offspring production
                     Rate of success in intrasexual competition for mates

Specialization and flexibility in life histories and the fast -slow continuum
   -   Thesis: Specialization and flexibility fundamental to understanding human life span
          o Focus: Relationship between brain evolution and life-history evolution
   -   Having large human brain:
          o Pro: allows humans to respond to environmental variation, learn
          o Con: large period of development to make it functional constrains human life course by
               requiring specializations for a slow life history

Embodied capital and life-history theory
   -   Graphical presentation of analytical models of life-history evolution based on embodied capital
           o Treat processes of growth, development, maintenance as investments in stocks or embodied
           o Stocks depreciate with time, so maintenance can be seen as investments in embodied capital
   -   2 tradeoffs:
           o Present-future reproduction: between investments in own capital and reproduction
           o Quantity-quality: between embodied capital of offspring and numbers
   -   Benefits of modeling life-history evolution in terms of capital investments: analysis of such
       investments is well developed in economics with well-established results

Capital investments and endogenous mortality
   -   Formula determines that a some age, a steady state is reached where capital is at its optimum level
       (capital and mortality rates remain constant)
   -   Two results:
           o Environmental change that increases productivity of capital:
                    Increases optimal level of capital investment (length of investment period)
                    Decreases mortality through increases in “s” (given by formula)
           o Morality rates:
                    Increases optimal capital stock
                    Produces reinforcing increase in “s” (given in formula)
   -   Note: “s” = energy

Embodied capital and the evolution of human life histories
   -   The brain allows modern humans to manage their food selections that are high in nutrients.
          o High levels of knowledge, skill coordination, and strength are required in order to gain
              necessary resources.
                   Productivity level increases with age (skills, work efficiency, and knowledge)  ROI
                       at older age
                   Males and females cooperate allows women to allocate their time to childcare,
                       increasing the survival and reproductive rates of the children.
          o Humans are specialists in that they consume the highest-quality plant and animal resources
              in their local environment and rely on creative, skill-intensive techniques to exploit them.
Digestion and diet

   -   Humans consume calorie-dense, low-fiber foods that are rich in protein and fat.  High meat
       consumption (hunted food)
   -   Extracted resources: invetebrate animal products, roots, nuts, seeds, palm fiber, growing shoots.
       Both hunted food + extracted resources are mostly acquired by humans.
   -   Collected resources: fruits, leaves, flowers, and accessible plant parts. Collected resources types of
       food are mostly acquired by chimpanzees.

Food Type                         Humans Food Consumption  Chimpanzees Food Consumption
Hunted food (vertebrae meat)                          50%                             2%
Extracted food                                        32%                             3%
Collected food                                          8%                           95%

   -   Human digestive system: large intestines to digest cellulose + long small intestine to digest lipids +
       carbon fatty acids to produce fatty acids
           o Specialized to digest meat + low-fiber diets

The brain and cognitive development
   -   Humans take ~2.5x as long to complete cognitive development as do chimps but humans learn faster
        2yrs old child abilities = 4yrs old chimps
          o Human require environmental input to complete development
          o Logic & reasoning: ages 16-18  initial productivity among modern hunters and gatherers
          o Elongated development in humans – associated with slowed aging of brain

Physical growth
   -   Human’s physical growth: faster  slower  faster than chimps/ gorillas. Human brain is 2x as big
       at birth as a chimpanzee’s neonate. Then growth more slowly following infancy/ early childhood.
           o Children are not able feed themselves until they completed growth. They rely on family food
           o Growth rates also are slow since children don’t need large bodies (less physical work)
                     They learn via observations + play
           o Growth rates will increase when brain is almost ready + adult body size is reached

Production, reproduction, and energy flows
   -   3 phases of net production of chimps
           1. Newborn to ~5yrs old  dependence on mother’s milk  net production: negative
           2. Independent juvenile growth to 13yrs of age for females  net production:0
           3. Applies to females  reproductive  net production: surplus
   -   Human net production:
           o Net production is negative to age 14  increased consumptions due to physical growth
           o Net production in humans > net production is chimpanzees
           o Humans require 1,750 calories a day. Whereas chimps only require 250 calories/day
   -   More difficult tasks are done by adults than juveniles for both humans & chimps
   -   Sexual division of labor: men and women have different skills that complement each other
           o Hunted food acquired by men – in charge of hunting food so women can focus on childcare.
              Men supply 97% calories to offspring.
            o   Gathered food acquired by women

   -   Chimp’s mortality rate rises after their lowest point prior to reproduction. 30% of chimps reach 20
       (the age when humans produce as much as they consume).
   -   <5% of chimps reach 45 (human’s peak net production)
   -   Human life span is about 65-75 years

The Life Span
The human life course and human life span
   -   Thesis: human life course is an integrated adaptation to a specialize niche
           o Life span and other physiology/biochemistry/parental investment etc. coadapted to a
               learning intensive feeding niche giving humans the most nutrient dense/highest quality food
   -   6 distinct stages:
           o 1) Early fetal stage – 5:
                     linguistic competence (comprehension) achieved
                     Large fat reserves maintained to support brain growth
           o 2) Childhood:
                     Slow physical growth
                     Large energetic allocation to building immune system
                     Cognitive development facilitated by play and practice
                     Very low mortality and productivity
           o 3) Adolescence:
                     Physical growth accomplished rapidly
                     Reproductive system mature
                     Final stages of cognitive development occur (brain and body ready for adult
           o 4) Males:
                     Early adulthood to prime adulthood in mid 30s:
                     Physical strength and information processing peak in early adulthood
                     Productivity increases, mortality rates constant and low
           o Women:
                     Reduction in productivity in interests of fertility and parental investment
                     Tradeoff: between resource acquisition and childcare
           o 5) Middle age: parenthood and grandparenthood
                     Dependency on parents peak age 40
                     Productivity at peak, but net productivity is negative, supported by resource transfer
                       from other families
                     Dependency and productivity diminishes through middle age
           o 6) Old age: age 60+
                     Physical deterioration rapid
                     Brain ageing evident
                  Sharp increase in mortality rate
                  Parenting finished and work effort decreases with productivity
          o   Note: 6th stage selected for fitness benefits
                  Trade off with menopause

Flexibility and variation in the human life span
   -   Physiological adaptations:
          o Increased nutrition, decreased work and disease load
                   Growth rates increase and faster maturation
                   Aging may be slowed in response to better nutrition, decreased work load and
                   Increased risk of heart disease, diabetes etc. might be result of evolved responses
   -   Behavourial responses to modernization:
          o Increased economic payoff to educational capital, decreased mortality rates owing to
              improvements in public health

Building blocks for adequate theory of senescence and the life span
   -   Senescence: increasing mortality rate with age
   -   Hamilton formalize William’s argument:
          o Sensitivities to fitness to changes in mortality rates decrease with age of action
          o Results suggest that reproductive value is not the critical determinant (both depend on
              probability of reaching that age, expected future reproduction at that age)
   -   What causes senescence?
          o Adding to quantity until optimal quantity is reached, but because of assaults from outside
              agents, it’s subject to decay
                   Ex. Costs of producing new cells and cost of repairing is different
                            During development, optimal life history program has to equalize marginal
                               fitness returns from:
                                    o Adding to new capital
                                    o Repairing existing capital
                                    o Reducing current mortality

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