Ch 5: Human Life History

Pre-Adult Maintenance: Energy must be partitioned between competing biological needs. This is a Zero-sum game where energy spent on one cannot be spent on another.
- Basal Metabolic Rate (BMR): The minimum energy required to keep the body functioning at rest.
- Immune Function: Critical for survival; often prioritized over growth during periods of infection.
- Cellular / DNA Repair: Processes that mitigate oxidative stress and maintain somatic integrity.
Ecology and Energy Allocation:
- Energy Budgets: Managed through the intake of calories vs. expenditure in various life domains.
- Energy Reserve (Fat): Humans exhibit high levels of neonatal adiposity (fatness) compared to other primates, serving as a buffer for the high metabolic demands of the brain.
- Growth vs. Reproduction: Life history theory suggests a fundamental trade-off; organisms cannot grow and reproduce at their maximum potential simultaneously.
Activity and Stages:
- Pre-natal Focus: High reliance on placental nutrition and maternal metabolic efficiency.
- Hormonal Influence: Endocrine signals like testosterone and insulin-like growth factors ( $IGF-1$ ) regulate the tempo of growth.
- Sexual Dimorphism: Differences in stature and muscle mass typically emerge most strongly during puberty due to diverging hormonal profiles.

Anthropometric Methods:
- Height/Length: Measured as crown-to-heel or standing height.
- Body Composition: Assessed via Skinfolds (subcutaneous fat), Bioelectrical Impedance (electrical conductivity of lean tissue), and DEXA (the gold standard for bone mineral density and fat mass).
Statistical Standardization:
- Z-scores: A method of standardizing growth data by calculating the number of standard deviations a measurement is from the population mean ( $Z = \frac{x - \mu}{\sigma}$ ).
- Centiles: Useful for tracking a child's growth trajectory relative to a reference population. Crossing centiles can indicate health issues or nutritional shifts.

Infancy (0-30 months):
- Nurture: Total reliance on maternal lactation and external care.
- Neurodevelopment: A unique human trait is the continued rapid brain growth post-birth. At birth, the human brain is approximately 25% of its adult size; by age 2, it reaches nearly 75%.
- Metabolism: Over 50% of the infant's Resting Metabolic Rate (RMR) supports brain metabolism.
Childhood (2-6 years):
- The Weaning Gap: Period after cessation of breastfeeding but before the child can process adult foods. Requires specialized provisioning (high-protein, easy-to-digest foods).
- Adiposity Rebound: An increase in Body Mass Index (BMI) after it reaches its lowest point in early childhood; early rebound is often associated with later obesity risk.
Juvenility (6-11 years):
- Adrenarche: The maturation of the adrenal cortex occurs around age 6-8, leading to increased levels of DHEA, which may influence cognitive and social development even before puberty.
- Social Learning: This phase is a "quiet" growth period where energy is redirected toward social and economic competency.
Adolescence (11-15+ years):
- Growth Spurt: Unique among primates, humans Experience a significant acceleration in linear growth (stature) during this phase.
- Secondary Sexual Characteristics: Development driven by the reactivation of the HPG (Hypothalamic-Pituitary-Gonadal) axis.

Early Weaning and IBI: Humans wean earlier (approx. 2.5-3 years) than Great Apes (approx. 5 years), leading to shorter Inter-Birth Intervals (IBIs) and higher fertility rates.
Cooperative Breeding: Human mothers receive help from others (fathers, grandmothers), which offloads the energetic cost of childcare.

Eco-Geographical Rules:
- Bergmann’s Rule: Body size tends to be larger in colder climates to conserve heat (lower surface area to volume ratio).
- Allen’s Rule: Appendages (limbs) are shorter in cold climates and longer in hot climates to regulate thermal exchange.
Height Variation: Influenced by the interaction of genetics and environment (Standard of Living). Short stature in some populations (e.g., Pygmies) may be an adaptation to high-pathogen environments or thermal stress, prioritizing early reproduction over extended growth.

Proximate Factors:
- Nutrition: Caloric and micronutrient (e.g., Zinc, Vitamin A, Iodine) availability.
- Infectious Disease: Synergistic relationship with malnutrition; illness impairs nutrient absorption and redirects energy to immunity.
Secular Trends: The observed trend over the last 150 years where children Reach adult height earlier and grow taller than their parents. This is largely attributed to improved sanitation, vaccination, and consistent nutrition.

Antagonistic Pleiotropy: Genes that enhance fitness in youth (e.g., high sex hormones) may have deleterious effects later in life (e.g., increased cancer risk).
Disposable Soma Theory: The body (soma) is "disposable" and should only be maintained enough to ensure reproductive success; hence, repairs are never 100% efficient, leading to senescence.

The Mother Hypothesis: Older women stop reproducing to invest more time in the survival of their existing offspring.
The Grandmother Hypothesis: Post-reproductive women increase their inclusive fitness by helping provision their grandchildren, allowing their own children to have shorter IBIs.
Constraint (Atresia): Menopause may simply be a result of the physiological exhaustion of the finite oocyte supply (follicular atresia) coupled with a human lifespan that has outpaced ovarian longevity.