Human Life History and Adaptability

I. Human Life History

Life History Theory:
- The study of an organism’s life cycle, including survival and overall way of life.
- Examines the strategies an organism employs to allocate energy towards various biological functions:
  - Growth
  - Maintenance
  - Reproduction
  - Care of offspring
  - Avoiding death
  - And other essential life processes.
The Human Life Cycle Stages:
- A. Prenatal Stage (Fertilization to Birth): This stage is divided into three trimesters:
  - 1st Trimester:
    - Characterized by rapid cell differentiation into various embryonic tissues.
    - Formation of crucial organs and physiological systems begins.
  - 2nd Trimester:
    - The main feature is significant growth in length, typically from approximately 8 inches to 14 inches.
  - 3rd Trimester:
    - Marked by rapid weight gain and the final maturation of organs.
    - Development of reflexes and responses to external stimuli.
- B. Human Brain Growth:
  - The vast majority of human brain growth occurs after birth.
  - Reasons for Postnatal Brain Growth:
    - Obstetrical Dilemma: The conflict between the relatively large size of a human infant's head and the narrowness of the human female's birth canal necessitates that much brain growth happens outside the womb.
    - Stimulating Environment: Postnatal brain growth allows for development within a stimulating environment, crucial for complex learning and adaptation.
- C. Infancy (Birth to 3 Years):
  - The primary period of nursing.
  - Functions of Breastfeeding:
    - Provides adequate nutrition essential for rapid growth.
    - Transfers vital antibodies from mother to infant, boosting the immune system.
    - Contributes to natural birth spacing, allowing the mother's body to recover.
  - Considered the most rapid growth period (as shown on the human growth curve).
  - Significant development of cognitive abilities and motor skills.
  - Eruption of deciduous (baby) teeth occurs during this stage.
- D. Childhood (Weaning to Puberty):
  - An extended period unique to humans compared to many other species.
  - Critical time for learning and acquiring complex skills.
  - Children remain dependent on adults for food and care.
  - Growth levels off during this stage (as observed on the human growth curve).
- E. Adolescence:
  - Characterized by significant hormone changes, leading to:
    - Adolescent Growth Spurt: A rapid increase in height and weight.
    - Sexual Maturation: Development of primary and secondary sexual characteristics.
    - Menarche: The first menstruation in females, marking the onset of reproductive capability.
  - Females generally tend to mature earlier than males.
  - The onset of puberty is highly variable among individuals.
Skeletal Growth:
- During growth, the ends of long bones (epiphyses) are separate from the main shaft (diaphysis).
- Growth in height ceases when the epiphyses unite (fuse) with the diaphysis, indicating skeletal maturity.
F. Adulthood:
- Marked by the completion of epiphyses fusion, cessation of growth in height, and full sexual maturity.
- Biological changes primarily focus on the continuous replacement of cells and tissues.
- Characterized by stability through homeostasis, the body's ability to maintain internal equilibrium.
- Represents the period of greatest resistance to disease and stress.
G. Aging (Senescence):
- Refers to the physiological decline in various body functions.
- Involves a reduction in the body's ability to maintain homeostasis.
- Common changes include:
  - Bone loss (typically begins after age 40).
  - Decrease in muscle mass.
  - Reduction in immune function and efficiency.
H. Post-Reproduction:
- Menopause: The cessation of menstruation in females, typically occurring around age 50.
- Humans have a uniquely long post-reproductive period, especially in females.
- Hypotheses for the long post-reproductive period:
  - May be an