1. The Biology of Modern Homo sapiens
Chapter Sixteen

2. Human Growth and Development Today and in the Past
Human growth continues through late teens or early 20s
Three major spurts are typical, including first two trimesters in utero, first four years, and the adolescent growth spurt
Puberty
Pronounced increase in growth rate
Changes in the reproductive organs and secondary sexual characteristics

3. Trends in Growth and Development
The tendency over the last hundred or so years for each succeeding generation to mature earlier and grow larger
While no one knows for sure what causes this trend, some researchers believe that the following are responsible:
changes in nutrition
better sanitation
improved health services
less tedious lifestyles

4. Human Brain Growth 25% of its adult size at birth 50% at six months
75% at 2.5 years
90% at 5 years
95% at 10 years
Such a small amount of growth before birth is unusual for primates and mammals
Selective advantages of such an underdeveloped brain
Exit through narrow pelvis modified for bipedalism
Brain develops in stimulating, cultural context

5. Human Adjustment Adjustments Non-genetic changes
One reason that humans survive in a wide diversity of habitats
Three types
Behavioral adjustments
Cultural responses to environmental stresses (e.g., housing, food)
Acclimatory adjustments
reversible physiological changes to environmental stress (e.g., shivering in an arctic habitat, sweating in a desert habitat)
Developmental adjustments
Variation in the pattern of growth and development that provides a means of adjusting to environmental stress

6. Adaptive Significance of Human Variation
Human variation is the result of adaptations to environmental conditions.
Physiological response to the environment operates at two levels:
Long-term - evolutionary changes characterize all individuals within a population or species
Short-term - temporary physiological response is called acclimatization

7. Slash-and-burn Agriculture
A traditional land-clearing practice whereby trees and vegetation are cut and burned
In many areas, fields are abandoned after a few years and clearing occurs elsewhere
Sickle-Cell Allele
Generally regarded as a benign condition
If one parent has sickle-cell anemia and the other has sickle-cell trait, then the child has a 50% chance of having sickle-cell disease and a 50% chance of having sickle-cell trait

8. Sickle-Cell Allele
With the spread in Africa of slash-and-burn agriculture, in the last 2,000 years, penetration and clearing of tropical forests occurred
As a result, rain water was left to stand in stagnant pools that provided mosquito breeding areas close to human settlements
DNA analyses have confirmed such a recent origin and spread of the sickle-cell allele in a population from Senegal, in West Africa

9. Sickle-Cell Allele
A recent study estimates the origin of the Hemoglobin S (HbS) mutation in this group at 1,250 to 2,100 ya.
At least in some areas, malaria began to have an impact on human populations only recently
But once it did, it became a powerful selective force
The increase in the frequency of the sickle-cell allele is a biological adaptation to an environmental change

10. Lactose Intolerance
In all human populations, infants and young children are able to digest milk
In most mammals, including humans, the gene that codes for lactase production ‘switches off” in adolescence
The geographical distribution of lactose tolerance is related to a history of cultural dependence on fresh milk products

11. Homeostasis and Acclimatization
Homeostasis - a condition of stability within a biological system
interaction of physiological mechanisms that compensate for changes
stress is the body’s response to any factor that threatens homeostasis
Acclimatization - physiological responses to changes in the environment
responses may be temporary or permanent
acclimatization is subject to natural selection and genetic drift

12. Distribution of Skin Color in Indigenous Populations

13. Ultraviolet Rays
Ultraviolet Rays penetrate the skin and can eventually damage DNA within skin cells
The three major types of cells that can be affected are squamous cells, basal cells, and melanocytes

14. UV Radiation
Considering the cancer-causing effects of UV radiation from an evolutionary perspective:
Early hominins lived in the tropics
Solar radiation is more intense than in temperate areas to the north and south
Early hominins spent their days outdoors
Early hominins did not wear clothing that would have protected them from the sun
Selective Advantages of Dark Skin
Protects from folate degradation
Especially important for pregnant mothers

15. Thermal Environment
Mammals (and birds) have evolved complex physiological mechanisms to maintain a constant body temperature
Humans are found in a wide variety of thermal environments, ranging from 120° F to -60° F
Human response to heat
Long-term adaptations to heat evolved in our ancestors:
Sweat Glands
Vasodilation

16. Distribution of Variability
Gloger’s rule:
there is a tendency for more heavily pigmented populations to be located toward the equator and lighter populations farther from it
Bergmann’s rule:
the average weight of the members of the population increases and the surface area of the body decreases as the average environmental temperature decreases
Allen’s rule:
the relative size of the protruding parts of the body increase as the average environmental temperature increases

17. Bergmann and Allen’s Rules Examples
(a) These Samburu women have characteristic of many inhabitants of sub-Saharan Africa
(b) The Inuit women are shorter and stockier

18. Human Response to Cold Short-term responses to cold: Shivering
Vasodilation – capillaries near the skin’s surface widen to permit increased blood flow to the skin
Vasoconstriction - narrowing of blood vessels to reduce blood flow from the skin
Increases in metabolic rate to release energy in the form of heat

19. High Altitude
Multiple factors produce stress on the human body at higher altitudes:
Hypoxia (reduced available oxygen)
Can result in headaches, hallucinations, loss of blood pressure, etc.
Intense solar radiation
Cold
Low humidity
Wind (which amplifies cold stress)

20. Infectious Disease
Caused by invading organisms such as bacteria, viruses, or fungi
Throughout evolution, disease has exerted selective pressures on human populations
Disease influences the frequency of certain alleles that affect the immune response
Impact
Before the 20th century, infectious disease was the number one limiting factor to human populations
Since the 1940s, the use of antibiotics has reduced mortality resulting from infectious disease

21. Spread of Disease Vectors Pathogens Endemic Pandemic
Agents that serve to transmit disease from one carrier to another
Mosquitoes are vectors for malaria, fleas are vectors for bubonic plague
Pathogens
Any agents, especially microorganisms such as viruses, bacteria, or fungi, that infect a host and cause disease
Endemic
Continuously present in a population
Small bands of hunter-gatherers were not faced with continuous exposure to endemic disease because their population numbers were low
Pandemic
An extensive outbreak of disease affecting large numbers of individuals over a wide area

22. The Continuing Impact of Infectious Disease
Humans and pathogens exert selective pressures on each other
Microorganisms evolve
A pathogen must not be so virulent as to kill its host too quickly
Selection acts to produce resistance in host populations and/or to reduce the virulence of disease organisms