Modern Human Biology:

1
Chapter 16

• Modern Human Biology
• Patterns of Adaptation

2
Chapter Outline

• The Adaptive Significance of Human Variation
• Infectious Disease
• The Continuing Impact of Infectious Disease
• New Frontiers in Research Molecular Applications
  in Modern Human Biology

3
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.

4
Homeostasis

• A condition of stability within a biological
  system, maintained by the interaction of
  physiological mechanisms that compensate for
  changes.
• Stress is the bodys response to any factor that
  threatens its ability to maintain homeostasis.

5
Acclimatization

• Physiological responses to changes in the
  environment.
• Responses may be temporary or permanent,
  depending on the environmental change.
• Because it is under genetic influence,
  acclimatization is subject to evolutionary
  factors such as natural selection or genetic
  drift.

6
Geographical Distribution of Skin Color in
Indigenous Populations
7
Ultraviolet Light and Skin Cells
8
Skin Color

• Influenced by three substances
• Hemoglobin, when it is carrying oxygen, gives a
  reddish tinge to the skin.
• Carotene, a plant pigment which the body
  synthesizes into vitamin A, provides a yellowish
  cast.
• Melanin, has the ability to absorb ultraviolet
  radiation preventing damage to DNA.

9
Three Points Skin Color and Evolution

1. Early hominids lived in the tropics, where solar
   radiation is more intense than in temperate areas
   to the north and south.
2. Unlike modern city dwellers, early hominids spent
   their days outdoors.
3. Early hominids didnt wear clothing that would
   have protected them from the sun.

10
Folate and UV Radiation

• Folate is a B vitamin that isnt stored in the
  body and must be replenished through dietary
  sources.
• In pregnant women, insufficient levels of folate
  are associated with numerous fetal developmental
  disorders, including neural tube defects such as
  spina bifida.

11
Folate and UV Radiation

• Studies have shown that UV radiation depletes
  folate serum levels both in laboratory
  experiments and in light-skinned individuals.
• These findings have implications for pregnant
  women, for children, and for the evolution of
  dark skin in early hominids.

12
Question

• Which of the following has not been linked with
  light skin color in humans?
• cloudy skies
• folate
• all of these choices
• clothing

13
Answer c

• None of the following has been linked with light
  skin color in humans
• cloudy skies
• folate
• clothing

14
Vitamin D Synthesis
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.

16
Human Response to Heat

• Long-term adaptations to heat evolved in our
  ancestors
• Sweat Glands
• Vasodilation
• Bergmann's rule - body size tends to be greater
  in populations that live in cold environments.

17
Bergmanns Rule

• In mammalian species, body size tends to be
  greater in populations that live in colder
  climates.
• As mass increases, the relative amount of surface
  area decreases proportionately.
• Because heat is lost at the surface, it follows
  that increased mass allows for greater heat
  retention and reduced heat loss.

18
Allens Rule

• In colder climates, shorter appendages, with
  increased mass-to-surface ratios, are adaptive
  because they are more effective at preventing
  heat loss.
• Conversely, longer appendages, with increased
  surface area relative to mass, are more adaptive
  in warmer climates because they promote heat loss.

19
Examples of Bergmanns andAllens Rules

• (a) This African woman has the linear proportions
  characteristic of many inhabitants of sub-Saharan
  Africa.
• (b) By comparison, the Inuit woman is short and
  stocky.

20
Human Response to Cold

• Short-term responses to cold
• Metabolic rate and shivering
• Narrowing of blood vessels to reduce blood flow
  from the skin, vasoconstriction.
• Increases in metabolic rate to release energy in
  the form of heat.

21
Vasodilation

• Expansion of blood vessels, permitting increased
  blood flow to the skin.
• Vasodilation permits warming of the skin and
  facilitates radiation of warmth as a means of
  cooling.
• Vasodilation is an involuntary response to warm
  temperatures, various drugs, and even emotional
  states (blushing).

22
Vasoconstriction

• Narrowing of blood vessels to reduce blood flow
  to the skin.
• Vasoconstriction is an involuntary response to
  cold and reduces heat loss at the skins surface.

23
High Altitude

• Multiple factors produce stress on the human body
  at higher altitudes
• Hypoxia (reduced available oxygen)
• Intense solar radiation
• Cold
• Low humidity
• Wind (which amplifies cold stress)

24
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.

25
Impact of Infectious Disease

• 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.

26
Impact of Infectious Disease

• In the late 1960s, the surgeon general declared
  the war against infectious disease won.
• Between 1980 and 1992 deaths from infectious
  disease increased by 58.
• Increases in the prevalence of infectious disease
  may be due to overuse of antibiotics.

27
Question

• The number one cause of death among humans until
  the 20th Century was
• AIDS.
• TB.
• malaria.
• infectious diseases.

28
Answer d

• The number one cause of death among humans until
  the 20th Century was infectious diseases.

29
Vectors

• Agents that serve to transmit disease from one
  carrier to another.
• Mosquitoes are vectors for malaria, just as fleas
  are vectors for bubonic plague.

30
Endemic

• Continuously present in a population.

31
Zoonotic

• Pertaining to a zoonosis, a disease thats
  transmitted to humans through contact with
  nonhuman animals.

32
Antibodies

• Proteins that are produced by immune cells and
  serve as major components of the immune system.
• Antibodies recognize and attach to foreign
  antigens on bacteria, viruses, and other
  pathogens.
• Pathogens are substances or microorganisms, such
  as bacteria, fungi, or viruses, that cause
  disease.

33
Environmental Factors

• Global warming may expand the range of tropical
  diseases.
• The spread of disease is associated with
  encountering people this includes crossing
  borders and penetrating remote areas.
• The increasingly large human population leads to
  overcrowding, unsanitary conditions and the
  spread of communicable disease.