The Biology of Modern Homo sapiens Chapter Sixteen

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

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

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

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

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

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

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

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

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

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

Distribution of Skin Color in Indigenous Populations

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

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

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

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

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

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

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)

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

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

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