anth40 growth&adaptation lecture

How We GrowA Look at the Adaptive

Significance of Human Growth

Presentation by:Helen Corless

Learning goals for today's lecture

1. Identify and describe the various life stages, especially in relation to growth

2. Understand the relationship between growth velocity and sexual maturation during puberty for boys vs. girls; be able to identify indicators of sexual/physical maturation

3. Understand the concept of adaptation in an evolutionary context, and recognize human growth as a form of adaptation

4. Be able to identify and discuss two examples of biological plasticity (adaptability)

5. Understand surface area to volume relationships with respect to adaptations in body size and shape

Patterns of Human Growth

Auxology - the study of human growth and development

All animals undergo stages of growth anddevelopment that are to some degree under genetic control. Growth and development can be very sensitive to environmental conditions, however, as we shall see...

Stages of Human Growth

Four Stage Model of Growth (Schultz)• Prenatal or Gestational Stage • Infancy• Juvenile Stage• Adulthood

In this model, stages are defined with reference to the appearance of the permanent teeth• Infancy ends and juvenile stage begins with eruption of

first permanent tooth • Juvenile stage ends and adulthood begins with

appearance of last permanent tooth


Tooth eruption patterns are useful for comparing stages of growth across species of primates, but they do not reflect patterns of growth unique to humans Five Stage Model of Growth (Bogin)• Prenatal or Gestational Stage• Infancy• Juvenile stage• Adolescence• Adulthood

*Adolescence marked by rapid growth and maturation of the body (species-specific adaptation)

Prenatal or Gestational Stage

• Period of rapid growth and development• Begins with conception and ends with birth

o About 40 weeks in humans

• Developmento Zygote: sperm + eggo Blastocyst: implants around day 6o Embryo: rapid grwth & cell differentiation (3-8wks)

oCNS development begins around week 3o Fetus: weeks 9 to 40 of gestation

• Length of this stage is not a function of body size• Susceptibility to teratogens (e.g. alcohol -> FAS)


• Humans are born with brains about the size of an adult chimpanzee

• Human head represents about 1/3 of total infant body size

• Human females give birth to infants whose heads are very large compared to the size of the mother's pelvis

• Chimps don't have this problem

If birth is so difficult, then why didn't female humans develop larger pelves?


• Too large of a pelvis would reduce bipedal efficiency, too small of a pelvis would restrict infant brain size

Evolutionary trade-off

• Shape as well as size of pelvis is a critical factor in delivery of a child: tight fit between newborn's head and pelvis & baby must rotate as it passes through the birth canal

• In 2001, 24.4% of all births were C-section deliveriesEvolutionary significance?

"Lucy"Australopithecus (~3.2 million yrs)

Evolution of the Pelvis

Homo erectus(~1.2 million yrs)

Homo sapiens (modern day)

353mm 385mm

Newborn head: 320-370mm

Newborn head: ~318mm

Juvenile Stage

• Begins around 5 or 6 yrs of age with appearance of first permanent tooth (Schultz model)

• Most occurs after brain size has reached adult proportions (around 6-7 years of age)

• Length of this stage varies widely among mammal species

• Growth during this stage is high but not exponential (leveling off in growth velocity)

Why do we have such long childhoods?

Juvenile Stage• Delays onset of sexual

maturity and ability to reproduce ("cost")

• Necessary training period in

which younger animals can learn their adult roles and the behaviors necessary to survive and reproduce ("benefit")

• Benefits of social life > costs of

delayed maturation

Adolescence

• Onset of puberty marks end of juvenile stage and beginning of adolescence

• Puberty here refers to the adolescent growth spurt, the period of rapid growth and maturation of the body (though literally means the appearance of pubic hair)

• Adolescent growth spurt not seen in other mammals

• Most children in industrialized societies go through puberty between ages 10 and 14, but it tends to occur earlier in girls than it does boys

Why do we need adolescence?

May also provide an extended time period for human children to better learn adult behavioral roles

"The complex social and cultural life of humans, mediated by language, requires an adolescence, which is an extended period of social learning and development" -Stanford and Allen, p476

In support of this, recent research has found that there is some growth in gray matter (neurons) of parts of the brain during puberty.

Interpreting this Graph:

• X-axis could read "growth velocity" • Exponential growth during infancy that slows

down during the first three years of life.• High growth velocity during juvenile stage

but it is leveled off• Puberty (growth spurt) occurs earlier in

females around 11-13 years of age; males tend to have this growth spurt later, around age 14.

• Growth trails off as individuals move into their adult years

Interpreting Previous Graph

• Shows percentiles for normal growth in boys (left) vs. girls (right), according to the CDC

• As you can see, boys on average have greater height and weight attainment than girls

Sexual Maturation

• Puberty is also characterized by maturation of primary and secondary sexual characteristics

Primary - anatomical parts involved in sexual reproduction

Secondary - distinguish between two sexes but not directly part of reproductive system

• Puberty generally occurs about 1 year before onset of menarche in females

• Menarche - the onset of a girl's first menstrual period; marks the beginning of her reproductive life

• Girls experience the growth spurt before becoming fertile, but the reverse is true for boys

Tanner Stages

of Sexual Maturation

Sexual Maturation

CNS

CNS

Menarche

PHVPHVB2,PH2

B5,PH5Adult ovulation

frequency

Interpreting the Previous Graph

• In both boys and girls, puberty begins with changes in the activity of the hypothalamus and other parts of the central nervous system ("CNS puberty")

• CNS also marks time when growth velocity changes from deccelerating to accelerating (3 years before PHV)

• In girls, first outward sign of puberty is the development of breast buds (B2) and appearance of pubic hair (PH2)

Sexual Maturation

CNS

CNS,G2

PHVT, PH2

Sperm

G5, PH5

Muscle spurt

Interpreting the Previous Graph

• In boys, pubertal development starts with increase in LH and the enlargement of testes, followed by enlargement of the penis (G2)

• About a year later, Testosterone levels rise (T), followed by appearance of pubic hair.

• About a year before peak height velocity (PHV) is achieved, sperm may be detected in the urine

• Adult stages of genital and pubic hair development follow the growth spurt (G5 & PH5)

Sexual Maturation

• From 1850s to 1970, the average age of menarche in Europe and North American population decreased from around 16-17 years to 12-13 years.

• In cultures undergoing rapid modernization, changes in the age of menarche of been measured over short periods of time:

Urban Bundi girls in Papua New Guinea: age of menarche dropped from 18yrs to 15.8 yrs in a twenty year period

• Menarche is earlier in children from well-off families than in the underprivledged; later in girls from larger families than those from smaller ones

Precocious Puberty

Onset of puberty at an unusually early age Medical definition: before age 8 in girls, before age 9 in boys Causes: Obesity? Chemicals? Growth hormones?

Implications for Life History Theory unclear

Adulthood

• When does it really begin?• Menopause - the post reproductive period in the lives of

woman, after the cessation of ovulation and menses (unique to humans and a species of whale)

• Humans are unique among mammals in having a significant part of life span extend past female reproductive years

Why do we have such long periods of post-reproductive life?

Artifact of extended lifespan?Grandmothering hypothesis (inclusive fitness)

Life History Theory

Life history theory - the study of an organism's strategy to allocate its energy toward growth, maintenance, reproduction, raising offspring to indepedence, and avoiding death Examples of some major life history characteristics include:– Age at first reproductive event– Reproductive lifespan and aging– Number and size of offspring

(adapted from Bogin 1999)

Life History Theory

Variations in these characteristics reflect different allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions.

For any given individual, available resources in any particular environment are finite. Time, effort, and energy used for one purpose diminishes the time, effort, and energy available for another. (adapted from Bogin 1999)

Life History Theory

Human life history, with nearly 2 decades of infant dependency prior to social and sexual maturation, has long been considered advantageous because it provides: 1.An extended period for brain development2.Time for the acquisition of technical skills (e.g. tool

making)3.Time for socialization, play and the development of

complex social roles and cultural behavior (adapted from Bogin 1999)

Growth and Adaptation

Let's get something clear first...

Adaptation - a trait that increases the reproductive success of an organism, produced by natural selection in the context of a particular environment Adaptation, in the context of evolution, happens over multiple generations as gene frequencies change toward a better fit with the environment (via natural selection)


BUT adaptation is a more general phenomenon, as all species exhibit some degree of:

Biological Plasticity - the ability on the part of individuals to physiologically respond to changes in the environment

Ex: when not enough food, an animal will become thinner

Ex: when exposed to a lot of sunlight a person's skin will tan

(More examples to come!)


Adaptability - the ability of an individual organism to make positive anatomical or physiological changes after short- or long-term exposure to stressful environmental conditions

Adaptability refers to physiological changes that occur in individuals who have lived under a certain set of environmental conditions; their bodies reflect influence of environment on development.

DO NOT CONFUSE ADAPTABILITY AND EVOLUTION

Adaptability is about phenotypic changes in response to environmental conditions, it does not necessarily increase reproductive fitness and does not change the organism’s genotype

Adaptability enables populations in different environments to differ phenotypically without changing their genotype, as individuals within the population adapt to same local conditions.

DO NOT CONFUSE ADAPTABILITY AND EVOLUTION

Adaptability - occurs in individuals (single generation) as phenotypic changes occur in response to environmental conditions

Adaptation - occurs in populations, over multiple generations, as certain traits that increase fitness within a certain environment are selected for (natural selection)

Adaptability occurs within a single generation via biological plasticity; adaptation occurs over multiple generations via natural selection (but

adaptation must be heritable and confer reproductive advantage)

Patterns of Human Growth

• Patterns of growth that emerge under different environments provide clear examples of biological plasticity

Fetal ProgrammingSecular Trends

• In medicine, any deviation from normal growth patterns is seen as evidence of ill health

• In anthropology, we challenge you to look at growth responses to non-optimal environments in the context of non-genetic adaptation or adaptability (i.e. responses to environmental conditions that can actually enhance survival)

Fetal Programming/Barker's Hypothesis

Fetus makes physiological adaptations in response to changes in its environment to prepare itself for postnatal life. This may explain the well established links between reduced birthweight and increased risk of coronary heart disease, diabetes, hypertension, and stroke in adulthood.

Secular Trend in Growth

• Observed trend of increased growth and maturation in children of industrialized countries (population level)

In Europe and N.America: increase in stature in children ages 5-6 of about 1-2cm per decade since 1900

Started in Japan post-WWII and is showing up in parts of developing world

• Result of better nutrition (more calories and protein in diet) and a reduction in the impact of disease during infancy and childhood

• The decrease in age of menarche is another hallmark of the secular trend in growth

Increase in overall height attainment among Swedish boys from 1883-1968. Notice also the earlier onset of puberty as indicated by the dotted arrows.

Declining age at menarche over the last few hundred years has been observed in many European countries and, more recently, in developing countries

Secular Trend in Growth

• Recent migration studies of Mayan refugees from Guatemala to the US provide evidence in support of the secular trend

Mayan children raised in California and Florida were an average of 5.5cm taller and 4.7kg heavier than their counterparts in Guatemala

• Such a rapid change in growth attainment (i.e. in one generation) shows that shorter stature among native Guatemalans is a response to environmental conditions, not a genotypic adaptation

• Individual stature is result of a complex interaction between genetics, SES, and nutrition

Environmental Influences on Growth (explained by Tanner)• Much of the mean differences in body size between

populations is due to the effects of environmental conditions; also may be true of differences between individuals within populations

• "A child may receive numerous insults during growth and yet survive, but bodily adaptations for survival are made which may result in smaller body size"

• "If a whole population receives similar insults, such as a diet low in calories or protein, a small mean adult size results; if environmental conditions improve, the size both of children and adults increases"

Climate Effects on Human Growth

Bergmann's Rule: body size and climate Describes a tendency of body mass variation within groups

(species), such that people in cold climates will have larger body masses than people in hot climates

Remember: as volume increases, surface area decreases as a proportion to volume

Larger animals have a smaller surface area to volume ratio than smaller animals, so they radiate less heat per unit of body mass, and stay warmer in cold climates

Higher surface area to volume ratio in smaller animals in hot and dry climates helps facilitate heat loss through the skin and helps cool the body

Children lose heat faster than adults because they have a high surface area to volume ratio


Allen's Rule: body shape (appendages) and climate Humans from cold climates tend to have shorter limbs than

humans from warmer climates This is about temperature regulation; if you have long legs

and arms, this increases your surface area and helps with body cooling in hot climates

If you have short arms and legs, then you will conserve heat better due to smaller surface area to volume relationship

Tutsi people of Africa have a large surface area to volume ratio (long limbs), which helps them release heat

Inuit people of Alaska have a small surface area to volume ratio (short limbs), which helps them retain heat

Final thoughts...

Be wary of our tendency to think "Bigger is Better" as this is not necessarily true when it comes to adaptation. For example, in temperate industrialized countries the best-off members are larger than the worst-off and taller women have more successful reproductive histories than short women; however, in Peruvian Andes, it is small mothers who have more surviving offspring (Tanner). Remember, adaptation to current environmental conditions is the important factor, as this will largely determine reproductive fitness.

anth40 growth&adaptation lecture

Documents