Anth40 growth&adaptation lecture

How We Grow
A 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 and
development 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

Stages of Human Growth
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

• Development
o Zygote: sperm + egg
o Blastocyst: implants around day 6
o Embryo: rapid grwth & cell differentiation (3-8wks)
o CNS development begins around week 3
o 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?

Anth40 growth&adaptation lecture


• 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 deliveries
 Evolutionary significance?

Evolution of the Pelvis
Newborn head: Newborn head:
~318mm 320-370mm

353mm 385mm

"Lucy" Homo erectus Homo sapiens
Australopithecus (~1.2 million yrs) (modern day)
(~3.2 million yrs)

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

B2,PH2 PHV
PHV

CNS
Menarche

B5,PH5
Adult ovulation
CNS 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

Sperm
T, PH2 PHV
G5, PH5
CNS

Muscle
spurt

CNS,G2

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.


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 development
2.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

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 Programming
 Secular 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

More Related Content

What's hot (20)

Viewers also liked (16)

Similar to Anth40 growth&adaptation lecture (20)

Anth40 growth&adaptation lecture

Editor's Notes