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Human Evolution
and PREHISTORY
Chapter Thirteen:
MODERN HUMAN DIVERSITY
Link to the Canadian Association for Physical Anthropology
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What Are The Causes Of Physical Variability In
Modern Animals?
Is The Concept Of Race Useful For Studying
Human Physical Variation?
Are There Differences In Intelligence From One
Population To Another?
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PHYSICAL VARIABILITY
For most traits within the gene pool of Homo
sapiens there are variants of genes (alleles), thus
creating a rich potential for new combinations
of traits in future generations
Therefore, Homo sapiens is a polymorphic
species, e.g. the A-B-O blood system
Being polymorphic allows for the possibility of
adapting to many different environments
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PHYSICAL VARIABILITY
Polytypic
When a polymorphic species is divided into
geographically dispersed populations and genetic
variability is unevenly expressed
e.g. distribution of skin colour
Evidence suggests that the human species has been
polymorphic since Homo erectus migrated out of
Africa
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THE MEANING OF RACE
Early anthropologists tried to classify humans into
races on the basis of physical features, e.g. nose size,
skin colour
The problem was that no “pure” types could be
found and visible traits were found to occur over a
continuum
Within-group variation was found to be greater than
between-group variation, e.g. skin colour in Africa
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RACE AS A BIOLOGICAL CONCEPT
In biology, a race is defined as a
population of a species that differs in
the frequency of different variants of
some gene or genes from other
populations of the same species.
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RACE AS A BIOLOGICAL CONCEPT
1. Race is an arbitrary concept, i.e. there is
no agreement on how many genetic
differences it takes to make a race
2. No population exclusively “owns” a gene
or variants of a gene
3. Individuals of one race will not
necessarily be indistinguishable from
those of another
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ORIGINAL STUDY
Race Without Colour
Different populations of humans are visibly different
Geographically variable traits have arisen by natural
selection, sexual selection, accidental mutation
Therefore, if, for example, we used lactase-deficiency
as a means of classifying humans, we would have
different “races” than if we used skin colour to
classify them
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THE CONCEPT OF HUMAN RACES
1.
2.
3.
4.
Traditional racial classifications are based on a poor
understanding of the genetics of our physical traits
Race is also a cultural category, e.g. the terms used in
the Canadian census of 2001
The confusion of biological and social factors becomes
a basis for excluding whole categories of people from
certain roles or positions in society, even to the
extreme of extermination
Canada became officially multicultural in 1971, a
context that accounts for human diversity
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THE CONCEPT OF HUMAN RACES
Pressures exist in the scientific community to apply the
concept of race
Race and ethnicity data for clinical drug trials
Use of socially defined racial categories in
forensic identification
** for most anthropologists the concept of race has
no biological utility in understanding variation;
they study trait and gene frequencies and their
distribution
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Some Physical Variables
1. Much of human biological variation is related to
climatic adaptation, e.g. correlation between
body build and climate
2. People in northern climates tend to have greater
body bulk relative to their extremities than
people in southern climates
3. Nose and eye shape and the epicanthic eye fold in
populations of Asian origin also show a climatic
adaptation
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SKIN COLOUR: A CASE STUDY IN
ADAPTATION
Factors associated with skin
colour:
1.
2.
3.
4.
Transparency or thickness of
skin
Carotene pigment
Reflected colour from the
blood vessels
Amount of melanin pigment
in a given area
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Natural Selection and Skin Colour
Melanin protects against ultraviolet solar
radiation
Natural selection has favoured dark
pigmentation at the equatorial latitudes as
protection against intense UV radiation
UV radiation can cause severe sunburn and
decomposition of folate
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Natural Selection and Skin Colour
Natural selection has favoured light
pigmentation in northern latitudes
With little melanin in the skin the UV
rays can penetrate and stimulate
vitamin D production, thereby
preventing calcium regulation
problems, e.g. rickets
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Evolution of Skin Colour

It is likely that lightly pigmented skins are a recent
evolutionary development in humans and dark skins are
likely ancient

However, comparison of populations outside the tropics
suggests that 60,000 years is not enough time to produce
advanced depigmentation

The combination of a high frequency of sweat glands and
lack of heavy body hair would have been advantageous on
the savanna as a means of eliminating excess body heat

With little body hair, selection would have favoured dark
skins
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Distribution of Skin Colour
Inheritance of skin is complex and not well understood;
therefore genotypic and allelic frequencies cannot be
studied in the context of distribution
However, the phenotypic geographic distribution tends to
be continuous (see Figure 13.3 in text)
The exceptions have to do with population movements
and/or the practice of selective mating
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Distribution of Skin Colour
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THE SOCIAL SIGNIFICANCE OF
RACE
1. Racism is a social problem based on
emotions and best explained in terms of
collective psychology
2. The racist responds to social stereotypes,
not to known scientific facts
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Race and Behaviour
It is typical for certain “races” to be attributed with
certain characteristics, e.g. “cold” Scandinavians
These behavioural characteristics can be explained in
terms of culture, not science
As an example, high crime rates are associated with
certain groups, but an examination shows that
underlying factors of poverty, justice and inequality of
opportunity are the cause
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Race and Intelligence
In the past intelligence tests were used to
answer the oft-asked question of whether
some populations are more intelligent
than others
Today these tests are considered to be of
limited use because they are applicable
only to particular cultural circumstances
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Race and Intelligence
Studies such as those with twins have indicated that
there is a degree of heritability with intelligence; but no
one knows what it is
It is clear, however, that environment is important
There is no general agreement on what intelligence is;
is it the results from an I.Q. test?
Intelligence cannot be measured and any hypotheses
about population differences cannot be proven
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Evolutionary Medicine
Human ills are related to incompatibility between
current lifestyles and environment and those
conditions under which humans evolved
•
E.g. coughs, fever, diarrhea are useful to the
body; when should they be stopped?
•
E.g. infant crying, in the absence of a physical
problem, is a fitness-maximizing behaviour to
bring the parent into contact
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CONTINUING HUMAN BIOLOGICAL
EVOLUTION
Today cultural adaptation has reduced the importance of
human physical variations developed through biological
adaptation,
e.g. vitamin D-fortified milk
Culture has imposed its own selective pressure through
specific cultural practices, e.g. global warming and the
spread of malaria into northern regions
Aspects of culture such as medical technology and financial
status can determine one’s fitness
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Lactose Tolerance
The ability to absorb lactose in milk depends
on the presence of the lactase enzyme
Failure to retain lactase into adulthood is
controlled by a recessive allele
There are significant population differences
for lactase retention and lactose tolerance
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Lactose Tolerance
A high lactase retention is found in
populations with a long tradition of
fresh milk in the diet
The use of milk supplements for
those populations, if suffering from
malnutrition, can cause great harm
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Thrifty Genotype
Until 6,000 years ago all humans were characterized by a
thrifty genotype
This genotype permitted efficient storage of fat and conserved
glucose as well as nitrogen for times of need
The nonthrifty genotype was selected for by regular access to
lactose (e.g. milk), as a protection against adult-onset diabetes
Populations that are lactose intolerant retain the thrifty
genotype and the incidence of diabetes soars when they are
introduced to Western-style diets high in sugar content
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Environmental Problems
1. The ozone screens out some of the sun’s UV
rays and today the ozone is deteriorating due
to the use of chlorofluorcarbons in aerosol
sprays, refrigeration and air conditioning,
and manufacture of Styrofoam
2. The thinning ozone layer is directly related to
the rising incidence of skin cancer
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Environmental Problems
Increased exposure to radiation, e.g.
X-rays, CT scans, radioactive
wastes
Are the safety regulations adequate?
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Environmental Problems
Increased exposure to mutagenic agents, e.g. pesticides,
hormone-disrupting drugs
1. Poisonings, deaths and cases of cancer can be
attributed directly to pesticides
2. Synthetic estrogen (DES) is routinely added to
animal feed, and cancer and reproductive
problems/deformities are related directly to DES
3. At least 51 chemicals are known to disrupt
hormones, e.g. in certain plastics, detergents, giant
water jugs
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Implications for Humans
Early onset of female puberty and
dramatic declines in human sperm count
It is obvious that cultural practices are
currently having an impact on human gene
pools.
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