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Human Evolution and PREHISTORY Chapter Thirteen: MODERN HUMAN DIVERSITY Link to the Canadian Association for Physical Anthropology COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Chapter Preview 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? COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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. COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Distribution of Skin Colour COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Environmental Problems Increased exposure to radiation, e.g. X-rays, CT scans, radioactive wastes Are the safety regulations adequate? COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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 COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. 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. COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED.