(Note for the reader: the NOTES for the remainder of the course will be more eclectic in style. The overall themes are drawn from the Hillson text and the one by Harrison et al used previously in this course. Additional references are sited throughout and the reader with further interest in any of the topics is referred to those sources for further information.)
I. DEFINITIONS AND METHODOLOGY p 106
Dental occlusion is the way in which the teeth fit together. Normal occlusion is described as perfection, with all of the teeth regularly and symmetrically arranged in their respective arches. The author raises an interesting contradiction: this ideal is described as 'normal,' yet few people have it. Malocclusions (and occlusal variation) are departures from this norm. Our jaw position for chewing is asymmetrical-very different from that in centric occlusion.
The oldest method of recording occlusion is the Angle classification familiar to clinical dentistry. A system developed by the FBI was published in 1971 by Baume et al; frequent reference is made to it in the literature as a standard for recording measurements.
II. VARIATION WITHIN JAWS p 110
This is familiar fare for the practicing dentist: diastemata, irregularities and rotations; transposition of teeth, impaction, congenital absence, supernumeraries, and retained deciduous teeth.
Winged incisors are often mentioned as a non-metric trait. The term refers to the bilateral mesial rotation of permanent maxillary central incisors. It is particularly common in persons with Native American Indian ancestry.
Also note the statement that dental polygenesis is less common that agenesis.
III. VARIATION BETWEEN JAWS p 114
These variations included midline deviation, open bite or extreme overbite.
IV. DEVELOPMENT OF MALOCCLUSION p 115
The heritability of occlusal features has been studied extensively. Pay close attention to the summary conclusions: There is heritability, but it is low. Environmental factors are more important than genetic factors.
V. THE BEGG HYPOTHESIS
P. R. Begg was a student of Angle along with C. Tweed and A. Brodie. Following in the tradition of Campbell, Begg studied Australian Aborigines. He concluded that substantial occlusal and interproximal wear were normal and proper for the human dentition. Begg also noted the importance of vigorous chewing during the growing phase of the teeth and jaws. These themes are greatly expanded in article 7.1. The Begg hypothesis will be discussed shortly.
..... CJ '99
I. MEASUREMENTS OF PEOPLE
A. Mendelian and Polygonic Traits
Many of the traits of interest in studying human variation are complex traits. These traits are affected by growth and by the environment in which that growth takes place. Therefore, they reflect a much more complex relationship between genotype and phenotype (Relethford p 357).
-In the 'Two World of Genetics' we discussed some of the differences between Mendelian and polygenic traits. They can be briefly summarized as follows:
-one gene locus
-NO, not influenced by the environment
-many gene loci
-visible morphological traits
-'Gaussian' (normal) distribution
-YES influenced by environment by environment
We can summarize these remarks verbally. Mendelian 'simple' traits are NOT influenced by the environment and tend to be NOT ambiguous. Traits are either one or the other. Mendelian traits such as serology and biochemistry are invisible.
Polygenic traits are influenced by environment. Polygenic traits such as standing height are visible, measurable, and are influenced by environment.
Shortly we introduce the term anthropometry which literally means 'the measurement of people.' Orthodontists do it all the time. The discipline of anthropometry began with Sir Francis Galton, the brilliant and somewhat eccentric cousin of Charles Darwin. Galton found his measurements (such as standing height) distributed in a Gaussian (bell curve) fashion. He developed the statistical methods to analyze the data obtained.
In hybrid populations with mating between persons with very different skeletal quantitative traits, the distributions in the offspring are normal (also called Gaussian in British literature) distributions. There are NO bimodal distributions in the offspring. This is understandable when traits are controlled by several genes and are subject to environmental influence.
B. Sex Diagnosis in Skeletal Remains
There is an abundance of resource material available on this subject. This section is a brief introduction to the topic based on the resources listed below. Male and female traits in skeletons vary in degree of expression--just as you would expect of any polygenic morphological features.
The portion of the skeleton most useful for sex identification is the pelvis. By itself, the adult pelvis can be correctly identified in about 95% of cases; the skull between 85 and 90% of cases. Here is a brief summary of male/female pelvic differences:
Male Subpubic angle: <90 Pubic Shape: triangular Subpubic angle shape: convex Greater sciatic notch: <68 Sacrum: smaller & more curved Female >90 rectangular concave >68 larger and straighter
Subpubic angle: <90
Pubic Shape: triangular
Subpubic angle shape: convex
Greater sciatic notch: <68
Sacrum: smaller & more curved
larger and straighter
Sexing a Human Skull, Adult Male
a. the chin is square
b. the angle of the ascending ramus is close to 90
c. the root of the zygomatic arch is likely to extend beyond the auditory meatus.
d. the mastoid process is large
e. the external occipital protuberance is prominent.
f. there is a rough, robust skull
g. there are marked muscle lines
h. the forehead is receding
i. the brow ridges are prominent
j. there are rounded orbital margins
Sexing a Human Skull, Adult Female
a. the chin is rounded
b. the angle of the ascending ramus is obtuse
c. the root of the zygomatic arch is not likely to extend beyond the auditory meatus
d. the mastoid process is small
e. the occipital protuberances are poorly developed
f. there is a smooth, gracile skull
g. the muscle lines are not prominent
h. there is a vertical forehead
i. the 'brow ridges' are absent or are poorly developed.
j. The orbital margins are sharp
Buikstra, J. & Mielke, J. "Demography, Diet and Health" in The Analysis of Prehistoric Diets. New York: Academic Press, 1985.
Ghesquiere, J., Martin, D., and Newcombe, F. eds Human Sexual Dimorphism. London: Taylor and Francis, 1985. (This is included here as a more general reference on sexual dimorphism)
Helmuth, G. A Laboratory Manual in Physical Anthropology. Toronto: Canadian Scholars' Press, 1993.
Nelson, H. and Jurmain, R. Physical Anthropology 5th ed. St. Paul: West Publishing Company, 1995.
Schwartz, J. Skeletal Keys. New York: Oxford University Press, 1995.
Wolfe, L., Lieberman, L., and Hutchinson, D. Physical Anthropology Laboratory Textbook. Raleigh: Contemporary Publishing, 1994.
In its early days, physical anthropologists did a lot of measuring--particularly of the skeleton. We've recounted that Francis Galton founded biometrics-body measurements and the statistical treatment of those quantities.
Anthropometry is a reliable method for analysing growth. It can produce valid, objective information when applied to people.. It is non-invasive and its instruments are relatively inexpensive. Anthropometric data can provide reliable data, at least at the population level, of the growth of several different body tissues.
Its chief disadvantage is that traditional techniques only measure the surface of the body of the living (with the important exception of radiography).
Study of skeletal material has been used to study skeletal material, especially those of populations in the past. Bones were of course clean and neat to measure. At times, however, there was also the erroneous assumption that bones were less susceptible to the environment. Many lines of research have shown quite the contrary; bone is very susceptible to its environment.
(For an interesting treatment of anthropometry as applied to growth and development, see Uliajaszek et al, eds. The Cambridge Encyclopedia of Human Growth and Development. New York: Cambridge University Press, 1998 pp 26-29.)
A. Bone and Function
In order to illustrate the responsiveness of bone to environment, we report here the odd case of the bipedal goat. This is taken from Roberts, D "The Pervasivenes of Plasticity" in Mascie-Taylor, C. and Bogin, B. Human Variability and Plasticity. New York: Cambridge University Press, 1995 pp10-11.
The animal was born with phocomelia (underdeveloped forelimbs) so that for locomotion it had to hop like a kangaroo. In response to these special demands, the goat pelvis became modified to resemble that of a biped.
It is well understood that in humans, quite small reductions in function can profoundly affect cortical and trabecular bone volume in both younger and older individuals. Immobilization of part or all of the body results in bones that are lighter and thinner.
B. Applied Biotechnology and Ergonomics
A practical application of anthropometry is ergonomics which is 'biotechnology'--the adaptation of the physical environment to our biological needs.
One example is the design of stairs. Have you visited an imposing public building and found the stairs awkward to climb? For the majority of us, stairs with:
11" treads T
7" risers R
are ideal. Carpenters who build stairs apply this formula to their craft: T + 2R = 25
Ergometrics is essential in the design of everything from motorcyclist helmets to computer work stations.
In the design of chairs, designers have traditionally provided a width of 18 inches. As Americans get heavier, this may have to be increased. Some ferryboat operators have observed that more people have to stand these days; they can't fit into the standard width chairs provided on the boats.
C. Anthropometry and Measurement of Variation
Anthropometry, the measurement of the human body, is an essential component of the study of growth, development, and variation.
The International Biological Program (IBP) has a basic list of 21 measurements which requires but six instruments: (1) anthropometer, (2) spreading caliper, (3) sliding calipers, (4) tape measure, (5) skinfold caliper, and (6) and the (weight) scale.
There are two types of studies of growth.
In a longitudinal study the same individuals are measured repeatedly over time. These are rigorous, requiring time, resources, and determination.
In a cross sectional study the population is measured once with as many people as possible to obtain statistical validity.
This section is from Poirier, F., Stinie, W. and Wreden, K. In Search of Ourselves 5th ed. Englewood Cliffs: Prentice Hall, 1994.
There have been many indexes used in anthropometry. One we will mention shortly is the cephalic index.
Frequently encountered measurements in anthropometryare skin folds/subcutaneous fat; body composition (Archimedes principle); bioelectric resistance (measures fat); radiology (Greulich Pyle/cephalometry); (I125/bone loss); blood; karyotyping (chromosomes); work physiology; reflectance and many others (see Poirier, p 538)
Height and weight are probably the most common of the measurements. (from Relethford, J. The Human Species. Mountain View: Mayfield Publishing Company, 1997.)
Stature is easy to measure; however, it is anatomically complex (stature = legs + pelvis + vertebra + skull). It is measured standing or recumbent (lying down).
In all populations, women are five to 8% shorter than men. The components vary in different ethnic groups and this raises a question: is there a correlation between stature and geography?
There is no significant correlation between height and geography; both tall and short peoples are found in most of the world's major regions. The Mbuti pygmies of Africa are an especially short people; less well-known are diminutive folks in Malaysia and the Philippines. Birdsell (p 462 cited above) says that in Africa and a few rain forest mongoloid peoples have males under 5'.
The Tutsi and Nuer in Africa are very tall people. In the New World, the tallest people were in the Plains, East Coast; the shortest were in tropical central America.
Correlations have been established between stature and a variety of other measurements such as arm and leg length, abdomen, hip & shoulder girth, and weight. Stature correlates best of all with leg length. This has an application in archaeological studies.
The correlations between standing height and other body measurements offer a useful means of estimating height with skeletal material. This is especially useful in police investigation. This formula is especially useful: stature = 2.38 x femur length + 0.6141 femur length. That length is taken with the use of an osteometric board.
Technically, different equations are needed for the two sexes; possibly a different one for non-European populations is necessary. (See Harrison et al human Biology 3rd ed)
Weight is probably a better measure than height since it is volumetric (3D). Unlike stature, there is a geographical distribution by weight. There is a significant negative correlation between body weight and mean annual temperature for the various regions of the world.
The relationship between height and weight has a relationship to life expectancy and is of interest to life and health insurers.
The recommended Body Mass Index (BMI) is between 20 and 25. To calculate, multiply your weight in pounds by 700, divide by your height in inches, then divide by height again for your BMI.
Most adults in less developed societies carry very little body fat, while many North American sample populations carry considerable body fat. Some athletes have body fat as low as 5% in males, 10% in females. Extreme leanness depresses sperm production in males and ovulation in females-probably a factor in low fertility when humans lived as foragers. (See Shepard, R. Body Composition in Biological Anthropology. New York: Cambridge University Press, 1990.
Are you an apple or a pear? Divide your waist size by your hip size. The healthiest are <.80 for women, <.95 for men. (from Annual Editions Health 96/97)
2. The Length of Trunk and Limbs
Sitting height is taken with the subject seated, the back straight, sitting as erect as is possible. Lower limb length is approximated by subtracting this from total height.
Arm length is measured with the arm extended and from the acromion of the scapula to the tip of the 3rd finger.
A tendency for persons in tropic areas to have a linear build with long limbs whilst persons in the Arctic to have short limbs with a stocky build are expressed in Allen's and Bergmann's rules. We will save discussion of these variances for a future unit.
3. Measurements of Breadth
The width of the shoulders is from the two acromial points
Two measurements are used for the hip region:
(a) distance between the tuberosities of the iliac bones--the most lateral points on the iliac crest.
(b) bispinal breadth between the anterior superior spines of the ilia.
Circumferential measurements can be taken with tape at the thorax, abdomen, or hips.
There are quite pronounced differences between populations in the relations between transverse (side-to-side) measurements and stature. These measurements can be nicely illustrated in stick diagrams.
4. Anthropometry of the Head
Let us mention two kinds of anthropometry that are familiar to the clinical orthodontist.
a. Odontometrics is the measurement of teeth. The 'Bolton analysis' is a specific type of odontometrics with a clinical application. (For an especially comprehensive treatment of odontometrics, see Kieser, J Adult Human Odontometrics. New York: Cambridge University Press, 1990.)
b. Cephalometrics is literally 'measurement of the head.'
The following are measurements less familiar to the dental clinician.
c. Measurement of the Braincase
Measurement of the length and breadth of the head is one of the oldest measurements in anthropometry; it was introduced by Retzius over a century ago. The cephalic index is the breadth (at the parietal eminences) divided by the length (glabella, between eyebrows and the most posterior point on the occiput (opisthocranion).
This index distributes in a Gaussian fashion. Three general groups are commonly recognized.
Dolichocephalic (Gr long) is below 75
(Gr dolichos = long)
Mesocephalic (Gr in middle) 75-80
(Gr mesos = middle)
Brachycephalic (Gr short) above 80
(Gr brachys = short)
The cephalic index has been determined for a variety of populations (see Harrison et al Human Biology 3rd ed p 304).
Excellent diagrams appear in Boaz, N. and Almquist, A. Biological Anthropology Upper Saddle River: Prentice Hall, 1997 p 448)
The Cranial Index
The German-American anthropologist Franz Boaz undertook an important study of European immigrants to America in the early part of this century. Pioneering studies by Boaz and many since hae shown that showed that cranial index could change between parents and their offspring. Thus, the environment, and not genetics alone, could be important in determining the cranial index. Better nutrition and perhaps less illness due to infections and parasites seem to produce a larger size in first generation persons raised here.
(See Boaz & Almquist p 447, and Himes, J. "Secular Changes in Body Proportions and Composition" in Secular Trends in Human Growth, Maturation and Development. Chicago: University of Chicago Press, 1979. Boaz, the author of this text is NOT Franz Boaz; it is someone else.)
Has there been a trend in the cephalic index as seen in recent populations? To quote, "It is axiomatic in physical anthropology than man has become progressively round-headed or brachycephalic." See Roche, A., ed Secular Trends in Human Growth, Maturation, and Development Chicago: University of Chicago Press, 1979 p 43.)
d. Cranial capacity.
We often speak of it in hominid evolution; however, it really hasn't told us much about modern populations. In living individuals, braincase volume can be estimated from external measurements.
e. The Face.
Anthropologists measure total face height from nasion to the lowest point on the chin. Facial breadth is taken at the most lateral aspects of the zygomatic arches (bizygomatic width). Another width is the bigonial between the bony angles of the mandible.
III. POPULATION VARIATION AND THE NATURE/NURTURE PROBLEM
The pioneer work was by Franz Boas in 1911 in which he showed that the children of Sicilians, Russian Jews, and others reared in the USA were larger anthropometrically from the parent populations in Europe (cited above). Another famous study is of offspring raised in Japan who on average were considerably different and generally larger than their parents.
This has also been demonstrated with Mexican immigrants who themselves are a mixture of Indian and Spanish blood.
There is an important story here about immigrant prejudice. At the time Boa did his study, large numbers of Eastern Europeans were coming to America--there was strong anti immigrant feeling. The anti-immigration extremists sought to 'prevent genetic damage to the American population' and such data was meant as evidence of their inferiority. Studies in anthropology, however, demonstrated that when children were raised here, they resembled mainstream America.
Except for Native American Indians, we are all foreigners here!
Comment: environmental influence on growth was recognized by Boaz and has been by other investigators since that time.
Color, whether it is in teeth, skin, hair or eyes attracts our attention. The pigments have been difficult to analyze chemically. Geographical distribution of skin, hair, and eye color has been known for a long time.
A. Skin Pigmentation and its Color
Skin color depends on two main factors, the blood in the smaller vessels of the dermis--and the amount of dark pigment, melanin, in the dermis.
We often don't think of blood as a skin coloring agent, but oxygen deprivation does give a bluish color in fair-skinned persons, especially infants. It was a clinically important sign.
Melanin is made by melanocytes, which pass them onto epidermal cells. In dark skinned persons there are more melanin granules. Melanin pigments are widely distributed in plants and animals.
A suntan darkens lightly pigmented skin by stimulating the formation of additional melanin. Both light and dark skinned people do tan.
B. Measurement of Skin Color
Skin color is measured by reflectance spectrophotometry. The areas chosen are the middle of the forehead, back of the left forearm and the inner aspect of the left arm (it has the least exposure to the sun). The skin ought to be cleansed!
Light from a standard source is used. The reflected light is picked up by a photocell. The reflected light is compared to a pure white standard. Interestingly, the reflectance from highly melanized skin is the same as that from just the pigment in aqueous solutions. Field workers are urged to make sure that the skin is clean! (Poirier, p 550 cited above)
C. Inheritance of Skin Color
Skin color has been hard to analyze genetically. Possibly only three or four gene loci are involved in the control of skin color. Mating with respect to skin color is not random. The Internet advertisements by persons from India seeking an arranged marriage partner often say that they prefer 'a fair skin'--but it can work the other way, too. (My comment: once in a cultural anthropology informant report, the informant from Nigeria said that her parents would do a 'lights off' test on anyone they brought home!) Culture deeply affects mating patterns in many ways.
D. Skin Pigmentation and Natural Selection
In the Old World, the geographical distribution of skin pigmentation follows UV light exposure. There are exceptions. Among them are the pygmies who live in a deeply forested area--and have dark skins. Tasmanians south of Australia live in a temperate climate, and yet have had dark skins. In many areas outside of the Old World skin color does not correlate with latitude very well.
UV radiation is damaging to tissues. Light skinned people are vulnerable to skin cancer. Albinos born in tropical areas are especially vulnerable to skin cancer in equatorial areas. Many skin cancers in late life (I assume excluding melanomas) are rarely fatal. This brings up the Vitamin D hypothesis which is frequently proposed by anthropologists.
Vitamin D is produced in the skin and subcutaneous tissues when light penetrates and is absorbed there. The theory says that light skinned persons can better synthesize vitamin D and therefore a light skin is an advantage in northern latitudes. This issue is not resolved.
E. Hair Color
My receptionist once asked: why don't people have spots and stripes like dogs and cats? Do you have any ideas? Hair color is also caused by melanin granules. Spectrometry is (also) used to measure hair color.
In most parts of the world, hair colors are dark. North west Europe is the exception. Hair color often changes in childhood. Chinese and Africans have dark hair in youth. (This was evident to me when I visited Peru; the Quechua infants and small children had intensely black hair.)
Skin, hair, and eye color in general vary together, but there are exceptions. (see Birdsell p 478 cited above)
Blue-eyed Irishmen with black hair are common--maybe due to gene mixing & Viking repopulation after the last Ice Age. Green eyes often go with red hair. (see Poirier et al p 566)
As a rule, but not always, darkly pigmented skin is accompanied by dark hair. It is possible for two blue-eyed parents can have a brown-eyed child. (Poirier et al p 566)
A very unusual discordance in colors occurs in Australian Aborigines. Blonde hair (called tawny in the literature) is found most frequent in the center of Australia and is significant in this way. It is produced by a single gene locus. It is therefore Mendelian and NOT polygenic. It is dominant over the allele for dark brown hair. It is very different from European blondness. The homozygote for tawny hair--remains tawny (blonde) into adulthood; in the heterozygote, it fades at puberty.
It is suggested that it has some advantage. Whatever that advantage might be is unknown. Some speculate that if Australia had remained isolated, eventually all Aboriginals would have all become tawny blondes.
Similar blondness has appeared sporadically in New Guinea and Western Polynesia, but it never spread widely. (see Birdsell pp 479-481) The variation in lightness and darkness is probably controlled by many genes. Red hair seems to behave as a recessive depending on a single gene--but even with red hair there is a gradation of colors.
Eye color--the color of the iris depends in part on the granular pigment and in part on optical effects. Eye color is difficult to record accurately and is difficult to analyze genetically. Several genes seem to be involved, tho in general dark iris color is dominant over lighter color.
Albinism isn't a single genetic disease. It is at least three different types. In all three, abnormalities of the visual pathway to the brain produce a quivering of the eye (nystagmus) and poor depth perception. (see Poirier et al p 565)
V. HAIR FORM
Natural head hair form shows striking geographical differences. It is suggested that hair form may play a role in head thermoregulation but this is not confirmed. People also vary in the amount of body hair.
VI. Finger- and Palm Prints
Francis Galton was the first to study fingerprints and investigate their inheritance. The major patterns are under genetic control, but are not explicable by any simple system of inheritance.
There is high correlation between monozygotic twins. The ridge patterns of the palm also show individual and population differences.
VII. Mental Attributes
If there is any part of the body that illustrates environmental influence it is the brain. (see Lewontin, R. Human Diversity ch 6 New York: Scientific American, 1995 and Nash, J. "Fertile Minds" Time Feb 3, 1997. and Wall Street Journal 9 Apr 97 )
Nurturance affects brain capacity. Good nurturing and stimulation in the first three years of life activate more neural pathways.
Stimulation providers improves a child's scores on cognitive and language tests. On the other hand, poor child care impairs children for life.
There is a critical learning period for language--our most fundamental symbolic behavior. A second language can be learned even more quickly by a child during that time. If a first language isn't learned early, it never can be learned well.
Sensory experience seems to play an important role in brain wiring. Starting at age ten, the brain begins pruning unused connections. Children who don't play much or are rarely touched tend to develop smaller brains. Rats in challenging environments develop more synapses per neuron. Even older rats show brain connection differences in response to environment.
These observations have important parental and societal implications. We aren't born as blank slates--we interact and continually rewire in response to the environment. Talking to, holding, and reading to infants makes a difference. With deprivation, the ability to rebound declines quickly.
Intensive intellectual activity into advanced age seems to help the brain to continually rewire itself as shown in studies of Kentucky nuns. Early sensory experiences stay with us for life in many ways as illustrated by animal studies.
If a kitten's eye is covered for a time, the visual cortex will not develop correctly. If some song birds do not hear the species song in early life, they cannot sing. Goslings will 'imprint' on whatever they see during their critical time.
We take IQ tests and college entrance exams for granted; however, they owe their origins to helping developmentally disabled children. The IQ test was developed in France by Alfred Binet (1903) to identify children in need of remedial study. IQ tests tend to be culture-specific and have a built-in cultural bias. IQ tests tend to predict career success rather well, but for complex reasons.
Also, persons with a high IQ will seek out educational experience and benefit more from it.
Heritability of IQ is difficult to determine, tho it is highest with dizygotic twins. Studies of twins separated at birth have only been of limited help in disentangling nature from nurture..
One of the most effective ways to raise IQ is by adoption; strong evidence is available to support this finding. Success of a child correlates just as well with high family status as it does for years of schooling. Incidentally, wealth, status and power also seem to run in families. (see Lewontin Human Diversity 1995)
Resource for this unit:
Harrison et al Human Biology 3rd ed. Ch 12
In a very general way, polygenic traits tend to have more functional significance than do simple Mendelian ones. Those traits are probably acted upon by evolution more directly than are simple Mendelian traits such as the more obscure blood typing systems.
Variances between populations are rather small compared to variances within populations. The difference between continental groups amount to 5%. Variation tends to merge in clinal fashion between populations. Geographic genetic variation (such as ABO blood types) has long interested anthropologists and they have stimulated much speculation about their differential advantages in certain climates or geographic settings.
Genetic systems offer intriguing possibilities for relatedness--when put together as clusters and drawn out as dendrograms. Such clusters suggest, based on biochemical evidence, that modern humans emerged from Africa 50,000 years ago.
The Basques and Lapps seem to be geographic isolates as suggested by their language and genes.
Polynesians (and many Native Americans) are prone to diabetes and obesity when living on modern diets. This may reflect 'thrifty genes,' that confer survival in impoverished environments. (The Pima Indians have the highest incidence of type II diabetes in the USA).
Native Americans (Eskimos are not Indians according to some) resemble some Asiatic groups from whence they came; the founder population probably lacked B blood groups). The origin of Native American Indians remains a vigorous debate within anthropology.
..... CJ '99
Harrison et al Human Biology 3rd ed. Ch 13