In the following series of blog posts I aim to cover each of the main skeletal elements. Each post will have a single focus on a bony element, from the skull down to the bones of the foot. Firstly though we must deal with the variation that human osteologists and bioarchaeologists will see in individual skeletons, and in a population series. It is both useful and informative to learn the differences and the effects caused by the 4 main variation factors in the morphology of human bones. As it is by ascertaining the degree of influences that the variation factors can cause that we can begin to understand the individual, and the skeletal series of a population, in a more informative and considered way. The second part of this entry will focus on the basics of biomechanics, and the influences certain lifeways can have on bone.
The basic biology of bones was previously discussed in this post here, and of teeth here. Bone in its natural state must be recognised as a changing living organism throughout life that responds to stress, both nutritional and mechanical, and remodels accordingly. It is also must be remembered that bone is a composite material, and is able to heal itself.
Variation 1 : Ontogeny
Ontogeny is simply growth and development of an organism, in this case Homo sapiens. The archaeological record of skeleton remains include unborn individuals right through to individuals in their 70th year and beyond. Typically there are 7 classification groups of human age groups. They run from Fetus (before birth), Infant (0-3 years), Child (3-12 years), Adolescent (12-20 years), Young Adult (20-35 years), Middle Adult (35-50 years), and Old Adult (50+ years) (White & Folkens 2005: 364). Differences in bone structure, and in the growth of different bone elements often manifest themselves in changes in size and shape.
Adult with Two Juvenile Remains, From Southern Sahara
Basic Growth Profile for Homo Sapiens, Notice the Large Cranium and the Way the Body Catches Up.
Variation 2: Sexual Dimorphism
Humans are sexually dimorphic, that is there are differences between the female and male body size. Although not as distinct between our cousins such as the gorillas, female human skeletal remains are relatively smaller in both bones and teeth size (Jurmain et a 2010). Such skeletal variation is also manifest in the requirement of reproductive functions in the female skeleton, thus we are also often able to tell sex from skeletal remains (White & Folkens 2005: 32).
Generalised Male:Female Sexual Dimorphism
Variation 3: Idiosyncractic Differences
The idiosyncratic (or individual) differences found in skeletons are simply natural variations, in the understanding that every body is different, and rarely are people exactly the same (identical twins excluded). Idiosyncratic differences in bone affect the size and shape of the bone, and the topography of the bone surface. Again, such variation is very common in human skeletal remains (White & Folkens 2005: 32).
Disarticulated human bone from the site of Armana, ancient Egypt.
Variation 4: Geographic or Population-Based
As White & Folkens point out ‘different human groups can differ in many skeletal and dental characteristics’ (2005: 32). Thus this geographic variation can be employed to assess population affinities between skeletal series. This trait can be quite useful in determining commingling of certain populations in prehistoric skeletal series as certain environmental and genetic traits can be passed on.
Biomechanic Basics
So these are the four main variations we should be aware of when we are looking and studying individual skeletons or a series of a population. By considering these four main variations we can study the individual’s life pathway alongside other lines of investigation. What we must also take into account next are the basics of biomechanics. Biomechanics is the application of engineering principles to biological materials, whilst remembering that bone can remodel and change according to pressures put upon the bone. As Larsen states that ‘the density of bone tissue differs within the skeleton and within individual bones in response to the varying mechanical demands’ (1997: 197). It must be remembered that the response of human bone to ‘increased loading is in the distribution of bone (geometric) rather than density or any other intrinsic material property of bone’ (Larsen 1997: 197).
Importantly it is noted that Human bone is anisotropic, meaning its mechanical properties vary according to the direction of the load. Importantly, Wolff’s Law highlights how bone replaces itself in the direction of functional demand. A classic example of the remodelling capabilities of bone is that of the tennis player who has thicker cortical bone in their dominant arm. This manifests itself in thicker cortical bone alongside hypertrophy of the muscle attachment sites. One study carried out found that ‘males have a 35% increase in the cortical bone in the distal humerus of the playing arm vs the non-playing arm’, helping to exemplify Wolff’s Law (Larsen 1997: 196). That study was an example of bilateral asymmetry humeral loading. Alongside, it is the action of the main forces acting on human bone that help to change the bone, these include a) compression, b) tension, c) shear, d) torsion & E) compression + tension+ bending.
Wolff’s law states that healthy load bearing bone (LBB) responds to strain by ‘placing or displacing themselves (at a mechanical level) in the direction of the functional pressure, & increase or decrease their mass to reflect the amount of functional pressure’, often muscular strain and/or weight bearing pressures (Mays 1999: 3). As a part of this Frost (2004: 3) argues that the ‘mechanostat’, a tissue level negative feedback system, involves ‘two thresholds that make a bone’s strains determine its strength by switching on and off the biologic mechanisms that increase or decrease its strength’. However, Skerry (2006: 123) has argued that there are many ‘mechanostats’ operating on the LBB and that different elements throughout the skeleton require different strain magnitudes for maintenance. Furthermore Skerry (2006: 126) also notes that differences are apparent between the sexes, and that genetic constitution, concomitant disease, exercise & activity patterns must be considered.
A recent article has also highlighted how the femoral neck width of obese people changes to accommodate the added weight. In this case the width of the femoral neck has increased to dissipate weight throughout the bony area by increasing surface area and strength through redistribution of bone. This is an example of active bone remodelling adapting to changes that the person has gone through in life.
An archaeological example of the above will now be taken from Larsen 1997. ‘In the Pickwick Basin of northwestern Alabama, analysis carried out on both femora and humeri cross-sectional geometry has helped to reveal a number of differences between earlier Archaic Period hunter-gatherers and later Mississippian Period agriculturalists‘ (1997: 213). From the femora measurements it seems that the both female and male agriculturalists had a greater bone strength, whilst analysis of male humeri shows little difference between the two series. This has helped to show that activity levels increased for males but only in the lower limbs, as evidenced by the cross-section geometry. However, for females of both time periods both humeri and femora strengths increased. The article cited in Larsen (1997), Bridge 1991b, findings indicate that changes are from a greater range of activity undertaken by females than males. With palaeopathological signs of osteoarthritis, it is concluded that the shift to food production,in particular maize production, may have had a relatively greater impact physically on women in this setting.
The next post will focus on ethics in human osteology, and from there we will consider each of the anatomical skeletal elements in context of their relative limb.
Bibliography
Frost, H. M. 2004. (A 2003). Update on Bone Physiology and Wolff’s law for Clinicians. Angle Orthodontist. February 2004. 74 (1): 1-15.
Jurmain, R. Kilgore, L. & Trevathan, W. 2011. Essentials of Physical Anthropology International Edition. London: Wadworth.
Larsen, C. 1997. Bioarchaeology: Interpreting Behaviour From The Human Skeleton. Cambridge: Cambridge University Press.
Mays, S. 1999. The Archaeology of Human Bones.Glasgow: Bell & Bain Ltd.
Skerry, T. M. 2006. One Mechanostat or Many? Modifications of the Site-Specific Response of Bone to Mechanical Loading by Nature and Nurture. Journal of Musculoskeletal & Neuronal Interaction. 6 : 122-127. (Open Access).
White, T. & Folkens, P. 2005. The Human Bone Manual. London: Elsevier Academic Press.
These Bones Of Mine 