postcranial skeleton

The postcranial skeleton is defined as lying posterior to the cranium.  Visual morphology and pathology of bone shows there are generally two responses to any activity related stress, either bone is added or bone is removed.

 

The visual aid of high levels of sexual dimorphism of the postcranial skeleton enables researchers to reconstruct activities around behaviour, relating to subsistence strategies for prehistoric populations.  A significant decline in obvious sexual dimorphism is thought to be positive evidence of a substantial change in activity patterns.  However, other factors must also be taken into account (Wescott, 2006), such as biological, genetic and environmental, in order to form a more accurate picture of the reconstruction to take shape.

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It is also possible to reconstruct activities from looking at the pathology of the postcranial skeleton and again identify behavioural differences, such as gendered segregation.  By looking at male and female samples, pathological diseases such as Osteoarthritis/Degenerative Joint Disease (DJD) can be identified and enable researchers to identify patterns in gender segregation of work and other possible cultural differences.

 

This paper will focus primarily on activities related to subsistence as opposed to any other type of activity. It establishes how sexual dimorphism, environmental and cultural factors are reflected within the landscape of the bone, assisting in the interpretation of economic diversification over different time periods.  This paper will look at studies over a diverse range of historical times, including modern human remains.

 

 

Morphology

 

Musculoskeletal Stress Markers (MSM) have been used to identify habitual activity patterns relating to where the muscle, tendon or ligament was once attached.  Muscle use is an important factor in the remodelling of bone and maintaining strength (Weiss, 2012).  ‘Wolff’s Law’ states that when a muscle insertion is subjected to stress, what results is a significant increase in blood flow, resulting in the stimulation of cells.  These cells form new bone, resulting in hypertrophy.  However, there are disputes around bone remodelling as a result of trauma, or whether remodelling could be a result of repetitive action, without stress.

 

Taking into consideration analysis of skeletal human remains from archaeological and paleontological investigations, Ruff’s (2005) study of earlier humans in comparison to observations of Anatomically Modern Humans (AMH) showed a significant decrease in skeletal strength throughout the evolutionary process.  Ruff suggested this has been due to an increase in sedentism and a change in hunter-gatherer lifestyles.  He also noted that the pelvic structure and hip mechanical loadings impacted upon femoral shape and that activity patterns can affect both upper and lower limbs, in terms of shape and strength.  However, these patterns varied in terms of features by age.

 

 

Ruff identified that over the past two million years, there had been a decline in bone strength, leading to a 30% decrease in average femoral strength, proportional to body size.  In a comparison with modern tennis players, he found it possible to increase bone strength by mechanical loading and remodelling.  Therefore, Ruff suggested that skeletal strength is not one that has declined through evolution but that human bone continues to retain the ability to be strong and stimulate cells for additional bone growth in modern times.  This is consistent with a permanent evolutionary change to AMH bones.  Evidence has shown a steady decline in differences between males and females, beginning at hunter-gatherer groups, through to agricultural societies and onwards into the more modern industrial era (Ruff, 1987).

 

Ruff’s (1983) paper detailed an analysis of cross-sectional geometric parameters in two samples.  These were comprised of remains from the modern US population and remains from a late prehistoric/early historic population in Pecos Pueblo, New Mexico and focussed upon the bending strength of 10 specific femoral and tibial locations.

 

Clearly males and females are sexually dimorphic in terms of obstetric pelvic structure, therefore it is to be expected that females have a strengthened proximal femoral shaft, due to the M-L distance between hip joints.  This difference continues to be evident in cross sectional bone shape, more so in the area of the distal femur and into the knee region.  Ruff (2005) found this dimorphism became stronger dependent upon subsistence strategies.

 

Cross-sectional geometric properties were identified by sectioning the length of the femur and tibia, measuring from the distal end to the mid-femoral neck.  These measurements on the bone took place at 20%, 35%, 50%, 65% and 80% of the tibia and femur across both sample sets (Ruff, 1983b).  The structural characterisation shown in differing bone shapes and are likely due to methods of mechanical loading and patterns of behaviour.

 

 

Taking the ‘Biomechanical Model’ into consideration, theoretically, the more sedentary the group, the more the sample should exhibit a greater circular and gracile femoral diaphysis and less sexual dimorphism.  This correlates with Ruff’s modern US sample, as differences in gender were greatly reduced.  Females showed greater M-L bending strength in the proximal femur diaphysis across both samples.  Results from Ruff’s study indicated that females, throughout time, had no large changes in cross-sectional shape of the femoral midshaft.  However, males became more circular.  Using the study to contrast with other samples across time, it showed bones to be considerably more marked during the hunter-gathering and agricultural times.  The anteroposterior/mediolateral (a-p/m-l) loading of the lower limbs declined in males.  This is indicative of a society that was once mobile and divided its labour according to gender.  It would appear to have taken place as far back as the mid-Palaeolithic (Ruff, 1987).

 

Wescott’s (2006) study supported the theory that sexual dimorphism is greater in mobile populations but whilst Ruff linked the steady decline in dimorphism to a change in divisions of labour throughout the years, Wescott theorised a strong correlation between lower limb structure and ‘Terrestrial Logistic Mobility’ (TLM).  His study found less sexual dimorphism in the femur midshaft morphology was not always present in his sample and suggested that mobility patterns can be influenced by genetics, climate, terrain, age and varying levels of activity.

 

 

In sum, Wescott argued that the femoral midshaft shape and robusticity did not respond to what was to be expected of the Biomechanical Model.  Whilst dimorphism was greater amongst mobile groups, sexual division of labour should be left open to debate and differences in femoral shape, should be addressed whilst analysing the wide range of factors mentioned above.

 

In Marchi’s (2008) paper on the cross-sectional geometric properties of the femur and tibia at the midshaft, a Neolithic sample from Liguria in Italy was compared with other European wide samples from the Late Upper Palaeolithic, Mesolithic and Chalcolithic, in order to investigate relationships between lower limb mobility and robusticity.  Marchi does not dismiss the division of labour within this group, however in the absense of any archaeological evidence, drew upon modern ethnographic studies of contemporary herding societies, to confirm sedentism involving gendered division of labour, e.g. males involved in herding and females with domestic duties.

 

 

Marchi suggested that whilst his Ligurian sample consisted of a pastoral lifestyle, the high robusticity markings implied rugged terrain.  This could be related the samples appearing consistent with evidential traits of earlier highly mobile populations, similar to the comparative LUP sample.  The sexual dimorphism of the Ligurian sample was very high.  The overall picture provides complex and varying activity patterns.

 

In 2006, Stock (2006) conducted a climatic study of hunter-gatherer groups, where lower limbs were analysed.  It was found that the distal tibia correlated with diaphyseal strength and mobility, whilst the proximal femur correlated with climate.  This has been further substantiated by Ruff (1994) who claimed that proximal limb bones are climatically determined due to their influence by body shape, as climate will be an intervening factor in terms of what activities have taken place.

 

Marchi et al (2006) also evaluated the Neolithic Ligurian population’s humerus and upper torso.  These findings resulted in significant levels of sexual dimorphism within the population, implying varying degrees of mechanical loading between the sexes.  The division of labour was more liberally suggested due to the high sexual dimorphism in strength exhibited.  Furthermore, taking into account archaeological findings, faunal remains and ethnographic studies, Marchi et al theorised repeated action by only males.  This was recorded to be the frequent, intense and bimanual use of axes.

 

The male section of the sample exhibited high incidence of bilateral enthesopathy at the insertions of the costoclavicular ligament, biceps, latissimus dorsi and pectoralis major (try and find a diagram).  Furthermore, frequencies of dorsal and bisulcate patterns of the axillary border of the scapular was also recorded.  It has been suggested that repeated adduction and abduction of the upper limb was responsible for changes. 

 

 

Pathological Diseases

 

 

 

 

 

 

 

Osteoarthritis is generally identified by researchers as being a process of degeneration, which can increase in speed by mechanical stress factors.  Many researchers believe pitting and eburnation on the surface of the facet joint to be majorly characteristic of osteoarthritis (Ortner, 1981) (see Appendix A for examples).

 

 

Habitual occupational or recreational activities may induce changes.  Both pathological and abnormal changes were evidenced on the skeletal remains recovered from the Mary Rose ship (Stirland, 1997).  Dramatic changes taking shape in the lower spinal segments.  These changes were predominately located in the vertebral columns of men, aged late teens to their late twenties, identified via the epiphyseal line.  Stirland and Waldrum (1997) used a comparison with a medieval grave site in Norwich (Sirland, 1996).  It was identified that Norwich sample would not have had the same occupation as those on board the Mary Rose, they were found to have had almost identical pathological changes.  However, the major notable difference was that the Norwich sample, of mainly executed prisoners, were considerably older than the semi-permanent Mary Rose crew, many of whom were recruited as early as adolescence.

 

The Mary Rose sample provided evidence that degenerative changes can be triggered by stress over a significant period of time, as found to be the case of lower spine segments of a repetitive activity.  

 

Again, as with robusticity, these changes to postcranial bone can indicate behavioural patterns that are significant changes in subsistence and in turn, assist in identifying patterns associated with division of labour.  For example, it is apparent, amongst hunter-gatherer and agricultural prehistoric groups, that males are consistent with occurrences of Osteoarthritis at all major appendicular joints (Ruff, 1987).  This is in direct contrast of studies on modern samples of US white and black females. The modern sample, compared to the prehistoric, shows relatively little by way of sexual dimorphism.  Furthermore, in stark contrast, females within the modern sample showed slightly elevated levels of osteoarthritis.

 

As it appears that sexual dimorphism declines significantly with the intensification of agriculture, the modern sample not only displays a lack of significant gender differences but the picture of industrialised societies appears particularly blurred. Paradoxically, an increase in osteoarthritis was reported more so amongst females.  This provides a clear division across the spectrum of time, in terms of gendered differences relating to activities.

 

Derevenski’s comparison of Ensay and Wharram Percy, was specifically chosen in contrast of each other, as both communities were known for their distinct division of labour.  Evidence, on both samples, was visible of activity related stress on the spines.  The paper analysed osseous change of the apophyseal facet and osteophytosis of the vertebral body across the range.

 

The Ensay sample of 51 individuals, based in the Outer Hebrides of Scotland, was dated to be circ.16th to 19th Century CE.  The Wharram Percy site, in the Yorkshire Wolds, provided 59 samples dating to the medieval period.

 

The Ensay sample showed higher levels of stress in comparison to Wharram Percy.  The facet sclerosis/eburnation, osteophytosis and remodelling was indicative of disruption to normal patterns of osseous change to the spine and was thought to be evidence of the use of baskets known as ‘creels’. Creels (Appendix B) have been identified as being in use during this timeframe, taken from historical observations in 1796 and 1833 (Derevenski, 2000).  Creels were supported by a woven strap that was placed around the shoulders and across the breastbone.  The main bulk of the weight rested upon a ‘creel pad’, which would take a maximum weight capacity of up to 36kg.

 

Males in Ensay also showed markers for stress, however this was thought to be due to physical exertion of a demanding and difficult terrain.  This appeared to be evidence of agricultural work and not heavy lifting, with steep terrain creating difficulties for the use of traditional horse and cart.  Wharram Percy had a predominantly agricultural economy but unlike Ensay, had a less remote community and terrain that was much more amenable to traditional transportation methods.

 

It is widely thought, by historians, that Medieval labour was expected to be divided according to gender, as a matter of social complexities.  However, historical records do confirm that flexibility around this was permissible within peasant communities.  Males and females have been shown to have taken opposite tasks, maybe according seasonal demand, e.g. males dairying and females ploughing.

 

In contrast to Ensay, females within Wharram Percy are less affected than males throughout the spine.  However, application of the Chi-square value suggested that there were no significant differences between the sexes relating to the spine in Wharram Percy, whilst in Ensay, the variations of stresses at different points on the spine, suggested that each gender was subjected to different variants of stress.  For example, distribution of osteophytes showed an even pattern in the lower half for males, whilst females displayed an increase in frequency in the lower thoracic and lumber areas.

 

In sum, with both genders showing significantly more skeletal stress than those from Wharram Percy, it is necessary to conclude a link with the environmental and cultural factors, as well as gendered division of labour.

 

 

Conclusion

 

The changes that affect the morphology and pathology of bones can create a picture of lifestyles once adopted by groups and indicate behavioural patterns, whether enforced by culture, environment or biological necessity.

 

The MSM of a bone are able to provide researchers with indications of muscle use and remodelling of bone and increase in stress.  By comparisons between genders and over vast time periods, it is possible to see strength remodelling decrease and decline in accordance with major changes in subsistence and divisions of labour.

 

The cross-sectional genetic study, comparing the early Peco’s population with a modern US population, is also able to determine differing bone shapes.  These shape changes are likely to be indicative of patterns of behaviour associated with mechanical loading.  Ruff’s long-term study identified that females had no significant changes throughout time, while males were shown as more circular, declining in strength as society became less mobile.

 

However, it is also apparent that a distinct correlation exists between mobility patterns and factors of genetics, climate, terrain, age and activity/occupation.  This was evident in the remains of the Ligurian sample, which although was strongly sedenistic, displayed markings that appeared to be similar to those of a LUP hunter-gathering society.

 

As with occurrences of strength and bone increase, the removal of bone or changes to bone structure can also be evident in terms of pathological diseases.  Whilst looking at male and female data sets together is helpful, in order to see the segregation of gendered work, it is also apparent that gender and terrain can be found to be majorly impacting upon markers for stress, such as the Ensay sample.  Whilst the use of ‘creels’ can provide evidence of segregated work types, the difference in comparison to the terrain of Wharram Percy can also be clearly evident in the Ensay males, in terms of physical exertion from the rough terrain.

 

Looking at male activities in isolation of females can also assist in identifying pathological diseases and whether intensive, repeated activity could potentially enhance degenerative changes, as appears evident in the comingled remains of the Mary Rose crew.

 

What has been established is that it is possible to deduce lifestyle activities on the postcranial skeleton by looking at the morphological and pathological evidence.  Sexual dimorphism of remains is crucial to identifying facts in bone robustness and sexual division of labour that took place over time.  However, terrain, culture, subsistence, major economic changes and climate are all impacting factors and provide a wider, more complete picture, established from the visible signs of bone remodelling, decline and changes in bone shape.

 

This paper has concentrated upon identifying activities that correspond with subsistence.  In conclusion, researchers studying the postcranial skeleton across time, including later historical and modern samples, have found it is possible to utilise the remains in collaboration with other areas of study.  These areas, such as ethnographical evidence and historical records, add value to a more comprehensive picture.  This in turn can assist in understanding the lifestyles of prehistoric societies, where there is minimal supporting evidence available.  The continued study of the remains of ancient postcranial skeletons is crucial to understanding subsistence of the past, as behavioural and morphological variation differs greatly in comparison to modern samples.