Aside from some recent technological mishaps (now resolved!), which has resulted in a lack of posts recently, I’ve also been doing some preliminary research into human skeletal aging and human biological aging in general. Partly this has been out of general interest, but it was also background reading for a small project that I was working on over the past few months.
Knowledge of the aging of the skeletal system is of vital importance to the bioarchaeologist as it allows age estimates to be made of both individuals and of populations (and thus estimates of lifespans between generations, populations and periods) in the archaeological record. The aging of human remains, along with the identification of male or female biological sex (not gender, which is socially constructed) and stature in adults, when possible, provides one of the main cornerstones of being able to carry out a basic demographic analysis of past populations – estimates of age, sex, stature at death, the construction of life tables and the construction of mortality profiles of populations, etc. At a basic level inferences on the funerary treatment on individuals of different ages, and between different periods, can also be made. For example, in identifying the possible differential treatment of non-adults and adults in funerary customs or of treatment during their lifetime as revealed by their burial context according to their age-at-death.
Growing Pains
However, aging is not quite straight forward as merely understanding and documenting the chronological age of a person – it is also about understanding the biological age of the body, where the body undergoes physiological and structural changes according to the biological growth stage (release of hormones influencing growth, maturation, etc). Also of importance for the bioarchaeologist and human osteologist to consider is the understanding of the impact and the implications that the environment (physical, nutritional and cultural) can also have on the development and maturation of the skeletal system itself. Taken as such aging itself is a dynamic process that can depend on a number of co-existing internal and external factors.
For instance, environmental stresses (i.e. nutritional access) can leave skeletal evidence in the form of non-specific markers of stress that can indicate episodes of stunted growth, such as Harris lines on the long bones (identifiable via x-rays), or episodic stress periods via the dentition (the presence of linear or pitted enamel hypoplasias on the teeth) (Lewis 2007). Knowing what these indications look like on the skeleton means that the bioarchaeologist can factor in episodes of stress which may have led to a temporary cessation of bone growth during childhood or puberty, a period where the bones haven’t achieved their full adult length, due to a lack of adequate nutrition and/or physical stresses (White & Folkens 2005: 329).
It is recognised that humans have a relatively long adolescence and that Homo sapiens, as a species, senescence rather slowly. Senescence is the process of gradual deterioration of function that increases the mortality of the organism after maturation has been completed (Crews 2003). Maturation simply being the completion of growth of an individual themselves. In an osteological context maturation is complete when the skeleton has stopped growing – the permanent dentition, or 2nd set of teeth, have fully erupted, and the growth of the individual skeletal elements has been completed and the bones are fully fused into their adult forms.
This last point refers to epiphyseal growth and fusion, where, in the example below, a long bone has ossified from several centres (either during intramembranous or endochondral ossification during initial growth) and the epiphyses in long bones fuses to the main shaft of the bone, the diaphysis, via the metaphysis after the growth plate has completed full growth following puberty (usually between 10-19 years of age, with females entering puberty earlier than males) (Lewis 2007: 64). Bioarchaeologists, when studying the remains of non-adults, rely primarily on the development stage of the dental remains, diaphysis length of the long bones (primarily the femora) and the epiphyseal fusion stage of the available elements in estimating the age-at-death of the individual (White & Folkens 2005: 373).
A basic diagram showing the ossification and growth of a long bone until full skeletal maturation has been achieved Notice the fusion points of the long bones, where the epiphysis attaches to the diaphysis (shaft of the bone) via the metaphysis. Image credit: Midlands Technical College. (Click to enlarge).
After an individual has attained full skeletal maturation, the aging of the skeleton itself is often reliant on wear analysis (such as the wearing of the teeth), or on the rugosity of certain features, such as the auricular surface of the ilium and/or of the pubic symphysis, for instance, dependent on the surviving skeletal elements of the individual. More general biological post-maturation changes also include the loss of teeth (where there is a positive correlation between tooth loss and age), the bend (or kyphosis) of the spinal column, and a general decrease in bone density (which can lead to osteoporosis) after peak bone mass has been achieved at around 25-30 years old, amongst other more visible physical and mental features (wrinkling of the skin, greying of the hair, slower movement and reaction times) (Crews 2003).
Gaps in the Record
There are two big gaps in the science of aging of human skeletal remains from archaeological contexts: a) ascertaining the age at which individuals undergo puberty (where the secondary growth spurt is initiated and when females enter the menarche indicating potential fertility, which is an important aspect of understanding past population demographics) and b) estimating the precise, rather than relative, age-at-death of post-maturation individuals. The second point is important because it is likely that osteoarchaeologists are under-aging middle to old age individuals in the archaeological record as bioarchaeologists tend to be conservative in their estimate aging of older individuals, which in turn influences population lifespan on a larger scale. These two issues are compounded by the variety of features that are prevalent in archaeological-sourced skeletal material, such as the effects of taphonomy, the nature of the actual discovery and excavation of remains, and the subsequent access to material that has been excavated and stored, amongst a myriad of other processes.
So in this short post I’ll focus on highlighting a proposed method for estimating puberty in human skeletal remains that was published by Shapland & Lewis in 2013 in the American Journal of Physical Anthropology.
Identifying Puberty in Human Skeletal Remains
In their brief communication Shapland and Lewis (2013: 302) focus on the modern clinical literature in isolating particular developmental markers of pubertal stage in children and apply it to the archaeological record. Concentrating on the physical growth (ossification and stage of development) of the mandibular canine and the iliac crest of the ilium (hip), along with several markers in the wrist (including the ossification of the hook of the hamate bone, alongside the fusion stages of the hand phalanges and the distal epiphysis of the radius) Shapland and Lewis applied the clinical method to the well-preserved adolescent portion (N=78 individuals, between 10 to 19 years old at death) of the cemetery population of St. Peter’s Church in Barton-Upon-Humber, England. The use of which spanned the medieval to early post-medieval periods (AD 950 to the early 1700) (Shapland & Lewis 2013: 304).
All of the individuals used in this study had their age-at-death estimated on the basis of dental development only – this is due to the strong correlation with chronological age and the limited influence of the environment and nutrition has in dental development. Of the 78 individuals under study 30 were classed as probable males, 27 as probable females and 21 classed as indeterminate sex – those classed as a probable male or female sex were carefully analysed as the authors highlight that assigning sex in adolescent remains is notoriously problematic (the ‘holy grail’ of bioarchaeology – see Lewis 2007: 47), therefore only those individuals which displayed strong pelvic traits and were assigned an age under the 16 years old at the age-at-death were assigned probable male and female status. Those individuals aged 16 and above at age-at-death were assigned as probable male and female using both pelvic traits and cranial traits, due to the cranial landmarks being classed as secondary sexual characteristics (i.e. not functional differences, unlike pelvic morphology which is of primary importance) which arise during puberty itself and shortly afterwards (Shapland & Lewis 2013: 304-306).
The method involves observing and noting the stage of each of the five indicators (grouped into 4 areas of linear progression) listed above. It is worth mentioning them here in the sequence that they should be observed in, together in conjunction with the ascertained age at death via the dental analysis of the individual, which is indicative of their pubertal stage:
1) Mineralization of the Mandibular Canine Root
As noted above dental development aligns closer with chronological age than hormonal changes, however ‘the mineralization root of the mandibular canine may be an exception to this rule’ (Shapland & Lewis: 303). This tooth is the most variable and least accurate for aging, aside from the 3rd molar, and seems to be correlated strongly with the pubertal growth spurt (where skeletal growth accelerates during puberty until the Peak Height Velocity, or PHV, is reached) than any of the other teeth. In this methodology the stage of the canine root is matched to Demirjian et al’s (1985) stages, where ‘Stage F’ indicates onset of the growth spurt and ‘Stage G’ is achieved during the acceleration phase of the growth spurt before PHV (Shapland & Lewis 2013: 303).
3) Ossification of the Wrist and the Hand
The ossification of the hook of the hamate bone and of the phalangeal epiphyses are widely used indicators in medicine of the pubertal stage, however in an archaeological context they can be difficult to recover from an excavation due to their small and discrete nature. The hook (hammulus) of the hamate bone (which itself can be palpated if the left hand is held palm up and the bottom right of the hand itself is pinched slightly as a bony protrusion should be felt, or vice versa if you are left handed!) ossifies during the acceleration phase of the growth spurt in both boys and girls before HPV is attained. The appearance, development and fusion of the phalangeal epiphyses are also used to indicate pubertal stage, where the fusion has been correlated with PHV in medical research. With careful excavation the epiphyses of the hand can be recovered if present.
4) Ossification of the Iliac Crest Epiphysis
As this article notes that within orthopaedics it is noted that the ‘Risser sign‘ of the crest calcification is commonly used as an indicator of the pubertal growth spurt. The presence of an ossified iliac crest, or where subsequent fusion has begun, can be taken as evidence that the PHV has passed and that menarche in girls has likely started, although exact age cannot be clarified. The unfused iliac crest epiphyses are rarely excavated and recorded due to their fragile nature within the archaeological context, but their absence should never be taken as evidence that this developmental stage has not been reached (Shapland & Lewis 2013: 304).
5) Ossification and Epiphsyeal Fusion of the Distal Radius
The distal radius epiphysis provides a robust skeletal element that is usually recovered from archaeological contexts if present and unfused. The beginning of the fusion is known to occur during the deceleration phase of puberty at around roughly 14 years of age in females and 15 years of age in males, with fusion completing around 16 years old in females and 18 years old in males (Shapland & Lewis 2013: 304).
Results and Importance
Intriguingly although only 25 (32%) of the 78 individual skeletons analysed in this study had all five of the indicators present, none of those presented with the sequence out of step (Shapland & Lewis 2013: 306). The initial results indicate that it is quite possible to identify pubertal growth stage for adolescent individuals in the archaeological record based on the preservation, ossification and maturation stage of the above skeletal elements. Interestingly, the research highlighted that for all adolescents examined in this study from St. Peter’s Church in Barton-Upon-Humber indicated that the pubertal growth spurt had started before 12 years of age (similar to modern adolescents), but that is extended for a longer time than their modern counterparts (Shapland & Lewis 2013: 308). This was likely due to both genetic and environmental factors that affected the individuals in this well-preserved medieval population.
Further to this there is the remarkable insight into the possible indication of the age of the females entering and experiencing menarche, which had ramifications for the consideration of the individual as an adult in their community, thereby attaining a probable new status within their community (as is common in many parts of the world, where initiation ceremonies are often held to mark this important stage of sexual fertility in a woman’s life). This is the first time that this has been possible to identify from skeletal remains alone and marks a landmark (in my view) in the osteological analysis of adolescent remains.
As the authors conclude in the paper the method may best be suited to large cemetery samples where it may help provide a ‘broader picture of pubertal development at a population level’ (Shapland & Lewis 2013: 309). Thus this paper helps bridge an important gap between childhood and adulthood by highlighting the physiological changes that individuals go through during the adolescent phase of human growth, and the ability to parse out the intricate details our individual lives from the skeletal remains themselves.
Bibliography
Crews, D. E. 2003. Human Senescence: Evolutionary and Biocultural Perspectives. Cambridge: Cambridge University Press.
Lewis, M. E. 2007. The Bioarchaeology of Children: Perspectives from Biological and Forensic Anthropology. Cambridge: Cambridge University Press.
Shapland, F. & Lewis, M. E. 2013. Brief Communication: A Proposed Osteological Method for the Estimation of Pubertal Stage in Human Skeletal Remains. American Journal of Physical Anthropology. 151: 302-310.
White, T. D. & Folkens, P. A. 2005. The Human Bone Manual. London: Elsevier Academic Press.
These Bones Of Mine 