Osteology - Skeletal Variation

• All living humans have unique physical characteristic
• This variation is also seen in bones of the human skeleton and dentition

» There are 5 major sources of variation –

• Ontogeny (= growth)
  • Ontogenetic differences in size and shape during growth from foetus to adult
  • Basis for estimating age from skeletal remain
•  Sexual Dimorphism (= two shapes)
  • Variation in shape and size between males and females
  • Basis for assessing sexual identity
• Population (= spatio-temporal)
  • Differences between groups separated by space (geographic distance) or time
  • Basis for inferences about ancestry
• Idiosyncrasy (= individual variation
  • Differences in bone size, robusticity, proportions, form and so on between individuals of the same sex, age and population

As we get older the skeleton 'spaces out' so to speak. Variation can be seen in the changing size of the body. For example, we can see:

• Growth in disphysis - 
  • Primary ossification centre
• Osteogenesis and growth of epiphysis
  • Secondary ossification centre

Further, we can see changes in proportions:

• Face
  • Elongates through age and fully develops skeletal features (proportion)
• Cranial vault
  • Considerably changes shape
• Teeth
  • Estimate age through the use of growth, eruption, loss of deciduous teeth, growth, eruption of permanent teeth 

Sexual dimorphism can affect: size, morphology, and colour. In humans we specifically see:

• Sex
  • Individuals biological identity
• Gender
  
  • Social identity; masculine/ feminine/ neutral/ non-binary

Human skeletons show moderate sexual dimorphism.

In very general terms, female skeletal elements tend to be smaller-sized, lighter with less rugged muscle markings (gracile)

Male skeletal elements tend to be larger, heavier, with more rugged markings (robust)

BUT:

• In any population there are small males and large females
• Normal sexual variations can be visualised as two overlapping disribution curves

Sexual dimorphism is the skeleton is most extreme in skull and pelvis. Published figures indicate 8-95% accuracy (byers 202:171) 

So, what are the main dimorphic differences between male and female human skulls?

• Male skulls have a crest on the occipital protuberance - which females have neither of
• Males have a 90-degree gonal angle whilst females have a more obtuse angle to the gonial angle
• Males have a brow bone with a flat frontal bone whilst females have no brow bones with a curved frontal bone

This can be looked at further with the Frankfurt horizontal

When it comes to prehistoric populations the differences in the cranial length, but with modern populations we have to ask what actually makes a population. Is it: County? Geography Language? Religion? Something else? All of them?

For example, consider the word 'hispanic'...

• Some moder Cubans show:
  • Strong African affinity followed by Spanish but lack indigenous Ameridian affinity
• This suggests a complete replacement of the indigenous Cuban population
  • However, some Mexian are:
    • Lacking both the African and Spanish components but show a strong indigenous Ameridian affinity
• This displays a different biological pattern entirely!

This shows that with modern standards of population we cannot simply cut it down to specifics and need broader terms when working with this kind of variation. 

What does idiosyncratic mean?

• Idio- (Greek: personal)
• -syn- (Greek: with)
• -crasis (Greek: combination)

Idiosyncratic variation is the combination of unique individual characteristics

• Idiosyncratic variations are different between people of the same sex or age! Many factors are involved:
  • Genetics
  • Disease
  • Nutrition
  • Epigenetics/Environment

There are many different forms of cranial deformation as a result of cultural variation. For example:

• Circumferential deformation; wrapping reap of vault
• Anteroposterior deformation; pressure at rate of vault

There are other kinds of cultural variation though that do not involve the cranium. For example:

• African tribal tradition of neck rings - Depressing the clavicals rather than elongating the neck
• Asian tradition of foot binding (China) - Breaking the foot with a rock to bind it into resembling a lotus flower 

In modern times and in Western culture we do have our own examples of cultural variation such as:

• Scarification - burning, defrosting, cutting & removing skin
• Piercings
• Tattoos

When it comes to skeletal variation there are some strong conclusions that can be drawn upon.

1. There is no 'typical' in human osteology

• There will always be differences in our osteology for multiple reasons and some similarities even might not even be unique or representitive to the population

2. Many speciments may be repsentative

3. Variation in biological structures is normal and to be expected

4. Many variation have multiple possible origins

5. There are five main types of variation that exist in the skeleton