When examing new skeletal material the first thing to be done is to lay all the material out in correct anatomical position. This involves identifying the bones as to type (e.g. femur, mandible, vertebrate,etc.) and side (if applicable). As this is being done each bone is recorded on an inventory sheet. Once this is done a set of standard measurments (more about this below) is taken on whatever bones are in good enough conditon to measure. This is where the fun begins. There are certain other types of information that one wishes to obtain from the skeleton. In the early days of anthropology and archaeology the ethnicity, gender and stature (if possible)of the material was determined and not much else (and here I am being a little simplistic). Recent technological advances and the rise of forensic anthropology have changed things almost completely. Ethnicity, gender and stature are still recorded but there is more that we can learn (and some interesting wrinkles).
Before I go any further I need to talk about non-metric and metric methods of analysis. Non-metric traits are those that are, basically, either present ot absent. Below is a picture of a human pelvis. A probe is pointing to the greater sciatic notch.
Picture 1In females the notch is large in males it is somewhat smaller. The size of the sciatic notch has lead to "The Rule of Thumb". If you stick your thumb into the sciatic notch and have some room to wiggle it, it is female. If you have little to know room. it is male. This is another type of non-metric trait. In this case the non-metric trait is based on variable expression of the trait.
Metric traits are those that can, obviously be measured and there are quite a few of them in anthropology. Once upon a time they were used, and abused, with abandon. The cranial index (a measure of the length vs breadth of the cranium) is a good example. The cranial index was used widely during the early days of physical anthropology. Then it was realized that a wide variety of factors (including the environment) affect the shape of the human skull and these kinds of simplistic uses of metric traits were abandoned. These days anthropologists are much more sophisticated (more of that later). A good example of a metric trait is the size of the head of the femur.
Picture 2The diameter of the head of the femur is used to identify the gender of the skleton - diameters greater than 45mm indicate male, diameters smaller than 45mm indicate female (this method assigns gender accurately 93% of the time).
Whether one uses metric or non-metric traits in the analysis of skeletal material one should always take natural variation in populations into account. Most of the criteria for determing gender, ethnicity and stature were developed from two skletal populations (the Todd collection and the Terry collection). These collections were amassed during the 20's and 30's which means that that people makeing up the collections were born in 1800's - in some cases in the 1850's. Over time it was realized that the criteria were becoming less accurate. It turns out that things like better nutrition and healthcare were the cause. A second issue comes from applying these techniques to a different geaograhphic population than the one they were developed in. FOr example, criteria developed to identify ethnicity in the American Southwest are not as accurate when applied to populations in the great plains. With this in mind let us proceed to our analysis of the skeleton.
Gender
The following discussion concerning the determination of gender from the skeleton applies only to adult material. Determination of gender in subadult material is practically impossible. The criteria used to determine gender are reflections of secondary sexual characteristics that do not develop until puberty.
The best place to determine gender in the human skeleton is in the pelvis. In humans the pelvis been shaped by a compromise between bipedal locomotion and childbirth. Looking down on the pelvis in males the pelvic basin is narrower or heartshaped, whereas in females it is rounder.
Picture 3This is due to modification of the the three bones that compose the pelvis. In females the pubic bone is longer than in males, the sciatic notch is wider and sacroiliac articulation (the area where the sacrum joins with the ilium)
is raised.
Picture 4These three features increase the size of the pelvic basin and because of them there are certain other criteria diagnostic of gender. In the bottom row of picture three we see a ventral view of the male and female pelvis. Note the area labeled ventral arc. In females, due to the longer pubic bone, it is wider. On the other, dorsal, side the the pubic bone is a feature called the ventral arc - which occurs only in females.
Picture 5Returning to picture four you will notice a area labeled "pre-auricular sulcus" this feature occurs only in females (but not in all females). Another feature that aids in the assignment of gender are dorsal pits. These occur on the dorsal surface of pubic bone and are associated, along with scarring of the preauricular groove and scarring of the groove for the interosseous ligament, childbirth. However, there is no simple correlation betwenn the number of dorsal pits and the number of offspring born. There are few metric methods for determing gender from the pelvis and they are not used that frequently.
Turning to the skull there are a number of traits the, when viewed collectively, indicate gender. Starting with the shape of the mandible. In males the chin is squarish, in females more rounded and comes to something of a point. The skull proper, in males is more rugose with muscle markings being larger and rougher. The supraorbital ridges are larger in males - as are the mastoids. The upper edges of the eye orbits are sharp in females and blunt in males.
The posterior end of the zygomatic process (in the picture above the probe is touching the zygomatic process superior to the articular eminence) extends further back in males than females. Often, it extends past the external auditory meatus (the large whole for the ear).
Gender can also be determined metrically, although this requires the use of advanced statistics. Discriminant functions are the most frequently used method. Fortunately, for the mathmatically impaired quite a few discriminant fuctions have been compiled for this purpose so that all one has to do is plug the measurements into a formula. Several computer programs, such as Fordisc, have also been developed.
Ethnicity
This is one of the most controversial areas in physical anthropology. Back in the bad old days "race" was a rigid, typological catagory. These days the anthropological approach is much more sophisticated, based in a large part on the acceptance of populational thinking (imported mainly from biology). The impact of the concept of the cline can not be underestimated as well (for example skin color follows a cline, as do several other human traits). Be that as it may, how do we identify ethnic groups when all we have is a skeleton?
When I was in school discussions of ethnic affiliation revolved around three main groupings: Caucasoids, Negroids and Mongoloids. Towards the end of my time in school the focus became wider and more populations were included (mainly due to the efforts of Forensic Anthropologists who had to deal with a wider variety of populations in widely different regions). I will start with above three and then move outwards (in the next post of this series).
Caucasoid skulls are characterised by the following: nasal sill (running along the nasal aperture, retreating zygomatics (see picture) narrow nasal opening depressed nasal root (hard to explain and I couldn't find a picture), braincase with length and width nearly equal and minimal alveolar prognathism (if you were to lay a pencil lengthwise from the nasal opening to the jaw the pencil would touch the chin).
If you were to lay a pencil where the black line is in picture "c" you would be able to insert your finger in between the zygomatic bone and the pencil in caucasoid skulls. The zygomatic bones are angled slightly back towards the rear of the skull.
Negroid skulls are characterized by the following: high, straight forhead with bregmatic depression (bregma is the point where the coronal and sagittal sutures meet), long, relatively narrow braincase, wide interorbital breadth, poorly defined nasal margin (as opposed to having a sill) and alveolar prognathism (a pencil placed with one end on the nasal spin would not touch the chin. Note: the pencil tests and the "rule of thumb" come from Bass's influential Human Osteology: a Laboratory and Field Manual).
Mongoloid skulls are characterized by the following: projecting zygomatics (see above), edge to edge bite (unlike causasoid and negroid skulls which frequently have an overbite), shoval shaped incisors, nasal overgrowth, face realtively broad compared to height and less prognathism than caucasoid or negroid skulls. Mongoloid skulls can be difficult to distingish from negroid or caucasoid skulls.
The above lists of traits are subject to a certain amount of subjectivity. What one sees in a skull can be influenced by one's views on ethnicity. This is where metric analysis really earns it's keep. The most common techniques where pioneered by Giles and Elliot in their paper "Race Identification from Cranial Measurements" and Gill in his "A Forensic Test Case for a New Method of Geographical Race Determination". Giles' and Elliot's method is basically a three step process. First gender is determined. Then eight measurements are plugged into either of two formulas. The first formula distinguishes between American white and American black skulls (there are separate formulas for male and female skulls). The second formula distinguishes between American white and American indian skulls. If the skull could be any of the three then both formulas are used and the results are plugged into a plot. The x axis of the plot reflects the American white-indian scale. The y axis reflects the American white-black scale. The scores for the skull yielded by each formula are then used to plot where the skull is on the graph and the location yields the biological affinity. The result is accurate about 82% of the time in males and 88% of the time in females.
The main problem with the above method is that it tends to misclassify native americans. Gill's paper tries to address this. Gill focused on the midfacial region, that is, the region around the nose and eye orbits. He took six measurements and combined them into three indices. The score then yielded the biological affinity. This method can distinguish American whites from American indian skulls 80% of the time. The problem with the method is that it doesn't do a good job of distinguising between Eskimos, American blacks and American indians. It also doesn't do a good job of, say, distinguishing between plains populations of American indians and southwestern populations of American indians (I will take this up in more detail in the next post in this series).
Age
There are several different methods to determine age in a skeleton. If the skeleton belongs to a subadult one could use erruption patterns of the deciduous teeth or the examine the rate of epiphyseal closure.
The above photo shows the epiphysis of the femur before and after it unites with the femur. Different epiphysis unite with their bones at different times and the number of number of united epiphysis combined with the stage of the other epiphysis can yield and estimate of age.
There are also several different methods to determine age in adult skeletons. The most frequently used involves cranial suture closure. Generally, the coronal, sagittal and lambdoidal sutures are used. The picture below shows the location of the coronal and lambdoidal sutures (the sagittal suture runs along the top middle of the skull).
Each suture is examined on a 5 point scale running from completely open (0) to completely closed (4) a composite score of all the sutures is created which indicates age. Several variants of this method exist (some use different sutures of the skull and one uses suture closure of the palate).
One can also use the pelvic bones to determine age. For example, the surface of the pubic bone can be used to determine age.
An examination is made of the different aspects of the pubic bone. Each aspect is scored and the composite (as above) yields an age estimate. Above is a picture of some of the different stages. Note there are separate criteria for males and females. A similar technique can be used on the auricular surface of the ilium (mentioned above under gender).
The surface of the bone in this area undergoes some textural changes which are associated with the aging process. The criteria, such as granularity, striations and billowing are scored (as in the above techniques) and the composite score yields an age. Speaking from experience, this technique can be difficult to use.
Stature
Most techniques of stature determination are based on regression formulae. Basically, one measures the femur or tibia and plugs that measurement into the correct formula ( depending on ethnicity and gender) and you get the stature. To date no method has been developed to accurately predict weight based on the skeleton
The one area I haven't touched on is pathology. Which will be dicussed in the next post.
