Analytical Methods for Dating Human Bones.


1. Introduction

An aging process is inherent in organic material. See Carbon 14 dating (Libby, W. F. 1947). After death the radioactive C14 is not replenished from the atmosphere. There occurs disintegration at a constant rate. The quantity is halved after 5,570 years which is the Libby Value. There is a practical limitation of radiocarbon dating because the certain minimum quantity of organic carbon must be available in the sample specimen.

Calcined bone is undateable, whereas charred bone is potentially dateable. The absolute age of a skull or mandible is usually obtained indirectly when it comes from a deposit containing more suitable for radiocarbon dating.

Contemporaneity is not always justifiable due to mankind’s long established practices of burying the dead. The relative age of a fossil bone is determined by comparison of the chemical composition with that of other fossil bones of known ages. Either from the same site or some other area if they have been preserved under comparable conditions. As soon as bones are buried their composition is subject to chemical changes, some of which are slow, some fairly rapid.

2. Buried organic matter.

Fats, protein (collagen) and the fatty composition are lost quite rapidly. The protein disappears much more slowly. Under some conditions, such as permanently frozen soil or exclusion of air and bacteria, the protein may persist for tens of thousands of years.

The appearance and texture of bone is not a reliable guide to how much organic matter it contains. The organic content of fossil bones has become widely regarded as an unreliable criterion for their antiquity. Some bones that appear to be well fossilised do in fact contain considerable amounts of protein. Relative ages can be determined by comparison of the contents that include nitrogen, carbon, and chemically bound water. Also the quantity of mineral ash after burning which increases with age. It is convenient to assess the residual organic matter in fossil bone or dentine by determination of the nitrogen content. As bone protein or collagen decays it becomes fossilised and broken down into amino-acids. These are leached out or retained according to local conditions.

3. Changes in mineral matter of buried bones.

This depends on the composition of the percolating ground water which is of two kinds. Alteration of the phosphatic material of which bones are mainly composed is hydroxyapatite. The addition of new mineral matter, such as lime or iron oxide, changes the latter and leads to an increase in weight. The most valuable change however is the irreversible substitution of one element for another in the hydroxyapatite. These two elements are thus fluorine and uranium.

The fluorine is distributed is soluble fluorides in trace quantities in all ground waters. Over the passage of time bones and teeth in permeable deposits progressively accumulate fluorine. The fluorine becomes fixed in the bone and is not readily removed which provides the fluorine dating method. With regard to uranium mineral phosphates, including bones, all contain uranium.

4. Dating fossil man.

dating fossil mankind requires correct placing in the Pleistocene and Holocene periods. Also the determination of morphological evidence in terms of the hominid phylogeny. Also the correct ecological information of the sites. Considerations of fossil age raises two questions. Firstly the relationship of geological, faunal, archaeological sequences at the site in terms of chronological age in years BP. Secondly, the remains have to have contemporaneity with deposits. The association of faunal, climatic and archaeological information derived from the site can be the same as the specimen.

This allows assessment of relative age and answers the second question or determination of absolute or chronometric age. To determine contemporaneity evidence is required to show there is no unnatural disturbance of the specimen, no intrusive burial, and no derivation from older deposits. This requires photographs in situ and stratigraphic drawings. Furthermore, two chemical tests are needed. Buried bone accumulates fluorine by a rate determined by local conditions. These include soil concentration and length of burial. This shows also contemporaneity of hominid bones and animal bones buried at the same site. Nitrogen  decreases in buried bone and affects nitrogen concentrations. Therefore nitrogen disappears rapidly under oxidising conditions and may be absent.

5. Relative and absolute dating.

With regard to relative dating Comparative geology includes correlations with soil pollen content. Studies of the fossil fauna assemblage, comparison with other sites, and the different layers at the same site. Also comparison and recognition of stone implements. This allows the establishment of the relative position of the specimen in the geological, climatic, faunal and archaeological sequence.

Absolute dating implies chronometry and is the oldest method in use. The upper atmosphere, nitrogen, and the bombard of cosmic radiation results in a known proportion of radioactive carbon which is incorporated into atmospheric carbon dioxide. The occurs absorption of CO2 by vegetation and thence by animal tissues. Burial of bones means Carbon14 begins to decay at a known rate. Mathematical translation of the C14 content of organic material gives an estimate of specimen age. In practice the Carbon14 technique is limited to specimens 60,000 to 70,000 years of age. This is because the C14 content is too small then to estimate.

Another radiometric method is the Potassium-Argon technique or K/Ar. Naturally occurring potassium (K) contains an isotope which decays at a constant rate and produces Argon. This argon is held within the crystals of some potassic compounds. Argon content estimation of deposits around the specimen will give the indirect estimation of the age of specimen. In practice the method is limited to 20,000 years of age specimens.

6. Cave sediments and prehistory.

Cave excavations demand not only knowledge of prehistory but also of geological observation and research. Caves are places of manifold geological occurrences and the preservation of sediments. The exact analysis of the sedimentary accumulation in a cave permits the deduction of the sequence, and the causes of geological events.

Nearly all caves and cave deposits date from the later Quartenary period and the Upper Pleistocene and Holocene. Mankind has frequented and used rock shelters at all times and in many areas. In these locations the lived, buried their dead, and used them as ritual sites.

7.  Sex determination in earlier humankind.

No single measurable character of any bone will serve by itself to distinguish two different individuals, either racially or sexually. Critical examination of the combination of all of the characters gives results of value.

Anthropologists, often confronted with the problem of sexing bone find the only practical way is accomplished by combining measurements. These are those obtained by combing morphological observations in a fashion as logically valid as possible. From a genetical point of view observation and measurement of characters is required.

If observations and measurements are in keeping with normal patterns of growth, morphology, and the function of bone, they may prove to be of value in sex determination. We are still far from understanding to what extent, genetic, environmental, hormonal, or other factors are responsible for the final shape of bone. It is not advisable to trust to a single, metrical or morphological character.

In earlier populations, as in modern ones, we have examples of varying sex ratios. For prehistoric groups the sex ratio, combined with differential fertility and mortality, can greatly modify the outward appearance of a population. It shows therefore that it is not uncommon to find unequal proportions between sexes. Factors constantly change sex ratio so that this changes from one age to another, one generation to another. It follows that what is found in one archaeological stratum may not serve as a pattern for any postulations about human remains found in adjacent strata, whether above of below.

Sexual dimorphism in prehistoric remains and various modern humans do not have the same degree of sexual dimorphism. The pronounced sexual differences in size is a primitive condition. Upper Palaeolithic populations in Europe are similar to their descendants. One cannot judge prehistoric remains by the same criteria as for modern ones.

8. Methods of sex determination.

It is impossible to base sex determination on one character alone. Neither metric of morphological. Masculine characteristics in one bone may be accompanied by female traits in another. This is equally true for prehistoric and modern remains. There are three areas to analyse: pelvis; skull and face; articular surfaces of joints and long bones.

Originally lecture and course notes dated 1972 and research preamble 1975-76.


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