Long bone morphometrics for human from non-human discrimination

Forensic anthropologists are frequently required to confirm the human origin of complete and partial skeletal remains. This determination, however, can be difficult for bone fragments with few or no distinctive morphological landmarks. Current methods of distinguishing human from non-human bone fragments include microscopic, immunological and DNA testing, which are each limited to some degree (e.g. time consuming and expensive). The purpose of this study is to investigate an alternative morphometric approach to quantify the external structure of human long bones (humeri, femora, tibiae) compared to quadrupedal (sheep, dog, pig) and bipedal (kangaroo, emu) animals commonly found in Australia. Eight traditional linear measurements were taken on a sample of 50 human and at least 10 of each of the five non-human species; measurements were then analysed using ANOVA, canonical variates analysis (CVA) and direct discriminant analysis. The results from ANOVA and CVA indicate morphometric variation between the six species (five for the humerus), which were able to be correctly classified into their corresponding groups with a high degree of expected accuracy: humerus – 70-97.8%; femur – 70.9-97.3% and tibia – 72.4-96.2%. Direct discriminant analysis further separated the human from the combined non-human species, with a cross-validated classification accuracy of 95% or higher. More importantly, however, the technique also proved to be accurate if only a fragment of the diaphysis is analysed; classification accuracy 63 – 99%. As it may not always be possible to accurately determine the known midshaft of a bone (especially if only a relatively small fragment is available) the potential forensic utility of a method requiring a whole bone has to be considered. In order to assess whether the power to discriminate between humans and non-humans is significantly reduced if measurements are taken up to 2cm above and below the known midshaft, a series of measurements (antero-posterior diameter, medio-lateral diameter and midshaft circumference) were taken on 17 human and 50 pooled non-human species (40 for the ii humeri). Overall, it was found that there is only a small degree of size variation within 2cm above or below the known midshaft, with relatively low average standard deviation ranges for the three human and non-human bones: humerus – 0.420-2.2.05 mm; femur – 0.345-0.586 mm; tibia – 1.034-2.676 mm. Not knowing the precise location of the midshaft on classification accuracy was also shown to have a relatively small impact, especially for the tibia, which appears to be the 'bone of choice' for attempting to distinguish between the human and non-human species considered in the present study. This study has provided strong evidence that quantifying the external structure of long bones is a forensically viable technique for species discrimination. This project has also shown that morphometric methods can be successfully applied for human from non-human identification when remains are fragmentary and lack diagnostic muscle attachment marks or articular regions. Overall, the results of this study have outlined a forensically useful non-invasive method to distinguish human from non-human bones.