Anatomical Adaptations for Bipedalism: Arms & Legs

Most quadrupedal and arboreal primates have either longer arms relative to their legs or arms and legs of equal length, while most bipeds have relatively longer legs than arms. Based on this information, it is possible to reconstruct the positional behavior of a species by calculating the humerofemoral index. This index is the length of the humerus divided by the length of the femur, multiplied by 100: humerus length x 100/femur length.

Results of the humerofemoral index calculate the overall body proportion of an organism which can then be compared to others. The higher the index value, the longer the arms and the more likely a primate is to be arboreal. Most arboreal primates have ratios close to 100. For example, the mean ration for the common chimpanzee is 97.8. Humans average a lower ratio at approximately 71.8. The ratio of the famous A. afarensis (Lucy) is intermediate between modern humans and chimpanzees at 84.6^9,10.