Hominoid energetics: could load carriage have driven the early adoption of bipedal locomotion in human evolution?

a NERC-funded collaborative project with:

Andrew Chamberlain, University of Sheffield

Stephanie Davy-Jow, University of Sheffield

Bill Sellers, Animal Simulation Laboratory, University of Manchester

Rachel Payne, Royal Veterinary College, University of London

Habitual walking on two legs (bipedalism) is one of the unique features that distinguishes humans and their immediate fossil ancestors from the chimpanzees, gorillas and all other non-human primates. The evidence from fossil hominid leg bones and preserved trails of footprints shows that this change to bipedal walking happened very soon after the human evolutionary lineage diverged from the African apes, which suggests that bipedalism may have been an important catalyst for some of the other traits that define the human condition. The importance of bipedal locomotion is highlighted by the large number of theories that have been proposed to explain why walking on two legs is preferable to walking on all fours. Many of these theories argue that by using only your legs for walking you are able to free up your arms for some other purpose, and it is often suggested that this other purpose involves manipulating or carrying something – whether it is food, infants, tools or weapons. This is supported by the fact that when chimpanzees are observed walking upright it is often when they are carrying items of food.

However carrying, unlike walking, is an activity that leaves no direct trace in the fossil record. One of the few ways of testing such theories relating to the advantages and disadvantages of behaviours in long extinct animals is to create computer simulations. These simulations allow us to estimate the actual numerical values of the benefits of behavioural change in terms of energy (and hence food) saved. If such a change has a disproportionately large net benefit then we have some evidence to support our hypothesis, however if the effect is small then this would suggest that we need to look elsewhere.

The goal of this project is to produce a computer simulation of carrying behaviour in early human and human-like fossils and to use it to estimate the ease with which one of these animals could carry particular objects any given distance. This will be achieved by inputting morphometric data from fossil hominid skeletons into a computerised walking simulator so that it can simulate hominid bipedalism whilst carrying a variety of objects. The energetics of movement associated with carrying will then be determined from the model, and used to evaluate the benefits and costs of bipedal carrying.