It has long been recognized that the bipedal posture reduces the surfa
ce area of the body exposed to the sun. In recent years, a theory has
been developed by Wheeler that bipedalism evolved in the ancestor of t
he Hominidae in order to help relieve thermal stress on the animals in
open equatorial environments. Bipedalism was said to afford a distinc
t adaptive advantage over quadrupedalism by permitting hominids to rem
ain active in the open throughout the day. The heat load of the hypoth
etical hominid comprises the external environment as modelled by Wheel
er and the animal's internal environment (i.e., the internal heat gene
rated by its metabolic and locomotor activities, and its evaporative a
nd respirative cooling capacities). When these factors are integrated
in the calculation of the animal's thermal budget, the putative advant
age of the bipedal over the quadrupedal posture is considerably reduce
d. The simulations conducted in this study suggest that the increased
time afforded to early hominids in the open by bipedalism was relative
ly short and, therefore, of little or no adaptive significance. These
results suggest that thermoregulatory considerations cannot be implica
ted as a first cause in the evolution of bipedalism in the hominid anc
estor.