Credit: David Carrier, University of Utah.
These two photo sequences depict a key part of a University of Utah
experiment that showed why there is a fighting advantage to walking on two
legs and being tall -- something that may help explain why ape-like human
ancestors started walking upright and why women today tend to prefer tall
men. In the top three photos, a participant in the study kneels with four
limbs on the ground and then raises one arm to strike downward on a padded
block equipped with sensors to measure the force of the blow. The bottom
three photos show the same experiment, but with the blow delivered from an
upright position. The study found that blows delivered downward from a
two-legged posture are more powerful than downward blows from an all-fours
posture, or than any blows delivered upward, from the front or sideways.