Punching on All Fours and on 2 Legs (image) University of Utah Caption 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. Credit David Carrier, University of Utah Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.