THURSDAY, May 19 (HealthDay News) -- The human ability to stand on two legs may have developed because it improved males' ability to fight, a new study suggests.

University of Utah researchers used a punching bag fitted with a sensor to measure the force of punches by male boxers and martial arts experts as they hit the bag in four different directions: forward, sideways, down and up. The men struck the bag from a standing position and while on their hands and knees.

The men hit harder when they were in a standing position and when they were hitting downward rather than upward, showing that tall, upright males have a fighting advantage. The findings may help explain why our ancestors began walking upright and why women tend to prefer tall men, according to the study authors.

"The results of this study are consistent with the hypothesis that our ancestors adopted bipedal posture so that males would be better at beating and killing each other when competing for females," biology professor David Carrier said in a university news release.

"Standing up on their hind legs allowed our ancestors to fight with the strength of their forelimbs, making punching much more dangerous," he explained.

The findings also offer a "functional explanation" for why women tend to find tall men attractive.

"Early in human evolution, an enhanced capacity to strike downward on an opponent may have given tall males a greater capacity to compete for mates and to defend their resources and offspring. If this were true, females who chose to mate with tall males would have had greater fitness for survival," Carrier said.

The study is published in the May 18 online edition of the journal PLoS One.

More information

PBS has more about the origins of bipedalism.