Early dinosaurs stopped moving about on all fours and rose up on just their two hind legs, thanks to their large, muscular tails, a new study has found.
Bipedalism, the ability to walk on two legs, in dinosaurs was inherited from ancient and much smaller proto-dinosaurs, researchers said.
"The tails of proto-dinosaurs had big, leg-powering muscles," said Scott Persons from University of Alberta in Canada.
"Having this muscle mass provided the strength and power required for early dinosaurs to stand on and move with their two back feet. We see a similar effect in many modern lizards that rise up and run bipedally," Persons said.
Over time, proto-dinosaurs evolved to run faster and for longer distances.
Adaptations like hind limb elongation allowed ancient dinosaurs to run faster, while smaller forelimbs helped to reduce body weight and improve balance.
Eventually, some proto-dinosaurs gave up quadrupedal walking altogether, researchers said.
The researchers also debunk theories that early proto-dinosaurs stood on two legs for the sole purpose of free their hands for use in catching prey.
"Those explanations don't stand up. Many ancient bipedal dinosaurs were herbivores, and even early carnivorous dinosaurs evolved small forearms.
"Rather than using their hands to grapple with prey, it is more likely they seized their meals with their powerful jaws," said Person.
Researchers wondered if bipedalism can evolve to help animals run fast, why mammals like horses and cheetahs are not bipedal.
"Largely because mammals don't have those big tail-based leg muscles," Persons said.
The researchers also theorise that living in burrows may have helped our ancestors to survive a mass extinction that occurred at the end of the Permian period (from 299 to 251 million years ago).
However, when proto-mammals emerged from their burrows, and some eventually evolved to be fast runners, they lacked the tail muscles that would have inclined them towards bipedalism.
The study was published in the Journal of Theoretical Biology.
