Fishy Origins Of Hip Bones Not As Complex As We Thought

The evolution of swimming sea creatures into walking land animals was not as complex a process as previous explanations make it seem, according to a new study that examined the development of hip structures in fish.

Researchers say what appeared to be a large change in morphology could actually occur in relatively few steps. 

Nearly 400 million years ago the first tetrapods, or four-legged animals, stepped onto land. The transition from sea to land was made possible by the evolution of strong hipbones and a connection through the spine via an ilium - features that were not present in the aquatic ancestors of tetrapods.

A study on the evolutionary structure of hip bones has found the the differences in the tetrapods that evolved from fish and the modern, upright walking human are not all that different. The study found that most of the key elements necessary for the transformation to human hips were actually already present in our fish ancestors.

"Many of the muscles thought to be 'new' in tetrapods evolved from muscles already present in lungfish. We also found evidence of a new, more simple path by which skeletal structures would have evolved," said Catherine Boisvert of the Australian Regenerative Medicine Institute at Monash University, who authored the study along with Jean Joss and Professor Per Ahlberg.

Boisvert and her collaborators compared hip bone and musculature development - bones of the Australian lung fish and the Axolotl, commonly known as the Mexican Walking Fish. They concluded the transition from simple fish hip to complex weight-bearing hip could be done in a few evolutionary steps. The sitting bones would have evolved by the extension of the already existing pubis, the researchers report. The connection to the vertebral column could have evolved from an illiac process already present in fish.

"The transition from ocean-dwelling to land-dwelling animals was a major event in the evolution of terrestrial animals, including humans, and an altered hip was an essential enabling step," Boisvert said in a news release.

"Our research shows that what initially appeared to be a large change in morphology could be done with relatively few developmental steps."