A simple step from ocean to land

A comparative biology study has found that the evolution of the complex weight-bearing hips of walking animals from the basic hips of fish may have been a much simpler process than previously thought.

Tetrapods, or four-legged animals, first stepped onto land about 395 million years ago. This significant event in the evolution of terrestrial animals, including humans, was made possible by strong hipbones and a connection through the spine via an ilium - features that were not present in the fish ancestors of tetrapods.

Dr Catherine Boisvert of the Australian Regenerative Medicine Institute at Monash University, Macquarie University’s Professor Jean Joss and Professor Per Ahlberg of Uppsala University in Sweden examined the hip structures of some of human’s closest fish cousins.

They found the differences between humans and fish are not as great as they appear - most of the key elements necessary for the transformation to human hips were actually already present in our fish ancestors.

The researchers compared the development of the bones and musculature of the hip in the Australian lung fish, Neoceratodus forsterii, and the axolotl, Ambystoma mexicanum, commonly known as the Mexican walking fish.

Surprisingly, the results showed that the transition from a simple fish hip to a more complex weight-bearing hip could be achieved in a few evolutionary steps.

“Many of the muscles thought to be ‘new’ in tetrapods evolved from muscles already present in lungfish,” Boisvert said. “We also found evidence of a new, more simple path by which skeletal structures would have evolved.”

The researchers found that the sitting bones would have evolved by the extension of the already existing pubis. And the connection to the vertebral column could have evolved from an illiac process already present in fish.

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

This study was published in the journal Evolution and Development.