The Fishy Origins of Human Hips: Evolution from Sea to Land

The complex, weight-bearing hips that humans and other walking animals have were once thought to have evolved through a series of complicated processes. Now, though, scientists have discovered that the evolution from the basic hips of fish to these weight-bearing hips may have been a far simpler process.

Tetrapods, which are four-legged animals, first stepped onto land about 395 million years ago. Yet actually taking those first steps was no easy process. They had to develop strong hipbones that could support their weight without the help of buoyant water. In addition, these hipbones had to have a connection through the spin via an ilium, a feature that was not present in the fish ancestors of tetrapods.

In order to find out exactly how fish hips might have evolved into complex, weight-bearing hips, researchers examined some of human's closest "fishy" cousins, including the Australian lung fish and the Axolotl, commonly known as the Mexican Walking Fish. More specifically, they examined the bones and musculature of these species.

They found that, surprisingly, the differences between human hip bones and fish hips aren't nearly as great as they first appeared. In fact, it turns out that most of the key elements that are necessary for the transformation to human hips were already present in our fish ancestors.

"Many of the muscles thought to be 'new' in tetrapods evolved from muscles already present in lungfish," said Catherine Boisvert of the Australian Regenerative Medicine Institute at Monash University in a news release. "We also found evidence of a new, more simple path by which skeletal structures would have evolved."

So exactly how would these bones have evolved? The researchers discovered that the sitting bones would have evolved by the extension of the already existing pubis. The connection to the vertebral column could have then evolved from an iliac 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. 

The research could have huge implications for the study of evolution and understanding the process by which land mammals came into being.

The findings are published in the journal Evolution and Development.