It seems that our hips have fishy origins. A study from researchers at Monash University has shown that the evolution of the complex hips of walking animals from the basic hips of fish was much more straightforward than previously thought.
Four-legged animals first stepped onto land approximately 395 million years ago. This major change was only made possible by strong hipbones and a connection through the spine via an ilium. These features were not present in the fish ancestors of tetrapods.
After closely examining the hip structures of some of human’s closest fish cousins, the researchers discovered that the differences between us and them are not as significant as they seem. In fact, most of the important elements essential for the switch to human hips were already present in our fish ancestors.
Catherine Boisvert of the Australian Regenerative Medicine Institute at Monash University and her colleagues compared the hip growth of the Australian lung fish and the Mexican Walking Fish. They found that the change from simple fish hip to weight-bearing hip could be accomplished in a few evolutionary steps.
According to Boisvert, a large number of the muscles believed to be “new” in tetrapods actually evolved from muscles already found in lung fish. She said that they also discovered evidence of a more simple path by which skeletal structures would have developed.
The researchers learned that the sitting bones would have progressed by the stretching of the already present pubis. The connection to the vertebral column could have developed from an iliac process already existing in fish.
The transition from ocean-dwelling to land-dwelling animals, which involved a complex, weight-bearing hip, was a “major event in the evolution of terrestrial animals,” but the study’s findings suggest that what first seemed to be a significant change in morphology could actually be completed with very few evolutionary steps.
The study’s findings are described in detail in the journal Evolution and Development.