Frogs May Have Been The First Animals To Evolve Kneecaps 400 Million Years Ago

The researchers found evidence of soft kneecaps in eight different species of frogs. Kurit afshen/Shutterstock

Josh Davis 10 Jul 2017, 14:37

Frogs have been hiding a secret for the last 400 million years. It turns out that the amphibians' legs contain a sort of kneecap, something that scientists thought only evolved after tetrapods made the move to land full-time, and potentially re-writing how we thought the structures first began.

Those of you imagining fully formed patella such as the ones found in humans might be a little disappointed, as the kneecaps are basically formed from a mass of soft fibrous tissue. Because they are not ossified, or bony, biologists have never found them in amphibians before, even when using microscopes. But one group of researchers were able to find evidence of the primitive kneecap by analyzing tissue slices taken from the legs, and published their results in The Anatomical Record.

It was originally thought that the kneecap evolved when tetrapods first fully left the water and started laying eggs on land. The solid nature of the patella acting in effect as a lever, aiding in the lifting of the limbs as these animals spend their lives traipsing across the terra firma, is thought to have been the driving force as to how they came about in the first place.

But the discovery of a primitive kneecap in a group of animals even more ancient than birds, mammals, or reptiles, might alter why we think the structure first evolved some 400 million years ago. The fact that the kneecap in the amphibians is soft and flexible may offer some clues as to why it first emerged.

“It does matter what kneecaps are made out of,” explains the Royal Veterinary Colleges’ John Hutchinson, to New Scientist. “Bone is a good lever, better at resisting compression than fibrocartilage, so animals using their kneecaps as levers rather than cushions would benefit from bony kneecaps.”

Yet even when frogs are resting, their limbs sit in the same position as humans legs do when they are about to jump. This means that they are under constant stress, and so need to be good at absorbing these forces. This is likely why the amphibians' patellae are so soft and fibrous, all the better to absorb the huge amounts of energy from jumping and hopping, and therefore protect the animals' joints.  

Others, however, remained to be convinced. Whether or not the patch of tissues identified in the amphibians can be truly classed as kneecaps is up for debate, and while the researchers found evidence for these structures in eight species of frogs, it may be premature to assume that all 7,000 species also have them, and therefore evolved in the ancestors to them all. As always, more work will need to be done.

[H/T: New Scientist]

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