Frogs may have evolved the first kneecaps on Earth

7 July 2017

By Andy Coghlan

Frogs legs have sprung a big surprise – contrary to textbook biology, they have primitive kneecaps.

The kneecaps are made of dense, fibrous cartilage rather than bone, and appear to be much better suited to soaking up the strains of leaping and jumping than the bony human patella.

They may have been missed until now because they are not clearly visible on frog leg bones, even with a microscope, says Virginia Abdala of Argentina’s Institute of Neotropical Biodiversity, who led the investigation. The researchers analysed full skeletons of 20 species, but they were only able to see kneecaps in the eight specimens from which they took tissue slices for analysis.

One implication of the discovery is that kneecaps like this began to evolve in the Devonian period 400 million years ago, when the first four-legged animals reached land, the researchers say.

“Until now it was thought that the evolution of kneecaps coincided with the arrival of tetrapods that lay eggs on land or retain fertilised eggs in the body,” says Abdala. This investigation shows that the process really started with fibrocartilage in frogs, she says.

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