How ribs might have been vital in the evolution of walking

An analysis of a fossil found 20 years ago has revealed new details which might explain how vertebrates evolved to walk on land nearly 400 million years ago.

Tiktaalik, a 375-million-year-old fish,  discovered in Canada, is a “fishapod” – a missing link between fish and the first four-legged tetrapods to walk the Earth. All land animals with a backbone (and those which evolved to be water dwellers again, like whales) can trace their ancestry back to pioneering fish like Tiktaalik – from dinosaurs, frogs and birds to humans.

Four skeleton ancient fish tetrapod ancestor scan on black
Reconstruction of Tiktaalik. Credit: Proceedings of the National Academy of Sciences (2024).

The new analysis of Tiktaalik’s skeletal structure is published in the Proceedings of the National Academy of Sciences.

Tiktaalik was discovered in 2004, but key parts of its skeleton were unknown,” says first author Tom Stewart, assistant professor at Pennsylvania State University. “These new high-resolution micro-CT scans show us the vertebrae and ribs of Tiktaalik and allow us to make a full reconstruction of its skeleton, which is vital to understanding how it moved through the world.”

The reconstruction shows that Tiktaalik’s ribs likely attached to its pelvis. It’s thought that this helped the fish support its body on land, which would have been crucial in the eventual evolution of walking.

Most fish have vertebrae and ribs that are the same length along their spine. But limbed vertebrates have vastly differently sized ribs. This allowed specialised functions in different parts of the trunk including a mechanical link between ribs and the pelvis and hind limbs that support the body.

Pelvic fins of fish are evolutionarily related to hind limbs in tetrapods. The researchers found that walking was able to develop with the help of a larger pelvis which formed part of the vertebral column to brace and support the body from new forces which come from walking on land.

Tiktaalik is remarkable because it gives us glimpses into this major evolutionary transition,” Stewart explains. “Across its whole skeleton, we see a combination of traits that are typical of fish and life in water as well as traits that are seen in land-dwelling animals.”

Original descriptions of Tiktaalik focused on the front portion of its skeleton.

“From past studies, we knew that the pelvis was large, and we had a sense that the hind fins were large too, but until now couldn’t say if or how the pelvis interacted with the axial skeleton,” Stewart adds. “This reconstruction shows, for the first-time, how it all fit together and gives us clues about how walking might have first evolved.”

Buy cosmos print magazine

Please login to favourite this article.