The mythical Tiktaalik roseae, the four-legged fish that took the first step towards land, was discovered 20 years ago in Canada.
Popularized on the Internet thanks to the memes of the fish reaching land and turning back, Tiktaalik is a "fishapod" - a missing link between fish and the first four-legged tetrapods to walk the Earth.
Scientists believe that all land animals with a backbone, from dinosaurs, frogs and birds to humans, as well as those that have evolved to become water dwellers again, such as whales, can be traced back to pioneering fish like Tiktaalik.
A new analysis of Tiktaalik's skeletal structure, published last week in the Proceedings of the National Academy of Sciences, has now revealed new details that may explain how vertebrates evolved to walk on land around 400 million years ago.
"Tiktaalik was discovered in 2004, but important parts of its skeleton were unknown," says Tom Stewart, a researcher at Pennsylvania State University and first author of the article, quoted by Cosmos.
Evrim Yazgin
"New high-resolution micro-CT scans show us Tiktaalik's vertebrae and ribs and allow us to make a complete reconstruction of its skeleton, which is vital to understanding how it moved around the world," says Stewart.
The reconstruction shows that Tiktaalik's ribs were probably connected to its pelvis. This configuration, the study's authors believe, helped the fish to support its body on land, which would have been crucial for the eventual evolution of walking.
Most fish have vertebrae and ribs of the same length along the vertebral column. But vertebrates with limbs have ribs of very different sizes.
This allowed for specialized functions in different parts of the trunk, including a mechanical link between the ribs and the pelvis and the hind limbs that support the body.
The pelvic fins of fish are evolutionarily related to the hind limbs of tetrapods. The authors of the study found that walking developed with the help of a larger pelvis, which was part of the spine, to support the body from the new forces resulting from walking on land.
"Tiktaalik is remarkable because it gives us a glimpse of this major evolutionary transition," explains Stewart. "Throughout its skeleton, we see a combination of traits typical of fish and life in the water, as well as traits that are seen in land animals."
"From previous studies, we knew that the pelvis was large and we had a feeling that the posterior fins were also large, but until now we couldn't tell if or how the pelvis interacted with the axial skeleton," adds Stewart.
"This reconstruction shows, for the first time, how everything fits together - and gives us clues as to how walking might have evolved," concludes the researcher.
So we're all here thanks to Tiktaalik - who, unlike his cousin who peeked out onto dry land and decided to turn back, dared to take his first steps out of the water. Thanks to his good ribs.
