The story of how our ancestors evolved to walk on land has remained largely unchanged for the last two decades, but the discovery of a new fossil could re-write our evolutionary history.
Scientists have found the remains of a new species of lobe-finned fish, known as Hongyu chowi, in a quarry in Ningxia, north China.
The bizarre fish shares characteristics with two other separate groups of lobe-finned fish – one of which evolved into early four-legged animals.
The fossil suggests that more species were evolving to live on land than scientists had previously thought.
Scientists have found the remains of a new species of lobe-finned fish, known as Hongyu chowi, in a quarry in Ningxia, north China
Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing suggest that Hongyu chowi doesn’t easily fit into the current understanding of the lobed-finned fish evolutionary tree.
The fossil comes from the late Devonian period of China, known as the Age of Fishes, and is between 360 and 370 million-years-old.
Scientists previously thought that lobe-finned fish evolved into four-limbed tetrapods around 360 million years ago.
Tetrapods include every extinct and living four-limbed animal, including amphibians, reptiles and mammals.
But hongyu chowi has changed this perception.
Measuring about 1.5 metres (five feet) in length, the fish has features from two species groups that were previously thought to be distinct.
Fish began to develop four limbs to walk on land around 360 million years ago, during the Devonian period, which is known as the Age of Fishes
Hongyu chowi’s size and some of the shapes of the fossil’s bones are similar to rhizodont fish, an extinct group of predatory lobe-finned fish.
Rhizodont fish were thought to have evolved away from the tetrapod stem group before they formed limbs.
But the fossil is also similar to another group of lobe-finned fish, the elpistostegids, which are a close relative to the early tetrapods.
HOW FISH WALKED ON LAND
The lobe-finned fish had fins contained muscle and bone that eventually involved into the four-limbs that would allow animals to walk from water on to land
Tetrapods evolved between 390 and 360 million years ago in the Devonian Period
The word tetrapod means ‘four feet’, and the group includes amphibians, reptiles, and mammals.
They evolved from a common finned animal, thought to be the lobe-finned fish.
Early tetrapods used their limbs as paddles to move in the water.
As they evolved, tetrapods developed a vertebral column and pelvic girdle that could support their bodies on land.
The shoulder girdle and gill skeleton of the Hongyu chowi are similar to those seen in early four-legged animals.
In the new study, researchers said these similarities could mean that the Hongyu chowi is a form of rhizodont that independently evolved the shoulders and gill cover support of a four-legged animal.
But the findings could also indicate that rhizodonts are closer relations to tetrapods and elpistostegids than was previously thought.
Our ancient tetrapod ancestors used four limbs to drag themselves on to land (artist’s impression pictured)
In both cases, the discovery reveals a significant increase in shared characteristics between separate groups on the evolutionary tree.
In their study, published in Nature Ecology and Evolution, the researchers, led by Dr Min Zhu, wrote: ‘In either case, the overall result reveals a substantial increase in homoplasy in the tetrapod stem group.
‘It also suggests that ecological diversity and biogeographical provinciality in the tetrapod stem group have been underestimated.’
The researchers found the bizarre Hongyu chowi fish fossil, in a quarry in Ningxia, north China