Fish uses fins to walk and bound
Scientists have filmed an African lungfish using two fins to walk and "bound" along the bottom of its tank.
The lungfish appeared to use their pelvic fins as hind legs - stepping along the tank bottom.
This suggests, the researchers say, that some fundamental features of walking on land arose in similar fish before the animals made the transition to land.
The findings are published in the journal PNAS.
The team studied the lungfish Protopterus annectens because this group of species is so closely related to tetrapods - land animals with four limbs and backbones, including humans, birds mammals and reptiles.
What the group hoped to learn about was one of the greater shifts in animal evolution.
"One really important event that happened in evolution was that fishy kinds of things evolved into tetrapods, and onto the land out of the water, in the Devonian period, about 360 million years ago," said lead author of the study Heather King from the University of Chicago.
"Since they're so closely related and everything else in their group is extinct, they're a really great animal to look at."
Researchers believe that these "lobe-finned" fish provide a living glimpse at the evolutionary stage between aquatic animals and the earliest creatures that walked on land.
"The water-to-land transition - a huge event over a long period - is easier [to understand if we] break it down into smaller events," Ms King explained.
She said that these experiments indicate what the order of this sequence of smaller events may have been.
Lungfish do not have any of the anatomical features associated with walking on land. They have no sacrum - the supportive bone at the base of the spine - and no digits (fingers or toes) on their limbs.
But in the videos captured by the researchers, the lungfish seemed to overcome their anatomical limitations by bending their fins along their length to form regions of support that functioned as "feet".
The fact that they were able to propel themselves along the base of a tank - using the floor as a substrate - suggests that this ability arose before the evolution of digits and before animals made the transition from water to land.
The findings have led Ms King to study the anatomy of the fishes' fins further, to compare them with the limbs of tetrapods.
She hopes that further studies can analyse the motions of the other four species of lungfish - which may show how we walked up onto the land in the Devonian.