Ancient fish learned to walk underwater before they arrived on land

Hundreds of millions of years ago, the first land animals emerged, facing new challenges unlike any their aquatic ancestors had experienced. But their marine predecessors had solved one big problem for living on land: they already knew how to walk.

That's the finding of researchers at the University of Chicago, who have studied the motion of the African lungfish, an eel-like creature that serves as a living fossil for some of the most primitive marine organisms. It has thin, scrawny limbs on its pelvis, and the new study has revealed the lungfish uses these limbs to lift itself above the bottom surface and to move itself forward.

Previously, these abilities were thought to be the exclusive domain of tetrapods, the planet's earliest limbed land-dwellers. This finding rewrites a crucial piece of our planet's evolutionary history, suggesting organisms already possessed the ability to walk before they ventured onto the land. This also means that ancient trackways seemingly formed by tetrapods might actually be the work of lungfish or its extinct cousins.

Reseacher Heather King explains that they could not have made this finding without the lungfish around to serve as a living representative of these otherwise extinct creatures:

"The lungfish is in a really great and unique position in terms of how it is related to fishes and to tetrapods. Lungfish are very closely related to the animals that were able to evolve and come out of the water and onto land, but that was so long ago that almost everything except the lungfish has gone extinct. This is all information we can only get from a living animal. Because if you were just to look at the bones, like you would with a fossil, you might not ever know these motions could occur."

Under video analysis, the lungfish displayed the ability to both walk — which means they alternated the movement of their limbs — and bound — which means both limbs moved at the same time. The lungfish displayed all the range of movement necessary to make many of the tracks previously attributed to tetrapods. The lungfish is able to get away with such scrawny limbs likely because its watery surroundings mediate the pull of gravity, and it can also fill its lungs with air to increase its buoyancy.

Co-author Neil Shubin explains just how unlikely this finding really is:

"If you showed me the skeleton of this creature and asked me to make a bet on whether it walks or not, I would have bet it couldn't. Their fins seem like the furthest thing from walking appendages possible. But it shows what's possible in an aquatic medium where you don't have to support yourself with gravity. This shows us — pardon the pun — the steps that are involved in the origin of walking. What we're seeing in lungfish is a very nice example of how bottom-walking in fish living in water can easily come about in a very tetrapod-like pattern."

Via PNAS. Image of speckled lungfish by Joel Abroad on Flickr.