Walking may have had wet start

Lurching lungfish suggest underwater start for four-legged locomotion

African lungfish walk and bound along the bottoms of water tanks on their slender, whiplike pelvic fins, a new study finds.
Because lungfish are closely related to some of the earliest four-legged terrestrial vertebrates known as tetrapods, such findings may indicate that transitional creatures learned to scuttle across the floors of ancient seas before they took to land and developed more complex limbs with digits, biologist Heather King and colleagues at the University of Chicago suggest online December 12 in the Proceedings of the National Academy of Sciences.

“The cool thing about the lungfish is that it’s walking underwater,” says King. “And if lots of tetrapods were also doing this it could mean that the first step in the evolution of vertebrate walking took place underwater.”

Around 400 million years ago, certain species of bony fish — called the lobe-finned fishes for their characteristic muscular, fleshy fins — began to evolve features such as larger limbs with digits, which allowed them to move onto land. “We have a whole series of fossils that show this transition from lobe-finned fishes to tetrapods,” says King. Until now, however, scientists didn’t have a clear idea of the order in which these features emerged.

“The fossil record has limitations,” says study coauthor Neil Shubin. “Living organisms are far weirder than you can imagine, and the more you look the more you find.”

To get a closer look at one of the last living species of the lobe-finned fishes, the research team plopped African lungfish (Protopterus annectans) one at a time into a tank with a plastic mesh bottom and trained several cameras on them. From side and overhead angles, the scientists captured video of the lungfish ambling and leaping using their pelvic fins.

“If you asked me to tell you what those fins do just by looking at them, I’d give you a thousand functions, and walking wouldn’t be one of them,” says Shubin.  “But that’s what they do.”

Watching the strange aquatic critters hop and hobble may be the best way to study this important transition in the history of vertebrate locomotion.

“We’re really beginning to see the sequence in which tetrapod gaits came about,” says Shubin. “And we’re seeing that they came about underwater.”

Shubin sees the aquatic environment as an important testing ground for vertebrate locomotion. “Water is unconstrained,” he says.  “Gravity is not as limiting, so you can get by with a variety of motor styles.” The extra buoyancy of primitive lungs, which likely evolved from the air bladders found in fish, probably helped get these early transitional organisms on the move, too.

“It seems possible that tetrapod walking would begin with underwater walking in lobe-finned fish,” says evolutionary biologist Per Ahlberg of Uppsala University in Sweden. Though he has no criticism of the actual study of African lungfish and what they do, he disputes the paper’s secondary claim that some of the fossil tracks traditionally attributed to early tetrapods might have been generated by lobe-finned fish learning how to stroll.

“There would be a lot of body drag evident, and finprints wouldn’t look like footprints,” he says. “But underwater walking in fish has been well documented.”  

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