The West African lungfish has been found to walk on its fins, according to a new study.
While other fish are known to stroll—and some even have "hands" (pictures)—this is the first time the behavior has been seen in a fish related to the first land-walkers. The find could mean that our ability to walk originated underwater, researchers say.
In the lungfish, "this ability is surprising, because lungfish don't have feet!" study leader Heather King said via email.
Based on observations of the fish's movements in glass tanks in the lab, the study showed the lungfish were able both to push off a solid surface and move along it using their pelvic fins. (Watch a brief video of the lungfish walking.)
"We found that the lungfish uses a range of gaits, from walking (alternating the limbs) to bounding (moving the limbs synchronously)," said King, a biologist studying at the University of Chicago, who collaborated with past National Geographic Committee for Research and Exploration grantee Neil Shubin on the study. (The National Geographic Society owns National Geographic News.)
Living Links to Missing Links?
The finding offers new insight into how animals with backbones first hauled themselves from prehistoric seas hundreds of millions of years ago, King said.
That's because lungfish and other so-called lobe-finned fishes are believed to be close living relatives of the earliest known tetrapods—four-limbed animals with backbones.
"This study tells us that walking behaviors are not exclusive to tetrapods" in that period and suggests that the evolutionary route to land walking began with their finny forebears.
The findings further suggest that fossil tracks previously credited to early land tetrapods with feet—or at least toelike digits—may instead have been left by lobe-finned fishes moving along the waterbed.
Disproving Key Fossil Interpretation?
The new walking-fish discovery casts doubt, for example, on fossil "footprints" from Poland, which in 2010 were attributed to a 395-million-year-old land creature, King said.
"Some of the patterns shown in these fossil trackways are similar to the patterns made by the limbs in the lungfish," she said.
The lack of signs of a dragging tail in the Polish tracks can be explained by the fact that lungfish lift their bodies off the bottoms of rivers, lakes, and swamps while walking—possibly aided by air-filled lungs that give the fish their name, she observed.
"There were many fishes alive 397 to 391 million years ago that share important functional features with lungfish," King said, "and it is possible that they were responsible for some of the fossil trackways that are now attributed to tetrapods."
The walking-fish study was published online Monday in the journal Proceedings of the National Academy of Sciences.