‘Weird’ walking fish could shed light on how humans evolved to stand upright

The sea robin, a bony fish that lives at the bottom of the ocean, has six leg-like structures that it uses to scurry across the sea floor, and sometimes dig for prey.

Although they look like limbs, scientists say the “legs” are actually an extension of the creature’s wing-like fins on each side.

Researchers have now discovered that sea robins grow their legs using the same genes that also plays a role in the development of human limbs.

They also found one particular species, called Prionotus carolinus, is highly sensitive to touch and can taste prey hidden within the sea floor.

The team said its findings, published in two separate papers in the journal Current Biology, could help shed light on how humans evolved to walk upright some six million years ago.

David Kingsley, a professor at Stanford University in the US, said: “Land animals evolved from fish ancestors, so some kind of relationship between human limbs and fish fins is not a surprise.

“However we were delighted to see that the gene that makes some parts of the fins develop in a new way in sea robins is also a gene that controls some bones in humans.”

Sea robins, also known as gurnards, can be found in tropical seas worldwide, and feed on worms and crustaceans.

Some species can be found in British waters, where they are often caught as bycatch and discarded.

Prof Kingsley said that like human ancestors who evolved to walk upright to adapt to life on land some six million years ago, sea robins developed unique bodies to adapt to the environment on the ocean floor.

He said: “In both cases, the evolution of walking legs required alterations to skeletal structures, muscular innovation, and sensory and motor control.

“Although the details of how this happened in fish and humans are likely very different, we hope to learn general principles about how old genes and old structures can be repurposed in nature in order to build new traits in wild species.”

Researchers from Harvard and Stanford universities decided to study what they describe as “weird creatures” after learning that sea robins are often followed by other fish because of their unique ability to find hidden prey.

Corey Allard, a biologist at Harvard University, said: “Sea robins are an example of a species with a very unusual, very novel trait.”

The team found that P. carolinus has sensors in their legs, known as papillae, that help them uncover and taste hidden prey.

These tiny protrusions act in a similar way to taste buds in human tongues, the researchers said.

Nicholas Bellono, an associate professor at Harvard, said: “This is a fish that grew legs using the same genes that contribute to the development of our limbs and then repurposed these legs to find prey using the same genes our tongues use to taste food – pretty wild.”

But they also found that other another species, known as Prionotus evolans, did not have papillae, and use their legs for movement rather than hunting.

The researchers said that P. carolinus’ legs were more shovel-like in shape whereas P. evolans’ were rod-shaped.

Prof Kingsley said: “We were originally struck by the legs that are shared by all sea robins and make them different from most other fish.

“We were surprised to see how much sea robins differ from each other in sensory structures found on the legs.”

Prof Kingsley said that as part of the next steps, the team is looking to explore how sea robins evolved to have these sensory abilities in their legs.