How Walking Fish May Explain Ancient Fish Moving onto Land

Around 375 million years ago, the forebears of every land vertebrate pulled themselves from water onto dry ground.

Exactly how they achieved this shift with fins that had not yet developed into true legs remains one of life’s major evolutionary mysteries.

New research has uncovered a possible answer in an unlikely source: the awkward, flopping movements of present-day fish that are able to travel across land when required.

Led by the University of Cambridge, the study used computer models alongside a specially designed robotic fish to examine how several unrelated species of walking fish move over land.

The researchers discovered that, although these species evolved separately on widely divergent evolutionary branches, they all rely on much the same fundamental walking pattern.

This straightforward mechanical approach has repeatedly evolved, and it could be the very method used by ancient fish as they first made their way ashore hundreds of millions of years ago.

Fish that can walk

A number of fish species alive today can travel over land when circumstances demand it.

Bichirs, lungfish, catfish, sculpin and snakeheads are much more at home in water, yet can propel themselves over dry terrain when necessary.

“If you’ve got the ability to walk on land and your predator doesn’t, then you can escape and hopefully the predator moves on,” said lead author Michael Ishida from Cambridge’s Department of Engineering.

“You’ve also got the ability to move from one shallow-water environment to another, like tide pools for example.”

Until now, no one had compared several unrelated walking fish species to determine whether they use shared underlying principles of locomotion.

Ishida, who works in Fumiya Iida’s laboratory and collaborated with biologists and palaeontologists, set out to investigate this question.

Recurring kinds of motion

The researchers first developed a computer model using the movements of Polypterus senegalus - the grey bichir, which is native to Africa - as well as those of other walking fish.

Whichever species informed the model, it consistently produced the same pattern.

“We kept seeing this recurring kind of walking motion, although it’s very primitive,” Ishida said.

“A number of different fish, spread out across the evolutionary tree, and not closely related to each other, all do it. It’s such a simple movement and can recur from a very basic starting point.”

They called this movement the “undulating tripod gait”. The fish first secures itself with its front fins or head, then swings its tail to drive its body forwards around that fixed point.

“It looks like a swimming fish dumped onto land,” Ishida said. “A swimming fish uses its body to propel itself through the water, so if you take that, put it on land, give it some ability to shuffle its front fins, that’s exactly what it’s doing.”

Building a robot to test it

Computer modelling can only provide evidence to a certain extent, so the team created a physical robot fish to put the findings to the test.

“We tried all kinds of different gaits on the robot, and every other gait we tried was slower,” Ishida said.

“Any time we changed how the body bended, or what sequence it was bended in, it was worse. It was surprising that the optimal walking pattern in the simulation and robot matched what the real fish actually do.”

Agreement between three separate sources of evidence - observations of real fish, computer modelling and robotics - makes the result particularly persuasive.

The gait is not merely widespread among walking fish: it seems to be the mechanically optimal way to move over land without fully developed legs.

That likely explains why it has evolved independently so often throughout evolutionary history.

What this reveals about ancient fish

The broader significance of the findings reaches back 375 million years to fossil fish such as Tiktaalik - the well-known “fishapod”, whose fins displayed early forms of the wrist and finger bones that would ultimately develop into tetrapod limbs.

Tiktaalik and related species were among the fish undertaking the first cautious journeys onto land, and palaeontologists have spent decades trying to establish how they travelled.

The Cambridge researchers propose that computer modelling and robotics could similarly be used for fossil species, drawing on known anatomy to simulate their movements and assess how they may have walked.

If the undulating tripod gait is as mechanically fundamental as this study indicates, it is a credible possibility for the way these ancient creatures first pushed themselves from shallow water.

The gait is ungainly and primitive, bearing no resemblance to the walking style of land animals today. Yet 375 million years ago, it may have been sufficient.

Click here for a short video demonstration of the fish-like robot locomotion.