The evolution from fish living in water to vertebrates living on land was one of the most pivotal moments in the history of the animal kingdom. Now scientists using robot fish suggest that these ancient pioneers may have used their tails like crutches to help them move across land. The scientists detailed their findings in the July 8 issue of Science.

The first terrestrial vertebrates, or tetrapods, evolved roughly 385 million to 360 million years ago, ultimately giving rise to amphibians, reptiles, birds and mammals. Scientists have discovered relatively few fossils dating back to this ancient time, so little is currently known about how tetrapods accomplished this move to land.

To learn more about this critical transition, scientists investigated living fish that previous research suggested were similar in biology to the earliest terrestrial vertebrates. This included the African mudskipper (Periopthalmus barbaratus), which lives in tidal areas near shore and sometimes crawls onto land with its fins, occasionally jumping with its tail. They also developed both computer models and a 3D-printed robotic imitation of these fish called MuddyBot.

“This study really brings a lot of tools to bear on the question of how vertebrate animals started to crawl on land, and not concrete or pavement, but a surface composed of complex materials,” says study senior author Daniel Goldman, a physicist at the Georgia Institute of Technology in Atlanta.

The three investigative approaches complemented each other. Animal experiments help scientists understand animal locomotion. But they don’t generally allow researchers to make animals repeat actions, or manipulate their shapes in methodical ways, or have them perform unnatural movements. Computer models offer insights into the underlying mechanics of locomotion, but fail to capture much about the complex interactions between an animal and its environment. Robot models of animals can offer the best way to learn about animal locomotion, but need to be informed by living animals and computer models.

The researchers experimented by placing the animals, robot, and computer model on the sandy beaches that early terrestrial vertebrates might have encountered. They unexpectedly found that fish tails might have played a critical role during ancient forays onto land. On flat ground, tails provided little benefit. However, on slopes, as one might see on riverbanks, tails were used significantly more often to help propel mudskippers forward. For instance, at slopes of 10 degrees, tails were used in roughly one-third of all “steps,” while at slopes of 20 degrees, they were used during more than half of all steps.

Observed fish movements echoed these results. At sandy surface inclines of 10 to 20 degrees, the mudskippers performed an interesting flip with their tail in synchrony with their limbs to propel themselves, Goldman adds. “As far as we could tell, no one had described this behavior before.” The tail, he explains, had not been implicated before as a propulsive element as part of vertebrates’ movement to land.

Experiments with the robots suggested that while ancient terrestrial vertebrates likely used their fins to move around on flat surfaces, when it came to climbing sandy slopes, they would have benefited from coordinating the motions of their fins and tails. The tails essentially act like crutches to help prevent slides down slopes.

“I think it is exciting to integrate diverse approaches like robotics, motion analysis of living fishes, and granular drag measurements to elucidate how fossils over 350 million years ago might have moved,” says John Nyakatura, an evolutionary biologist and comparative anatomist at Humboldt University of Berlin, who did not take part in this research.