The walk of life: Tails might have helped vertebrates evolve from sea to land
CLEMSON — Vertebrate life evolved from sea to land when it crawled ashore some 350 million years ago spawning the vast diversity of walking, land-dwelling, air-breathing creatures. That transition has led scientists to study how ancient fish used their fins as crutches to hoist themselves ashore.
However, new evidence suggests that life could have moved forward with the help of what was behind it: a tail.
In a study published this week by Science, researchers used high-speed video of the present-day land-loving fish, the mudskipper, as well as robotics and mathematics to determine how earlier vertebrate animals used their tails for stability and to push themselves up a sandy incline. This is the first comprehensive study of how tails might have aided the crutching movement.
“The invasion of land is one of the most important events in animal history. Without that event, we wouldn’t be here,” said Rick Blob, an author on the study and an expert in animal biomechanics at Clemson University, where he is an alumni distinguished professor of biology.
“Trying to understand what steps allowed it to happen ultimately tells us about ourselves and why we work the way we do. Without that background knowledge, we’re making a lot of guesses that are usually wrong.”
With information from Clemson, researchers at the Georgia Institute of Technology built a robotic model of the mudskipper to further study locomotion. Findings could help designers create amphibious robots able to move across granular surfaces more efficiently — and with less likelihood of getting stuck in the mud.
“Most robots have trouble moving on terrain that includes sandy slopes,” said Dan Goldman, an associate professor in Georgia Tech’s School of Physics. “We noted that not only did the mudskippers use their limbs to propel themselves in a kind of crutching motion on sand and sandy slopes, but that when the going got tough, they used their tails in concert with limb propulsion to ascend a slope. Our robot model was only able to climb sandy slopes when it similarly used its tail in coordination with its appendages.”
Previous research neglected the tail, Blob suggests, perhaps because it was too obvious.
“When we think of moving across land, we think of using limbs,” he said. In fact, when mudskippers moved across level sand their tails usually just trailed behind. But, he said, “it shouldn’t surprise us that fish would use their strongest asset, their tails.”
Animals are capable of overcoming challenges by getting a little creative.
“By observing what living animals can do, it helps us broaden our hypotheses. Maybe animals didn’t need the strongest limbs or fins because that tail’s going to give them a push up,” said Sandy Kawano, a post-doctoral fellow at the National Institute for Mathematical and Biological Synthesis in Knoxville, Tennessee, who worked on the study as a doctoral student in Blob’s lab at Clemson.
On a broader scale, Kawano said, studies like this one “help us understand how animals overcome a wider range of environments. There is a lot of concern about how animals will overcome changes in the environment (such as global warming). How will the animals fare? They might have a compensatory mechanism. There might be a way for them to compensate when things aren’t perfect.”
Blob and Kawano placed mudskippers on sand in an otherwise empty fish tank and videoed as the animals moved across the sand when the tank lay flat and at inclines of 10 and 20 degrees.
“The fish provided a morphological, functional model of these early walkers,” said Benjamin McInroe, who, as an undergraduate student at Georgia Tech used data from the fish to build a robotic model. “With the robot, we are able to simplify the complexity of the mudskipper and by varying the parameters, understand the physical mechanisms of what was happening. With the mathematical model and its simulations, we were able to understand the physics behind what was going on,” McInroe said.
Carnegie Mellon University researchers, who have worked with Goldman on relating the locomotion of other animals to robots, demonstrated that theoretical models developed to describe the complex motion of robots can also be used to understand locomotion in the natural world.
“Our computer modeling tools allow us to visualize, and therefore better understand, how the mudskipper incorporates its tail and flipper motions to locomote,” said Howie Choset, a professor in the Robotics Institute at Carnegie Mellon University. “This work also will advance robotics in those cases where a robot needs to surmount challenging terrains with various inclinations.”
Integrating three different models into one project, and the collaboration between biologists, physicists and engineers, produced a more comprehensive understanding than any single model or discipline could, Kawano and Blob said.
“Bringing these different faces of science together has driven a new insight and a new understanding,” Blob said.
For example, the robot demonstrated that the tail could be moved in ways that hindered the animal’s progress. Seeing that, compared to the mudskippers’ coordinated movements between tail and fins, gives a new appreciation for how the fish overcome challenging environments.
“If we can understand challenges as big as that, maybe we have a chance to understand some of the changes we’re going to see in the future,” Blob said.
The research was sponsored by the National Science Foundation, the Army Research Office and the Army Research Laboratory.