Clambering out of the water and up sandy shorelines was a monumental feat for some of the first creatures to walk on land. These ancient animals needed all the help they could get to push beyond the water's edge — and scientists say some of that help may have come from their tails.
After studying the movements of a unique fish called the African mudskipper, researchers realized that tails were an essential tool that could have allowed the first land-dwellers to gain traction on the sloped, sandy shorelines they would have encountered upon leaving their watery homes. The team reported its findings in Friday's edition of the journal Science.
"We can't go back in time and get continuous data," said Daniel Goldman, a physicist at Georgia Tech who worked on the study. "But there are are a number of organisms that exist today that … could resemble those animals of the Devonian era" — which ended about 360 million years ago.
The African mudskipper is a small amphibious fish that spends the majority of its life moving in and out of the water, taking care never to dry out. Roughly four inches long with two front legs and a tail, they can breathe through gills and through their skin, and they usually live in mudflats or mangrove estuaries.
As mudskippers climb out of the water, both front legs move in synchrony — reaching up and out to the side, then digging down into the sand to thrust their bodies forward.
"They're a fish that can do a push-up," said study lead author Benjamin McInroe, a graduate student in biophysics at UC Berkeley.
McInroe's colleagues watched mudskippers as they moved across sand, allowing them to make consistent, repeatable measurements.
On level ground, the mudskippers didn't use their tails and mostly kept them straight back. But as the researchers increased the slope of the sandy beach, the mudskippers relied on their tails to propel themselves forward and to keep from sliding downhill. By doing this, they nearly doubled the distance they were able to cover with each stride.
Next, the researchers 3D-printed a robot that used the same motion to navigate its way up tricky slopes, as if it were dragging its body around on a pair of crutches. Instead of letting it loose in sand, which could damage its delicate motors, they tested it in an environment filled with poppy seeds or tiny plastic beads.
Watching the robot navigate its terrain helped the researchers recognize some of the basic principles that would have helped ancient animals climb out of the water. They found that when the robot, or the mudskipper, couldn't use its tail, there was little room for error.
"On sandy ground, the tail was critical," Goldman said. "And it buffered against poor foot use and placement."
The researchers didn't create models of any of the creatures thought to be among the first to walk on land, such as Ichthyostega. But they do think that these results could shed light on the behavior of early land-dwellers.
Although the study provides some evidence that a tail would be helpful on difficult terrain, researchers should consider the body shapes of actual creatures from the fossil record, said Stephanie Pierce, a vertebrate paleontologist at Harvard who was not involved in the study. Only then can we come to any conclusions about how tails facilitated the transition from life in the water to life on land, she said.
The first vertebrates to emerge from the water had four legs, not two, Pierce said. Along with those hind legs came shorter tails relative to their overall body length.
It's hard to make generalizations "without analyzing the fossils themselves," she said. "But perhaps we can do that in the future."
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