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 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 4 inches long with two front legs and a tail, it can breathe through gills and through its skin, and it usually lives 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 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 the University of Californ ata, Berkeley.
McInroe’s colleagues watched mudskippers move across sand.
On level ground, the mudskippers mostly kept their tails straight back. But as the researchers increased the slope, they used the tails to propel themselves forward and to keep from sliding downhill. This nearly doubled the distance they could cover with each stride.
Next, the researchers 3-D-printed a robot that used the same motion to navigate 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 on poppy seeds or tiny plastic beads.
Watching the robot helped the researchers recognize some basic principles. They found that when the robot 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.”
Researchers should next 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 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.”
