Slowly rolling across the ocean floor, a humble single-celled creature is poised to revolutionize our understanding of how complex life evolved on Earth.
A distant relative of microscopic amoebas, the grape-sized Gromia sphaerica was discovered once before, lying motionless at the bottom of the Arabian Sea. But when Mikhail Matz of the University of Texas at Austin and a group of researchers stumbled across a group of G. sphaerica off the coast of the Bahamas, the creatures were leaving trails behind them up to 20 inches long in the mud.
The trouble is, single-celled critters aren't supposed to be able to leave trails. The oldest fossils of animal trails, called 'trace fossils', date to around 580 million years ago, and paleontologists always figured they must have been made by multicellular animals with complex, symmetrical bodies.
But G. sphaerica's traces are the spitting image of the old, Precambrian fossils; two small ridges line the outside of the trail, and one thin bump runs down the middle.
At up to 1.2 inches in diameter, they're also enormous compared to most of their microscopic cousins.
"If these guys were alive 600 million years ago, and their traces got fossilized, a paleontologist who had never seen this thing would not have a shade of doubt attributing this kind of trace to the activity of a big, multicellular, bilaterally symmetrical animal," Matz said.
"This is a very important discovery," Shuhai Xiao of Virginia Polytechnic Institute said. "The fact that protists can make traces has important implications for how we interpret many trace fossils."
The finding could overturn conventional thinking on a mysterious time in the evolution of early life known as the Cambrian Explosion. Until about 550 million years ago, there were very few animals leaving trails behind. Then, within 10 million years an unprecedented blossoming of life swarmed across the planet, filling every niche with hard-bodied, complex creatures.
"It wasn't a gradual development of complexity," Matz said. "Instead these things suddenly seemed to burst out of a magic box."
Charles Darwin first noticed the Cambrian Explosion and thought it was an artifact of a poorly preserved fossil record. The precambrian trace fossils were left by multicellular animals, he reasoned, so there must be some gap in fossils between the nearly empty Precambrian and the teeming world that quickly followed. But if the first traces were instead made by G. sphaerica, it would mean the Explosion was real; it must have been a diversification of life on a scale never before seen.
Genetic analysis of the water-filled G. sphaerica cells also reveals tantalizing clues that it could be the oldest living fossil on the planet.
"There's a 1.8 billion-year-old fossil in the Stirling formation in Australia that looks just like one of their traces, and with a discoidal body impression similar to these guys." Matz said. "We haven't proved anything, but we might be looking at the ultimate living macroscopic fossil."