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Single cell giant up-ends early evolution

Slowly rolling across the ocean floor, a humble single celled creature is poised to change our understanding of how complex life evolved on earth.

The grape-sized Gromia sphaerica, a distant relative of microscopic amoebas, had previously been discovered lying motionless at the bottom of the Arabian Sea.

But when Assistant Professor Mikhail Matz of the University of Texas 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 50 centimetres long in the mud.

The trouble is that these single celled critters, also known as giant deep sea protists, aren't supposed to be able to leave trails.

Trace fossils

The oldest fossils of animal trails, called 'trace fossils', date to around 580 million years ago, and palaeontologists believed 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 three centimetres in diameter, they're enormous compared to most of their microscopic cousins.

"If these guys were alive 600 million years ago, and their traces got fossilised, a palaeontologist 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," says Matz.

"This is a very important discovery," says Professor Shuhai Xiao of Virginia Polytechnic Institute. "The fact that protists can make traces has important implications for how we interpret many trace fossils."

The finding could overturn conventional thinking on the evolution of early life during a period known as the Cambrian explosion.

Until about 550 million years ago, there were very few animals leaving trails behind. Then, within ten 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," says Matz. "Instead these things suddenly seemed to burst out of a magic box."

Species explosion

Charles Darwin first noticed the Cambrian explosion and thought it was an artefact 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," says Matz. "We haven't proved anything, but we might be looking at the ultimate living macroscopic fossil."