For most of the time Earth has been inhabited, life took the form of single-celled organisms that just sat there in lumps, or floated around on water currents. But now fossils found in the African country of Gabon have turned up the earliest evidence of life showing some initiative and moving around of its own accord. It now seems that life was mobile some 1.5 billion years earlier than previously thought.

The earliest forms of life didn't seem to have bodies that fossilized well, so most of the fossil evidence we have of their existence is what are known as trace fossils, which are things like tracks and burrows. The oldest of these is roughly half a billion years old.

The newly-discovered fossils also fall into this category, but are much older – using geometrical and chemical dating the team says they're about 2.1 billion years old. That makes them the oldest known examples of locomotion in multicellular organisms, by quite a wide margin.

These trace fossils are made up of a series of tiny tubes in the rock, measuring just a few millimeters wide. From this and analyses of the surrounding material, the researchers can infer a few things about the organism that left them. For one, it may have been fairly slug-like, and it seems to have lived in oxygenated waters, suggesting it depended on oxygen – a key development in the history of life on Earth.

Fossilized microbial mats were also found nearby, suggesting a possible food source. But the most interesting tidbit might just be that this more complex organism might have been "multicellular" in the sense that it was a collection of single-celled organisms that clumped together to move around as a slug-like "colony", since they couldn't exactly move around much on their own. This could have been an important step in the evolution of multicellular lifeforms.

As far-fetched as it sounds, the phenomenon has been observed in modern amoeba as a survival mechanism when there's a shortfall of food. Either way, it's a fascinating discovery that helps fill in more blanks in the history of life on Earth.

It is plausible that the organisms behind this phenomenon moved in search of nutrients and oxygen that were produced by bacteria mats on the seafloor-water interface," says Ernest Chi Fru, co-author of the study. "The results raise a number of fascinating questions about the history of life on Earth, and how and when organisms began to move. Was this a primitive biological innovation, a prelude to more perfected forms of locomotion seen around us today, or was this simply an experiment that was cut short?"

The research was published in the journal PNAS.

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