(CNN) - A fossilized mucus trail led to the discovery of the oldest evidence for mobility on Earth, dating to 2.1 billion years ago, according to a study published Monday in the journal Proceedings of the National Academy of Sciences.
Previously, the oldest known traces of movement were dated to 570 million years ago, so this discovery significantly pushes back the timeline for mobile life.
Researchers have struck science gold twice now in Gabon's Fraceville Basin, where fossils are extremely well-preserved. The international research team found the earliest evidence for multicellular organisms there, and now, they have found fossilized tracks signifying mobility.
This means some multicellular organisms were able to navigate their primitive marine ecosystem, rich in organic matter, by moving through the mud.
In order to study the tracks, the researchers used a nondestructive imaging technique involving X-rays to analyze and reconstruct the traces in 3D. Remarkably, the millimeters-wide tubular structures are vertical as well as horizontal. Horizontal was expected; that would be natural movement as they pushed through the sediment layer. Vertical signifies more complexity and requires different movement. Although vertical movement was possible only in soft muds, like a mixture of clay and organic matter, the researchers were still surprised to see those varying tracks.
Over time, these sediments turned into black shale.
Although the tracks are incredibly well-preserved, it's difficult to say what the organisms that made them looked like. The researchers believe they acted like colonial amoebae or slime molds. These cluster together to form a slug when resources disappear, a formation that allows them to move to an environment that will supply what they need.
This isn't the oldest evidence for life on Earth, which is bacterial.
"Life on our planet could be nearly around 3.8 billion years old," said Abder El Albani, study author and professor at the Universite de Poitiers in France, in an email. "But for nearly 2 billion years it was bacterial. Even if their precise place on the tree of life is still a matter of discussion and deserves additional research, the Gabonionta are the earliest multicellular macroscopic organisms discovered so far.
But these more complex organism emerged during an interesting time.
The researchers studied the sediments in which they found the tracks of "Gabonionta," as they're calling the organisms. They were found under what would have been a shallow water column full of oxygen.
Previously, Earth was a more harsh place, with a carbon dioxide atmosphere and significant lack of oxygen.
But oxygen was prevalent during the time of Gabonionta, which probably used aerobic respiration to live, turning things like fats and sugars into energy. Mobility allowed these organisms to be more complex.
"What matters here is their astonishing complexity and diversity in shape and size, and likely in terms of metabolic, developmental and behavioural patterns, including the just discovered earliest evidence of motility," El Albani said.
But that in itself leads to another mystery. About 2.083 billion years ago, there was a significant drop in oxygen on Earth, leading to a long period of harsh conditions that would have cut short or limited this kind of evolution of complex organisms, El Albani said.
So did this complex organism and its movement lead to the evolution of better and more sophisticated movement, or was it cut short by the lack of atmospheric oxygen? Only time, and more research, will tell. For now, Gabonionta has no analog in later times, the researchers said.