Scientists have discovered the oldest fossil evidence of organism motility, or the ability of an organism to move independently. The discovery shows that development of motility began more than two billion years ago.

The fossils were found in Gabon, in rock called shale. Researchers described them as tube-like structures up to 170 millimeters long. They say the structures may have been created when the organisms moved through soft, wet dirt in search of food or a better environment.

The researchers also said the organisms might have been moving together as a group. The organisms remains a mystery as the fossils are a record only of their movement.

The researchers said the fossils dated back to a time when Earth was rich with oxygen. These conditions permitted simple life forms to develop more complexity.

Life on Earth started with single-celled bacteria about 4 billion years ago. The earliest life forms could not move under their own power. The Gabon fossils dated to the Paleoproterozoic Era are about 1.5 billion years older than the previous earliest evidence of organism motility.

Abderrazak El Albani, a scientist with the University of Poitiers in France, led the team in Gabon. The shale deposit there has proven to be a treasure for the study of ancient life. Several years ago, Albani and his team discovered it held the oldest fossils know of multicellular animals.

"What matters here is their astonishing complexity and diversity in shape and size, and likely in terms of metabolic, developmental and behavioral patterns, including the just-discovered earliest evidence of motility, at least for certain among them," Albani said.

The first vertebrates, or animals with backbones, appeared about 525 million years ago. Dinosaurs came about 230 million years ago and human-like species developed about 300,000 years ago.

Scientists believe this early time in motility may have faced a setback soon after the Paleoproterozoic Era, about the time of the Gabon fossils. The Earth saw a considerable loss of atmospheric oxygen at that time.

The research was published in the Proceedings of the National Academy of Sciences.

I'm Pete Musto.

Reuters News Agency reported this story. Hai Do adapted it for VOA Learning English. Caty Weaver was the editor.