Scientists have discovered in 2.1-billion-year-old black shale from a Gabonese quarry evidence of a revolutionary development in the history of life on Earth - the ability of organisms to move from one place to another.
The researchers on Monday described fossils of small tubular structures created when unknown organisms moved through soft mud in search of food in a calm and shallow marine ecosystem.
The fossils dated back to a time when Earth was oxygen-rich and boasted conditions conducive to cellular life evolving more complexity.
Life emerged in Earth's seas as single-celled bacterial organisms about four billion years ago, but the earliest life forms lacked the ability to move independently, called motility.
The Gabon fossils are roughly 1.5 billion years older than the previous earliest evidence of motility and appearance of animal life.
The Gabonese shale deposits have been a treasure trove, also containing fossils of the oldest-known multicellular organisms.
"What matters here is their astonishing complexity and diversity in shape and size, and likely in terms of metabolic, developmental and behavioural patterns," paleobiogeochemist and sedimentologist Abderrazak El Albani of the University of Poitiers in France said.
The fossils did not include the organisms themselves.
The tubular structures, up to 6.7 inches (170 mm long), were originally made of organic matter, perhaps mucus strands left by organisms moving through mud.
The researchers said the structures may have been created by a multicellular organism or an aggregation of single-celled organisms akin to the slug-like organism formed when certain amoebas cluster.
In comparison, the first vertebrates appeared about 525 million years ago.
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