A collection of short wiggly structures discovered in ancient rocks could be the earliest fossilised traces of organisms able to move themselves, scientists say.
If scientists are correct, the 2.1bn-year-old structures point to an earlier origin than generally thought for eukaryotes – cells with a membrane-bound nucleus and which make up plants, animals and fungi – previouslybelieved to have first emerged about 1.8bn years ago. It also pushes back the earliest evidence of self-propelled movement of eukaryotes by 1.5bn years – scooping the title from far younger multicellular lifeforms – and would be the first clear signs of motility for any type of organism.
“The question is: is this the first evidence of locomotion in some sort of complex living organism, and if it is, why is it that it took about 1,500m years before we start to see similar things in abundance in the geological record?” said Dr Ernest Chi Fru from Cardiff University, the co-author of the research. “Was it an experiment in locomotion that failed?”
Writing in the Proceedings of the National Academy of Sciences, the international team of researchers reveal how they used imaging techniques to probe what they describe as “string-shaped structures” found in a deposit of black shale – sedimentary rock that formed from a muddy marine bed.
Found in Gabon, Africa, in what is called the Francevillian Formation, and dated at about 2.1bn years old, the rocks have previously – and controversially – been reported to contain the earliest signs of multicellular life.
Now researchers say the rocks also contain the earliest evidence of locomotion: a host of structures of up to 6mm across and up to 170mm in length, some of which are intertwined, with rounded ends. While some run horizontally through the rock, others bend upwards.
Experts say the structures are probably the fossilised mucus trails of early multicellular organisms that travelled through sediment: many of the structures were found to be formed of pyrite (iron disulphide), probably produced as a result of activity by microbes that fed on the mucus.
The team say they considered, but ruled out, the possibility that the structures could have been formed by natural geological processes rather than living organisms, saying that the shape and texture of the structures, as well as chemical analysis of the pyrite points to a biological origin.
They add that while some might suggest the structures were formed by waves shifting microbial biofilms around, further inspection rules out such an explanation.
“What we see here does not conform to that sort of pattern,” said Chi Fru, adding: “The consistency of these twists and turns and the size of the burrows, it is very, very, very distinct and looks like what you would see for something living, not non-biological.”
The authors say the complexity of the movement shown by trails, and their dimensions, suggest they were made by a eukaryote, They add that it might have been similar, at least in behaviour, to slime moulds, which exist as individual amoebae but team up when food supplies dwindle to form a multicellular system that can travel. The team note that the newly revealed structures were often found close to the fossilised remains of microbial mats.
The researchers believe that “the microbial mats served as grazing grounds for this thing that was moving”, said Chi Fru.
While oxygen levels in the Earth’s atmosphere rose steeply several hundred million years ago – a change that has been linked to the emergence of complex life 540m years ago – there was a smaller spike in oxygen levels far earlier, during an interval known as the Lomagundi event. That, the researchers say, coincides with the dating of the Gabon fossils.
Dr Dominic Papineau, an expert in geochemistry and early fossils from University College London who was not involved in the study, said the findings were consistent with the structures being fossils. However, he said there was no signs that the organisms had tiny hairs or tail-like structures – signs of which do not preserve well in pyrite, he said – so it is not clear how the organisms moved.
“If they are correct, then their discovery challenges a few aspects of what we thought we knew about biological evolution,”said Papineau, although he added that it would tie in with the ancient spike in atmospheric oxygen, highlighting the importance of the gas.
However, Prof Graham Shields of University College London urged caution, noting that the structures might just be ripped-up or dried-out fragments of microbial mats or tube-like organisms called Grypania.
“I think the case for some biological connection is strong,” he said, adding the pyrite was probably produced through the activity of bacteria that use sulphur as fuel. “However, I don’t see much evidence for motility, ie wilful movement through a sediment in search of food, other than superficial resemblance to trails or burrows.”