Scientists love to put things in categories. In the natural world, life is divided into several categories: animals, plants, fungi, protists (such as algae or amoebas), bacteria and archaea (microscopic single-celled organisms).
But recent research has suggested this list might not be exhaustive. That’s because of a prehistoric cylindrical lifeform that some UK scientists think belonged to a now-extinct, totally unique branch of the tree of life.
The strange organism in question is called Prototaxites and lived approximately 440 to 360 million years ago.
“These organisms evolved during the period where we think the Earth initially became habitable for animals like ourselves,” Benjamin Mills, professor of earth system evolution at the University of Leeds, told BBC Science Focus. Mills was not involved in this research.
“It was a time when atmospheric oxygen reached breathable levels, which we attribute to plants and fungi spreading across the land surface and boosting global photosynthesis rates,” he continued.
Prototaxites could tower up to eight metres tall and stretch a metre wide, often resembling a massive, branchless tree trunk — though some were just a few inches tall.
Scientists have been arguing about what this thing was since the first Prototaxites fossils were discovered back in 1843, and recent research concluded it was a type of weird fungus.
However, in a new study that has not yet been peer-reviewed, scientists analysed the fossilised remains of Prototaxites taiti – a smaller species of Prototaxites, found in Scotland – and concluded that it was not a fungus after all, but instead something totally distinct.
“Prototaxites has always been a unique mystery, but assuming it was a fungus has helped us to place it within our current understanding of how life and the Earth have interacted over time,” said Mills. “But this new work shows quite confidently that the Prototaxites was not part of this plant-fungal radiation and was something else entirely.”
The scientists analysed fossils of P. taiti and compared it with the fossilised remains of true fungi from the same era, preserved at the same place.
Inside P. taiti were masses of tubes – just like modern fungi – but the scientists found these tubes were quite different from fungal tubes, branching off and reconnecting strangely.
Also, most fungal cells contain a chemical called chitin or chitosan, but when the scientists analysed the molecular composition of P. taiti, they found no evidence of chitin at all. Instead, the cells contained something called lignin – more similar to the wood or bark of plants.
“It’s worth pointing out that chitin can be present in tiny amounts in fungal cells, or even completely absent,” Neil Gow – professor of microbiology at the University of Exeter, who was not involved in the research – told BBC Science Focus.
However, the scientists concluded that P. taiti was not a fungus but came from a lineage that is now extinct.
“We know that these organisms didn’t survive,” said Mills. “But what if they and others like them had done, and had dominated the land surface? Would our oxygen-rich atmosphere have lasted?
“These questions are very important when we think about the possibility for habitable planets, or intelligent oxygen-breathing life, existing elsewhere.”
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About our experts
Benjamin J. W. Mills is a biogeochemist and professor of earth system evolution at the University of Leeds's School of Earth and Environment. He leads the Earth Evolution Modelling Group and is the chair of the Earth System Science Group of the Geological Society of London. He is interested in the changes in the surface environment that have made our planet habitable for complex life, and whether a similar trajectory might be possible on other worlds.
Neil Gow is a professor of microbiology and, until 2023, was the deputy vice chancellor for research and impact at the University of Exeter. His research focuses on life-threatening fungal diseases and his research group works on the fungal cell wall.