One billion years ago, the world's landmasses were joined together into an enormous supercontinent called Rodinia. Apart from a few fungi, life on land was scarce, but rivers and oceans were teeming with single-celled organisms.

Scientists from the University of Western Australia and Boston College recently uncovered some of these archaic creatures in 992 million-year-old rocks in Northwest Scotland. What's more, the microscopic fossils were preserved so remarkably that the researchers were able to see distinct cellular structures inside.

The work is published in Scientific Reports.

The discovered cells (pictured top) are called coccoids, which refer to "unicellular, non-flagellated, non-amoeboid organisms, with a definite shape, in general but not always ovoid." While it's impossible to conclude exactly what the microfossils are, Dr. David Wacey, a microbiologist at the University of Western Australia, and his co-authors think they are likely eukaryotic algae or cyanobacteria. That's because these microbes are known to take up rare earth elements, which suffuse throughout the fossils.

It's thanks to these rare earth elements, specifically taking the form of the minerals monazite and xenotime, that the cells are so marvelously preserved, the researchers say. Soon after death, the minerals precipitate and harden, fossilizing the cells and their innards.

Inside the cells, Wacey and his team saw shapes called intracellular inclusions (ICIs) that could have been cytoplasmic storage granules or even distinct organelles, but the researchers couldn't be certain.

Images of the cells containing REE phosphate. (A,B) Three cells within an open cluster that contain REE phosphate (white) associated with spheroidal organic ICIs (red arrows), plus occasional wisps of black organic material preserved in francolite or clay minerals.

"We interpret the ICIs... as largely plasmolyzed cell membranes and/or permeable wall layers which themselves contain the degraded remains of entire cells," they write.

Far older fossilized cells have been found in the past, but none with such exceptional preservation.

"We are reaching a new threshold in the study of Precambrian microorganisms, whereby the taphonomy of intracellular structure can now be addressed directly," the researchers write. "This has the potential to provide detailed insight to the biology and environmental chemistry of ancient ecosystems."

Source: David Wacey, Eva Sirantoine, Martin Saunders & Paul Strother. "1 billion-year-old cell contents preserved in monazite and xenotime." Scientific Reports volume 9, Article number: 9068 (2019) https://doi.org/10.1038/s41598-019-45575-4