Scientists, led by the UK’s University of Sheffield and Boston College in the USA, discovered a microfossil in the Scottish Highlands, which could be the oldest fossil of its kind on record.
The tiny fossil, dated to around one billion years ago, contains two distinctly different cell types and appears to belong to an ancient organism somewhere between unicellular and multicellular animals. This discovery could provide insight into how and where animal life evolved.
Palaeobiologist Charles Wellman, from the University of Sheffield, said:
The origins of complex multicellularity and the origin of animals are considered two of the most important events in the history of life on Earth; our discovery sheds new light on both of these.
We have found a primitive spherical organism made up of an arrangement of two distinct cell types, the first step towards a complex multicellular structure, something which has never been described before in the fossil record.
The fossils measure under 30 micrometers (0.003 centimeters) across. They were found in the Diabaig Formation at Loch Torridon, a sea loch on Scotland’s west coast in the Northwest Highlands, an area containing microfossils from lacustrine deposits dating back 1 billion years. The sedimentary deposits from the ancient lakebed have kept the tiny fossils in an incredible state of preservation, down to the subcellular level.
The new organism, called Bicellum brasieri, was preserved so well that its structure was clearly visible in several fossils. Its mature form appears to have comprised of a tiny sphere of tightly packed, spherical cells (named stereoblast), surrounded by a differentiated outer single layer of sausage-shaped cells.
Two populations displayed a mixture of cell types throughout the stereoblast. The scientists have interpreted this as a more juvenile form of the organism during the progression of differentiation, with the sausage-like cells developing and in the process of moving towards the stereoblast’s exterior.
Other multicellular organisms from around the same era have previously been discovered, including green algae and fungus. However, the morphology of the newly identified Bicellum is more consistent with Holozoa, the group that comprises animals and their closest unicellular relatives.
Bicellum could be a significant piece of Earth’s evolutionary puzzle. The organism could help us understand the shift from unicellular holozoa to more complex multicellular creatures and the origins of specific behaviors exhibited by complex animals.
Paleobotanist Paul Strother, from Boston College, explained:
Biologists have speculated that the origin of animals included the incorporation and repurposing of prior genes that had evolved earlier in unicellular organisms. What we see in Bicellum is an example of such a genetic system, involving cell-cell adhesion and cell differentiation that may have been incorporated into the animal genome half a billion years later.
One of the biggest debates about the origins of life is whether it occurred in freshwater terrestrial lakes or the salty oceans. This discovery could help fill in some of these gaps about where specific lifeforms evolved.
A marine environment seems likely for the first microbial life, given the growing evidence of a highly soggy early Earth as late as 3.2 billion years ago, and fossils have been identified dating back 3.5 billion years. Still, Bicellum suggests that lakes were also very significant.
The discovery of this new fossil suggests to us that the evolution of multicellular animals had occurred at least one billion years ago and that early events prior to the evolution of animals may have occurred in fresh water like lakes rather than the ocean.
Like many developments in the world, a complex mix of ingredients probably contributed to the evolution of our present Earth. The team is optimistic that the Diabaig Formation will contain even more clues to understanding the fascinating evolutionary tale. The findings were published April 13, 2021, in Current Biology.