A fossil found in billion-year-old rocks from the Scottish Highlands could represent the missing link between single-celled organisms and multicellular animals. If so, it could fill in some of the most significant gaps in our knowledge of the history of life.
“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,” Professor Charles Wellman of the University of Sheffield said in a statement.
“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,” Wellman continued.
Wellman and coauthors named the species Bicellum brasieri in Current Biology, the genus name a nod to its two type of cells. The species name honors co-author Martin Brasier, who died between discovery and publication.
The authors think Bicellum was probably a holozoan, rather than algae. Holozoans form a broad branch of the tree of life, on which animals are but one twig, with many single-celled organisms more closely related to us than fungi, or more distant relatives like plants, also included.
“The mature form of Bicellum consists of a solid, spherical ball of tightly packed cells… enclosed in a monolayer of elongated, sausage-shaped cells,” the paper notes. Like the whole object, the interior cells are nearly spherical, and 2-3μm across. The outer cells are a little narrower, but usually three to four times as long. The whole balls average 28.5μm (0.001 inches), roughly the thickness of a particularly fine human hair.
“However,” the paper continues; “Two populations… show mixed cell shapes, which we infer to indicate incipient development of elongated cells that were migrating to the periphery of the cell mass.” The shape even resembles that produced in an experiment designed to simulate how multi-celled organisms might have arisen.
If Bicellum teaches us nothing else, we can learn a lot from the rocks in which it was found. Their age tells us the first stirrings of multicellularity took place at least a billion years ago, while their composition indicates this occurred in freshwater lakes, rather than the ocean. The timing could help resolve the question of whether the oxygenation of the atmosphere was a necessary condition for animals to evolve. Better still, the discovery could help answer the fundamental question of how anything more complex than a single cell could evolve at all.
First author Professor Paul Strother of Boston College said, “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.”
Having gained the crucial advantage over its competitors of collaboration by different specialist cells, Bicellum or its close relatives may have dominated much of the Earth. Prior to the development of hard skeletons, however, life didn’t fossilize well, so the finding of Bicellum in the Torridonian deposit required a remarkable level of preservation that we can't count on finding often. An abundance of other microfossils has also been found beside Loch Torridon. The authors hope to find many more fossils of the same era, perhaps including Bicellum’s predecessors, or even more advanced species.