A fossil found in billions of years old rocks from the Scottish Highlands could represent the lack of connection between single-celled organisms and multicellular animals. If so, it may fill some of the most significant gaps in our knowledge of life history.
“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,” said Professor Charles Wellman of the University of Sheffield in a statement.
“We have found a primitive spherical organism consisting of an arrangement of two different cell types, the first step towards a complex multicellular structure, something never described before in the fossil record,”
Wellman and co-authors named the species Bicellum brasieri in Current Biology, the genus is called a nod to its two cell types. 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, where the animals are only a twig, with many single-celled organisms more closely related to us than fungi, or more distant relatives such as plants, also included.
“The mature form of Bicellum consists of a solid, spherical sphere of tightly packed cells … enclosed in a monolayer of elongated, sausage-shaped cells,” the paper notes. Like the whole object, the inner cells are almost spherical and 2-3 μm across. The outer cells are slightly narrower, but usually three to four times as long. Whole balls average 28.5 μm (0.001 inches), about the thickness of a particularly fine human hair.
“However” the paper continues; “Two populations … show mixed cell forms that we derive to indicate the beginning development of elongated cells that migrated to the periphery of the cell mass.” The shape is even similar to that produced in an experiment designed to simulate how multicellular organisms could have originated.
If Bicellum does not teach us something else, we can learn a lot from the rocks where it was found. Their age tells us that the first agitations of multicellularity took place at least a billion years ago, while their composition indicates that this happened in freshwater lakes rather than the ocean. Timing could help resolve the question of whether oxygenation of the atmosphere was a necessary condition for animals to evolve. Even better, the discovery can help answer the basic question of how something 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 reuse of earlier genes that had previously evolved into single-celled 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 genome of animals half a billion years later. ”
Having gained the decisive advantage over its competitors by cooperating from various specialist cells, Bicellum or its close relatives may have dominated much of the earth. Before the development of hard skeletons, however, life did not fossilize well, so the discovery of Bicellum in the Torridonian deposit required a remarkable level of conservation that we cannot rely on to find often. An abundance of other microfossils have also been found next to Loch Torridon. The authors hope to find many more fossils from the same era, perhaps including Bicellum’s predecessors or even more advanced species.
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