Bicellum brasieri, a freshwater protist that lived nearly one billion years ago, had two distinct cell types and could be the earliest multicellular animal ever recorded. Found in the Scottish Highlands, the microfossil reveals a new insight into the transition of single-celled holozoans into more complex multicellular animals.
Bicellum brasieri in mature form; all specimens were preserved in petrographic thin sections from the Diabaig Formation stratotype, Lower Diabaig, Scotland, UK. Scale bars – 5 μm in (A-J), 10 μm in (K). Image credit: Strother et al., doi: 10.1016/j.cub.2021.03.051.
“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, a researcher in the Department of Animal and Plant Sciences at the University of Sheffield.
Professor Wellman and colleagues examined thin sections of phosphatic lenses from the Diabaig Formation in Scotland that preserve populations of organisms trapped in ancient lake bottom sediments.
In several thin sections, they observed cell clusters that are composed of aggregations of two distinct cell types, indicating a condition that constitutes a step toward complex multicellularity.
Further investigation revealed a second set of cell clusters that appeared very similar in size and form but that lacked the fully differentiated second cell type.
Bicellum brasieri belongs to Holozoa, a clade of organisms that includes animals and their closest single-celled relatives, but excludes fungi. Image credit: Strother et al., doi: 10.1016/j.cub.2021.03.051.
“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,” Professor Wellman said.
“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 freshwater like lakes rather than the ocean.”
“Biologists have speculated that the origin of animals included the incorporation and repurposing of prior genes that had evolved earlier in unicellular organisms,” said Professor Paul Strother, a researcher in the Department of Earth & Environmental Sciences at the Weston Observatory of Boston College.
“What we see in Bicellum brasieri 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.”
The findings are published in the journal Current Biology.
Paul K. Strother et al. A possible billion-year-old holozoan with differentiated multicellularity. Current Biology, published online April 13, 2021; doi: 10.1016/j.cub.2021.03.051
http://www.sci-news.com/paleontology/bicellum-brasieri-09615.html