Billion-year-old fossil is the missing link that fills gap in evolution of complex animals

Charles Darwin: Scientist explains origin of life theory

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A team of international scientists from the UK and US unveiled their discovery today in the journal Current Biology. The billion-year-old microfossil, which was found in the Scottish Highlands, contains two distinct types of cells and may very well be the oldest multicellular animal on record. The “missing link” sheds new light on the origins of complex life, particularly between single-celled holozoa and more complex animals like sponges.

The term holozoa describes all living things – exclusive fungi – that include animals and some of their single-celled cousins.

A well-known example are the Choanoflagellates – single-celled critters believed to be the closest living relatives of animals.

The microfossil discovered by researchers at the University of Sheffield in the UK and Boston College in the US belonged to an organism somewhere between single-celled and multicellular animals.

The scientists have officially named their discovery Bicellum Brasieri.

The organism was very primitive and round, but its internal composition – with two distinct cell types – marks the first step towards more complexity.

The discovery also suggests multicellular animals began to evolve one billion years ago and in different environments than previously thought – freshwater lakes and not the oceans.

Professor Charles Wellman from the University of Sheffield and the lead investigator 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 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.”

Professor Paul Strother, the lead investigator of the research from Boston College, added: “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.”

The prehistoric fossil was discovered in the Northwest Scottish Highlands, at Loch Torridon.

The sea loch, which is about 15 miles (25km) long, was created by glacial processes.

Scientists were able to study the Bicellum Brasieri because the fossil was exquisitely preserved.

The preservation allowed the scientists to analyse the fossil at a cellular and subcellular level.

The scientists are now looking forward to examining the Torridonian deposits for more exciting discoveries, hoping they could shed more light on the evolution of multicellular organisms.

The research was mainly funded by the UK Natural Environment Research Council (NERC).

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