A billion-year-old fossil could be the oldest multicellular animal on record

A tiny fossil found in the Scottish Highlands could be a missing link in the evolutionary history of animals.

Dated to around a billion years ago, the microfossil shows evidence of two distinctly different cell types, and it appears to belong to an ancient organism somewhere between unicellular and multicellular animals. That probably makes it the oldest such fossil on record, a find that could provide insight into how and where animal life evolved.

“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,” said paleobiologist Charles Wellman. from the University of Sheffield in the UK. .

“We have found a primitive spherical organism composed of an arrangement of two distinct cell types, the first step towards a complex multicellular structure, something that has never before been described in the fossil record.”

The fossils – measuring less than 30 micrometers in diameter – were found in the Diabaig Formation at Loch Torridon, an assemblage containing microfossils from a lacustrine environment dating to 1 billion years ago. The stone deposits of the old lake bed have maintained the fossils in a remarkable state of preservation, down to the subcellular level.

The new body, named Bicellum brasieri, was so well preserved in multiple fossils, that its structure was clearly visible. In its mature form, it appears to have consisted of a tiny sphere of tightly packed, roughly spherical cells (called stereoblasts), surrounded by a single differentiated outer layer of elongated, sausage-shaped cells.

Bicellum brasieri. (Strother et al., Curr. Biol., 2021)

Two populations, however, show a mix of cell types throughout the stereoblast. The researchers interpreted this as a more juvenile form of the organism during the process of differentiation, with sausage cells growing and migrating out of the stereoblast.

Other multicellular organisms from the same era have been identified, including fungi and algae, but the morphology of bicelleaccording to the researchers, is more consistent with Holozoa, the group that contains animals and their closest single-celled relatives.

It means that bicelle could be an important piece of Earth’s evolutionary puzzle – helping us not only to understand the transition from single-celled holozoa to more complex multicellular animals, but also the origins of certain traits exhibited by complex animals.

“Biologists have hypothesized that the origin of animals included the incorporation and reassignment of earlier genes that had evolved earlier in single-celled organisms,” explained paleobotanist Paul Strother of Boston College.

“What we see in bicelle 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 discovery could also help fill in some gaps about the evolution of specific life forms. One of the biggest debates raging about the origins of life is whether it occurred in salty oceans or in terrestrial freshwater lakes.

Given that evidence is mounting for a very, very waterlogged early Earth just 3.2 billion years ago, and fossils dating back 3.5 billion years have been discovered, a marine environment seems probable for the very first microbial life, but bicelle suggests that lakes were important too.

“The discovery of this new fossil suggests to us that the evolution of multicellular animals occurred at least a billion years ago and that the earliest events predating animal evolution may have occurred in lakes. freshwater rather than in the ocean,” Wellman said.

Like many processes in this wondrous world of ours, it is likely that a complex mix of ingredients contributed to the evolution of the Earth we know and love today. The team hopes that Diabaig’s formation will contain even more clues to understand this fascinating story.

The research has been published in Current biology.