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Oldest complexityA three dimensional X-ray image of the outer (left) and inner (right) body of a fossil from the Gabonese site. A. EL ALBANI

Researchers have found what may be the earliest evidence of multicellular life on Earth. Large fossils uncovered in 2.1 billion-year-old rock from Gabon, in western Africa, appear to be incipient examples of macroscopic life in what was then a sea of single-celled microbes.

Scientists believe that multicellular life really took off much later, in the great expansion of animal body plans known as the Cambrian explosion 545 million years ago.

“The discovery is fantastic because it shows the existence of multicellular fauna 1.5 billion years earlier than what we know,” says team leader Abderrazak El Albani, a sedimentologist and paleobiologist at the University of Poitiers in France. “This is important to understand the evolution of life on Earth.”

Some researchers have suggested multicellular organisms arose as early as 1.6 billion years ago, but the evidence is controversial. El Albani and his colleagues were thus surprised to find large fossils in the newly excavated ancient Gabonese rocks. So far, the team has collected over 250 specimens that range in size from 1 to 12 centimeters.

Using detailed X-ray imaging called microtomography, the team created three-dimensional images of the fossils inside and out. The organisms had flat, round, soft bodies, with slits around the edges and complex, patterned folds inside. The creatures belong to new species that have never been described, the team reports in the July 1 Nature.

Other researchers agree that the large size, thickness, and three-dimensionality of the organisms suggest that they were, indeed, multicellular. “There does seem to be something more than just a clonal colony of bacteria,” says paleobiologist Philip Donoghue of the University of Bristol in the United Kingdom.

El Albani and his team believe the complex patterns and folding mean that cells must have coordinated their growth through chemical signaling, like all multicellular organisms do. The fossils could even be the first examples of eukaryotes, cells with membrane-bound nuclei, according to the team.

But since actual cells were long gone from the sediments, the team had to prove these fossils were not simply mineral formations that looked like animals.

Pyrite, a sulfur-containing mineral also known as fool’s gold, filled the fossils, providing evidence that sulfate-breathing bacteria had eaten away at living tissue. Carbon and sulfur isotopes also confirmed the fossils' organic origins.

Further analysis showed that the fossils couldn't have been more recent organisms that burrowed deep into sediments, because the surrounding rock was the same inside and outside the organisms’ folds.

Rock chemistry indicates the organisms lived about 30 to 40 meters deep in sea water. They likely breathed oxygen, which by that time had been building up in the oceans and atmosphere for 350 million years. Donoghue says it’s exciting that scientists are “edging back” the fossil record toward this so-called “great oxidation event” 2.5 billion years ago.

The research team plans to do more experiments to determine how these organisms lived and how to further categorize them.

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