Swedish scientists have uncovered 1.6 billion-year-old fossils that indicate multi-cellular life developed much earlier than previously thought — possibly 400 million years earlier.

Therese Sallstedt of the Department of Palaeobiology at the Swedish Museum of Natural History was examining fossilized, layered mats of cyanobacteria collected in an ancient rock outcrop in central India when she came across what appear to be primitive algae fossils.

Cyanobacteria are basic, single-cell life forms that have existed for nearly 3.5 billion years and are among the oldest fossils known.

"I was studying the mat biota, expecting to find more variations of bacteria," Sallstedt said in an email. She didn't expect to find well-preserved fossilized algae, which was much bigger and more complex than the surrounding bacteria. She became "very excited."

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The newly-discovered algae, by comparison, lived approximately 1.2 billion years before the first plants are thought to have appeared on land.

The rock samples come from an area dotted with dozens of Hindu temples between the towns of Jankikund and Chitrakoot along the Son River Valley in Uttar Pradesh and Madhya Pradesh. The surrounding rocks consisted mostly of calcium and magnesium carbonates. But the cyanobacteria and algae fossils were preserved in a layer of calcium phosphate, which preserves finely detailed cellular structures that can be explored with X-ray tomography.

"Phosphate is something of an almost magical substance when it comes to preserving fine-scale details, like intercellular features in fossils," Sallstedt said. "Sedimentary phosphate deposits such as this can therefore open up a window of sorts into other time periods since the preservation is so good."

Geological map of the Vindhyan basin, central India.

Geochronological research conducted by Stefan Bengtson, also of the Swedish Museum of Natural History, helped to date the calcium phosphate layer within which Sallstedt found the algae fossils, as well as rock layers above and below.

"Importantly we can see that the algae are part of the mats and could not in any way have been introduced later, which per definition makes them the same age as that of the rocks," she said.

The findings appear in the journal PLOS.

Images of the red algae fossils seen through an electron microscope.

Caroline Strömberg, an associate professor of biology at the University of Washington, said a better knowledge of the chronology of evolution allows for a deeper understand of what conditions were important for complex life to develop.

"Where did we come from? How did life evolve, and how did life come about from inorganic molecules? I think that is sort of a fundamental, existential question," she said. "That can help us perhaps locate those places that are a likely breeding ground for life on other planets. I think those are the questions that should matter to people. I hope."

For Sallstedt, who's working on a Ph.D., the find was an auspicious beginning to her career.

"It was a great moment finding those specimens," she said.

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