Complex life on Earth may have started at least a billion years earlier than thought, and now new evidence that seems to back this up further has been revealed.

Dating to 1.6 billion years old, a fossil found in Gaoyuzhuang, China, seems to show a small piece of seaweed. Except this was thought to have been impossible as at this time it was believed that the oxygen levels were too low for multicellular organisms to evolve.

But now a new study published in Nature Geoscience shows that there may have been an unknown oxygenation event during the Mesoproterozoic era, and that complex life kicked off a billion years earlier than thought.

Until recently the oldest known confirmed fossil of a multicellular – or complex – organism dated to roughly 600 million years ago, something that fits nicely with a spike in oxygen at around the same time. It is thought that for complex life to evolve, the level of atmospheric and oceanic oxygen needed to be relatively high, and up until this point in Earth’s history that had simply not occurred, thus meaning that multicellular organisms were unlikely to have evolved.

That is why, when researchers argued in 2016 that they had discovered a fossil that seemed to show multicellular life existing some 1.6 billion years ago – a full billion years older than expected – there was a lot of swirling doubt. The fossils were clearly visible to the human eye, measuring in at 30 centimeters (11.8 inches) long and 8 centimeters (3.1 inches) wide, but were made up of tightly packed cells.

With the oxygen levels at the time thought to be too low, some suggested that it might not be a single organism at all but part of a biofilm of bacteria or cyanobacteria as seen in stromatolites, while others said that the authors had simply made a mistake with their interpretation.

But a new study has found that the Mesoproterozoic era, which covers 1.6 billion to a billion years ago, may have been far more dynamic than previously understood. By studying the rocks that formed at the bottom of the ocean at this time, the researchers found that there was a hitherto unknown rise in oxygen concentrations at the beginning of the era.

While they are unable to calculate exactly how much the levels of oxygen increased, they suggest that it might have been just high enough to spark the formation of multicellular life, which needed this increase in oxygen in order to survive. Even earlier fossils, such as a 2.1 billion-year-old example from Gabon, have largely been discredited. 

Where this boost of oxygen came from, however, is a little harder to discern. As is why it then took such a vast amount of time to evolve into anything more complex. For now, palaeontologists will continue searching for even earlier signs of complex life, to try to understand this further.