Around 541 million years ago, life on Earth began to blossom as new, complex species started to appear. This is clearly evident in our fossil record, and the event is known as the Cambrian explosion. But it seems life appeared long before this took place. Over 100 million years before, in fact.

Researchers led by the University of California, Riverside have discovered traces of life in ancient oils and rocks found in Siberia, Oman, and India, which date back a vast 635-660 million years to the Neoproterozoic era. The findings are published in the journal Nature Ecology & Evolution.   

The team found a steroid compound called 26-methylstigmastane that’s only produced by one specific type of sponge – the demosponge. Therefore, it seems these sponges were bobbing about our seas over 100 million years earlier than expected – the famous Dicksonia fossil, which provides evidence for some of the earliest animal life, dates back 558 million years. 

"Molecular fossils are important for tracking early animals since the first sponges were probably very small, did not contain a skeleton, and did not leave a well-preserved or easily recognizable body fossil record," said lead author Alex Zumberge in a statement. 

"We have been looking for distinctive and stable biomarkers that indicate the existence of sponges and other early animals, rather than single-celled organisms that dominated the earth for billions of years before the dawn of complex, multicellular life."

The steroid compounds likely helped the sponges to live symbiotically with bacteria in a safe manner. Sponges tend to have mutually beneficial relationships with microorganisms that live inside them. For example, some sponges gain energy from photosynthetic cyanobacteria that receive protection from the sponges in return.

While it’s incredibly exciting to find the oldest ever signs of life, something is missing. Sponges have their own kind of skeletons made from silicon, which can become preserved as fossils, but we haven't found any of these fossils dating so far back.

Nevertheless, the fact we haven’t found fossils of the 660-million-year-old demosponges doesn’t mean the creatures didn’t exist, we may still find them.

“[Either] sponges were genuinely there that far back and nobody’s been able to find their fossils. Or they genuinely weren’t and the biomarkers are from something else,” palaeontologist Joe Botting told New Scientist.

Back in 2009, the same team of researchers found similar evidence of early life using a different steroid compound found in rocks in Oman. However, this sparked debate as to whether it really was solid evidence because some modern-day algae produce the same steroid compound. Since 26-methylstigmastane is unique to demosponges, the new findings provide much firmer evidence that the sponges, and therefore animal life, existed over 600 million years ago.

"This steroid biomarker is the first evidence that demosponges, and hence multicellular animals, were thriving in ancient seas at least as far back as 635 million years ago," said Zumberge.