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SPONGES were probably one of the earliest animal groups to evolve, but it has proved hard to work out exactly when in geological time they appeared.

Now, an analysis of ancient rocks and oils has turned up traces of steroids made by early sponges that indicate they may have been populating the ancient sea floor at least 120 million years earlier than we thought.

鈥淚f animals first appeared in a predominantly bacterial or microbial world, they would need to harness microbes and live symbiotically with them,鈥� says Gordon Love at the University of California, Riverside. That may be why sponges produce a vast array of sterols: steroids with antibacterial properties that could let them harbour microbes without harm.

The earliest sponges belong to a class called demosponges, which produce sterols that can be preserved in rocks as characteristic sterane molecules. Love and his team went hunting for these 鈥渕olecular fossils鈥� in rock and oil samples from Oman, Siberia and India that date to between 635 and 660 million years ago.

They found plenty of a sterane called 26-methylstigmastane that, as far as we know, is only produced by demosponges.

In previous work, Love had found another possible sponge biomarker, called 24-isopropylcholestane (24-ipc), in the same rocks. But some modern algae make a similar compound, so the ancient 24-ipc might not have come from sponges. Love says the evidence for sponges in the rocks is now clearer (Nature Ecology & Evolution, ).

But the finding hints at a big puzzle. Sponges have 鈥渟keletons鈥� made of silica fibres called spicules that give structure to their holey bodies. We don鈥檛 see fossil evidence of spicules until about 540 million years ago, at the dawn of the Cambrian period.

鈥淚f the biomarkers here are a genuine sign of sponges, then we鈥檝e got a huge problem with the fossil record,鈥� says Joe Botting at the National Museum Wales, UK.

It could be that older spicules are out there and we just haven鈥檛 discovered them yet. But Botting says there are many people hunting for them 鈥� and they have yet to find any.

So perhaps some other organism was making these compounds 660 million years ago, having evolved the same ability independently of sponges. Although we know that, today, sponges produce more sterols than all other complex life forms combined, and we haven鈥檛 found other living organisms that make the specific molecules that Love鈥檚 team found, there is a chance they are 鈥� or once were 鈥� out there.

Love says there is another possibility: the very first sponges may have lacked silica spicules that would have been preserved as microfossils. If so, this would explain why there are molecular signs that sponges were present 660 million years ago even though there is no microfossil evidence. In other words, we could be looking for fossils that don鈥檛 exist.

This article appeared in print under the headline 鈥淓arliest signs of animal life found鈥�