杏吧原创

Sponges were probably one of the earliest animal groups to evolve 鈥� but researchers have had trouble working 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 seafloor 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 鈥� steroids with antibacterial properties that could let them harbour microbes without harm.

The earliest sponges belong to a class called demosponges, which transform sterols into a compound called sterane that can be fossilised. 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 660 and 635 million years old.

Ancient steroids

They found an abundance of a sterane called 26-methylstigmastane, or 26-mes, that as far as we know is only produced by demosponges. Love and his colleagues used high-pressure hydrogen to break down organic polymers that bind this compound to the rocks it was embedded within, while preserving the molecular structure.

In previous work, Love had found another biomarker 鈥� 24-ipc 鈥� in the same ancient rocks. He and his colleagues claimed that this earlier discovery was also evidence that sponges were living in the oceans when the rocks formed 鈥� but some modern algae make a similar compound, and so other researchers argued that it wasn鈥檛 clear whether the ancient 24-ipc really did come from sponges.

Love says the evidence for sponges in the rocks is now clearer. 鈥淲e鈥檝e doubled up the evidence, and it鈥檚 probably greater than double because we鈥檙e coming out with a biomarker that鈥檚 exclusive to sponges,鈥� he says.

But the finding hints at a big puzzle. While sponges don鈥檛 have bones to leave behind, they do have 鈥渟keletons鈥� made of silica fibres called spicules that give structure to their holey bodies. But we don鈥檛 see , 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 that, at the moment, there is no way of unravelling,鈥� says Joe Botting at the National Museum Wales in the UK.

Sponge search

It could be that we just haven鈥檛 found the fossils yet. But Botting says there are many groups searching in rocks聽that should preserve such fossils, and they鈥檙e coming up empty. 鈥淚t鈥檚 not necessarily the case that they鈥檙e not there. The lack of evidence isn鈥檛 evidence of absence,鈥� he says.

It may also be that some other animal is responsible for synthesising these compounds, having evolved the same ability independently of sponges. While we know that sponges make more sterols than all other complex life forms combined, and we haven鈥檛 found other organisms that produce the specific molecules Love鈥檚 team found, it is possible they are 鈥� or were 鈥� once out there.

鈥淭here are two possibilities,鈥� says Botting. 鈥淪ponges were genuinely there that far back and nobody鈥檚 been able to find their fossils. Or they genuinely weren鈥檛 and the biomarkers are from something else. Something is really wrong.鈥�

Love says there is another alternative: the very first sponges may have lacked silica spicules that can be preserved as microfossils in rocks. If they did, 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, researchers like Botting could be looking for fossils that don鈥檛 exist.

To solve this standoff between palaeontology and biochemistry, Love suggests we look to more recent sponges. 鈥淲e鈥檇 have to see the biomarker in rock cores at the same time as sponge body fossils in younger rocks,鈥� he says.