Sponge fossils push back dawn of animal life

10 February 2009 by Tom Marshall

Scientists have found the oldest evidence for animals in the fossil record, pushing back the date of the earliest-known animal life by at least 100 million years, perhaps to as much as 750 million years ago.

Tropical sponges

The researchers analysed ancient rocks in the South Oman Salt Basin, off the south-eastern edge of the Arabian peninsula. They found chemical traces of early life, or 'biomarkers', showing the presence of demosponges, a class of sponge that accounts for around 90 per cent of sponges living today.

This is the first proof that multicellular animals evolved before the end of one of the greatest ice ages ever to affect the Earth, known as the Marinoan period of glaciation. The research appears in Nature.

The biomarkers form a continuous chemical fossil record of marine sponges living in the area's seas during the late Neoproterozoic period, which lasted from 1000 to 542 million years ago, and into the Early Cambrian, which lasted until around 513 million years ago.

We believe that we are converging on the correct date for the divergence of complex multicellular animal life.

- Dr Gordon Love

"We have unearthed the first fossil evidence for Cryogenian animals from the detection of anomalously high amounts of distinctive steroids produced by sponges," says Dr Gordon Love, a biogeochemist at the University of California and principal author of the study. "We believe that we are converging on the correct date for the divergence of complex multicellular animal life, between 635 and 750 million years ago," he adds.

The findings show that shallow waters over continental shelves contained enough dissolved oxygen to support multicellular animal life at least 100 million years before the rapid diversification of complex animals that palaeontologists call the 'Cambrian explosion', which happened about 530 million years ago.

The scientists analysed hydrocarbons in 64 samples of sedimentary rock drilled from 26 oil wells in the area. Using a technique called hydropyrolysis, they isolated the biomarkers from the hydrocarbon deposits locked in the rocks.

Hydropyrolysis allowed us to release key biomarkers from ancient rocks.

- Professor Colin Snape

Hydropyrolysis uses high-pressure hydrogen gas to extract biomarkers that are bound into the rock without damaging or deforming them as with previous pyrolysis methods, which rely on higher temperatures, would have done.

"Hydropyrolysis allowed us to release key biomarkers from ancient rocks with minimal structural alteration," says Professor Colin Snape, director of the Energy Technologies Research Institute at the University of Nottingham - one of the scientists who pioneered the technique and among the Nature paper's authors.

The biomarkers come from steranes - complex organic molecules associated with the presence of sponges. They were found to be present in large quantities - 300 to 13,000 parts per million of total saturated hydrocarbons, compared to less than one part per million in rock samples of similar age taken elsewhere.

The biochemistry of the samples puts firm limits on when the first sponges could have appeared, and the paper's authors write that the evidence "suggests that sponges were continuously prevalent in a wide range of Neoproterozoic environments before the known record of other animal fossils."

The sponge biomarkers support the hypothesis that complex animal life first rose to ecological prominence in the shallow coastal waters of the Cryogenian geological period, which lasted from around 850 million to around 635 million years ago. Such markers haven't been found in shale samples from deep waters, suggesting it took sponges longer to colonise these areas.

Evidence suggesting the presence of sponges in the ancient oceans has been found before - for example, traces of the organisms have been found in Australian sediments from between 543 and 549 million years ago. Preserved sponge tissue has likewise been found in Chinese rocks known to be less than 600 million years old.

But such physical preservation of sponges is rare, and this is why the analysis of biomarkers is such a promising method of tracking sponges' appearance in the ancient oceans. Many members of the Demospongia family don't have anatomical features that can be preserved as fossils, meaning the fossil record is partial at best.

Snape explains that hydropyrolysis was originally developed for use in petroleum exploration. Here, traditional methods using free biomarkers, which are not bound into the structure of the rock, fail due to biodegradation or the presences of contaminants, for example from drilling mud.

It can also be used in a wide range of other situations. One of these is detecting steroid abuse in athletes - the techniques used at present can provide ambiguous results because they do not access the full range of steroids present in urine. Hydropyrolysis can now solve this problem, meaning that scientists can tell with far more certainty whether illegal steroids are present. This research has been supported by a grant from the World Anti-Doping Agency; the team has now received funding to conduct further trials on real urine samples.

Another application is cleaning samples of charcoal before radiocarbon dating. It's important to remove all traces of more recent carbon from such samples before they are dated to avoid skewed results. At present researchers use chemical oxidation procedures on these contaminants, but it is hard to tell when they have all been removed - a problem the new technique gets around.

"We want to establish hydropyrolysis as the technique to use for both cleaning and measuring charcoals," Snape explains. "It provides a far more rigorous way of removing the contaminants that can distort the results of radiocarbon dating."