Living fossils in rock pools hold a key to life 3.7bn years ago

SPECIAL rock pools on the South African coastline have become windows on the world as it looked 3.7 billion years ago.

In the pools are green mats of what appear to be algae, but are living fossils. The discovery has excited scientists who found that the green mats are stromatolites, layers of cyanobacteria and diatoms, that live in water with high concentrations of calcium carbonate, and undertake photosynthesis.

The diversity of bacteria in each pool is so great that researchers believe there are new pharmaceutical substances to be discovered, including possible drugs to prevent cancer.

Stromatolites were considered rare, that was until they were spotted in abundance along the Eastern Cape coast, south of Port Elizabeth.

According to Professor Renzo Perissinotto, who holds a research chair in shallow water ecosystems at the Nelson Mandela Metropolitan University, he and his team have so far identified 540 rock pools that hold stromatolites.

These pools, he believes, might explain how the Earth evolved its oxygen-rich atmosphere and provide the reason why our earliest ancestors learnt to move.

The earliest stromatolite fossils have been dated to 3.7 billion years and were around at a time when the Earth was a very different place to what it is today.

“It would have been hellish back then,” explains Perissinotto, “the sky would have been a red or orange, with no oxygen, and seas would have been green because of the high iron content. There was lots of volcanic activity.”

However for the stromatolites, this was a happy time that lasted over two billion years, when conditions were perfect for them and they faced no competition.

But the theory is that their success ultimately led to their downfall.

Vast mats of photosynthesizing stromatolites filled the atmosphere with oxygen which in turn allowed metazoa (more complex and multicellular organisms) to evolve.

These metazoa, the theory goes, burrowed into the mats and fed on them.

“They would have grazed on them, like cows grazing on a field,” said Perissinotto.

Stromatolite formations declined sharply, but some survived and have clung on until the present.

Prominent living communities have been found in Western Australia, Ireland and the Bahamas.

But the South African stomatolites are the most extensive.

“Why it is so exciting is that the structures of the South African stromatolites are very similar to those found in the fossil record,” Perissinotto said.

In South Africa, stomatolites occur where freshwater mixes with seawater in the intertidal area.

But while multicellular organisms have been fingered as being responsible for the downfall of stromatolites, researcher Dr Gavin Rish- worth, also of Nelson Mandela Metropolitan University, has seen behaviour that suggests that it is more complicated than that. He has been investigating rock pools along the Eastern Cape and he has noticed multicellular organisms living in harmony with stromatolites.

“It didn’t make sense, as it was thought that they would destroy them. We needed to find out why they were coexisting” .

He came to the conclusion that these organisms were using these green mats to hide from predators and to take advantage of the oxygen-rich environment.

It may be a relationship that goes back hundreds of millions of years and multicellular organisms might be partly off the hook in causing the great stromatolite die-off of 500 million years ago.

Perissinotto believes this offers a glimpse into what was happening all that long time ago. “What might have happened is that during the day the metazoa took comfort from the stromatolites, with their increased oxygen levels, then at night they would go out to feed. And this required them to develop mobility.”

The s t r omatolite downfall, says Perissinotto might have come instead from the changing chemistry of the oceans.

But in the future, Perissinotto, his team and five international universities will be delving deeper into the secrets hidden in these rock pools hoping to obtain a better understanding of that time when life had just begun.