Row over when plants first emerged on Earth

Researchers taking a second look at ancient sediments have concluded that they do not represent the earliest traces of the rise of sun-fuelled photosynthesis/oxygen-producing organisms on Earth as previously thought, reports ScienceNOW website.

It says the findings come as a relief to some scientists, whose data had been in conflict with an earlier study with the same sediments that suggested oxygen first emerged in the planet’s atmosphere some 300 million years later.

According to the geological record, oxygen first rose in Earth’s atmosphere about 2.4bn years ago. The problem with using fossils to track down the organisms that gave rise to the oxygen is that fossils can take you back only about 2bn years out of the planet’s 4.5bn.

To figure out what was stirring before that, scientists must rely on traces of chemicals produced by biological processes and, by inference, in certain environmental conditions.

That’s what happened in 1999, when researchers examined samples of 2.7bn-year-old shale from Western Australia.

They found certain types of hydrocarbons that indicated oxygen-producing bacteria had existed at the time. But the discovery also created a conundrum, because it placed the rise of photosynthesis some 300m years before chemical studies of ancient rocks indicated oxygen began to spread in Earth’s atmosphere.

This discrepancy continues to bedevil researchers, who have struggled to mesh their data with evolutionary models.

An Australian team, writing in Nature, analysed microscopic solidified oil droplets contained in the shale, the researchers inferred the composition of the organic materials present as the shale formed.

The analysis clearly shows the hydrocarbons identified in the 1999 study could not have been derived from bacteria contained in the sediments, says geochemist and lead author Birger Rasmussen of the Curtin University of Technology, Perth, Australia.

Instead, they probably represent contaminants introduced from younger sedimentary rocks that somehow got mixed in with the shale layer or by the drills the researchers used to extract the samples, he says.

The conclusions are “pretty strong and a lot of eyes will be focused on how this shakes out,” says geobiologist Woodward Fischer of the California Institute of Technology, Pasadena, US.

He says the previous findings had been regarded as a standard time measurement for both the rise of oxygen and for photosynthetic organisms, and, therefore, any conflicting data produced by other researchers had been regarded as suspect.

But overall he’s convinced that the new results ultimately “will free up people’s minds again” to find the other sources of early oxygen.

Source: ScienceNOW

Date: 22/10/2008

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One Response

  1. Over thirty years ago, I heard that certain amino acids were found in an uncombined state from what are known as ‘white’ smokers, low temperature under sea hot springs. My immediate reaction, “That was where life originated” i.e. in a high energy environment with unstable compounds coupled with aluminoslicates and silica where early life would mimic, crystal growth. (It was only about a decade later that John Corliss from Corvallis actually wrote up the idea in a professional paper instead of just walking away from it.) With such a rich source of energy, life would have originated as a way to economically transform complex compounds to lower energy states.
    According to my beliefs, primitive life from the undersea hotsprings drifted upsection where they encountered the photic zone. Their primitive DNA was promptly fried by ultraviolet radiation. The predictable result—mutation and natural selection. The natural selection would favor compounds that could shield the cell’s DNA from disruption. The most likely candidate: Some kind of primitive chlorophyll that would intercept the high energy ultraviolet radiation and transfrom it into something that would not damage early life’s proto DNA.
    In this model of the development of chlorophyll, life around the high energy environment of chemosynthesis around the hot springs had no need for food because all the food that was needed could be provided by chemosynthesis. It was only when life drifted up into the chemosynthetically challenged surface waters that creating food was essential for propogating life beyond the range of chemosynthesis related life. First early life put on the sun block, and only later decided that by products of the sun block were mighty tasty.

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