Initially the micron-sized remains found deep in the rust-coloured rocks of the Pilbara in northwestern Australia made scientists specialising in the origins of life very excited. Now it is more likely to make them confused. The region is one of the last pristine relics from the earliest era of the Earth's crust, when the elements coalesced 2.7 billion years ago to become what is postulated to be the world's original supercontinent, known as Vaalbara.
It was what was inside the ancient shale of the Pilbara that excited researchers, who found the remnants of primitive algae, demonstrating that the oldest known proof of photosynthesis - and by extension, life on Earth - was more than 550 million years earlier than previously thought. But last month, the science journal Nature revealed that better methods of analysis had cast doubt on the veracity of the original findings, which were published nearly 10 years ago. Rather than being literally and figuratively set in stone, the chemical biomarkers that signify the existence of the cyanobacteria algae are now suspected of having infiltrated the rock strata after the sediments were laid down.
Even that is disputed, with others interpreting the new findings as potentially consistent with the theory that photosynthesising life forms existed 2.7 billion years ago, even though the Earth was then more than twice as hot as it is today and oxygen, now the second most common element of the atmosphere, did not become present in significant quantities for another 350 million years. This progression of research to obfuscate rather than clarify is hardly new in science, where one of the recurring themes in any highly theoretical field is for new discoveries to prove that scientists did not know as much as they originally thought they did.
After all, it was only within the last 150 years that science finally extinguished the theory that all life was the result of spontaneous generation. Since the days of Aristotle, it was thought self-evident that maggots appeared spontaneously in meat, aphids from dew on leaves, field mice from hay, and even crocodiles from rotting submerged logs. The discovery of bacteria was initially held as support for the theory and it took until 1861, when Louis Pasteur showed bacteria and fungi did not appear in nutrient-rich but sterile material, that spontaneous generation theory was finally rejected. But disproving one theory for abiogenesis - the origins of life - did not prove an alternative, and to this day scientists are pursuing a verifiable hypothesis to fill the scientific vacuum.
The irony is that while spontaneous generation has been scotched as an ongoing process, almost all of the theories jostling to replace it rely on a spontaneous generation of life at some point in the distant past when the most basic manifestation of life, in the form of photosynthesis, first took place. The sole dissenting view contends that life arrived via space debris such as a comet's tail. That is thought to have happened between 3.5 billion to 4.4 billion years ago, relatively soon after the hypothesised formation of the Earth 4.6 billion years ago and in a completely different environment than exists today. In the Hadean era, the first 800 million years of the Earth, temperatures were more than three times as hot as now - hence the name, a derivative of Hades - and the atmosphere had virtually no oxygen.
Although photosynthesis is postulated to have been present in the Hadean era, no compelling proof has been found in the few rocks identified as being of that age. John Olson, a University of Massachusetts biochemist, postulated in 2005 that there were signs in Greenland rock sheets dating back to 3.8 billion years ago, as the Hadean era ended and the Archean era began, of photosynthesis based on hydrogen rather than oxygen because of the latter's absence. His view was bolstered by what he dubbed "possible microfossils" in South African and Australian rocks around 3.4 billion to 3.5 billion years ago.
But the closest to proof remains in the rock samples from the Pilbara, where researchers initially hailed the discovery of chemical biomarkers indicating the presence of photosynthesising life forms 550 million years before the next oldest example. A conundrum was that the researchers in the late 1990s did not just find biomarkers called hopanes, which prove the presence of simple bacteria. They also found steranes, indicating the presence of more sophisticated cellular life forms that were previously not thought to have existed until a billion years later.
The double conundrum was that the steranes should have been destroyed by the heat and pressure that were evident from the rock samples, while the presence of oxygen-generating cellular life forms was at odds with the near total absence of oxygen in the atmosphere for another 350 million years. According Birger Rasmussen, a geochemist from Australia's Curtin University of Technology, the solution came from using a far more precise probe that could take carbon-isotopes samples just five microns across, or less than one tenth the width of a human hair.
It showed a far wider discrepancy in the carbon dating than had been found previously, which Dr Rasmussen said in the paper published in Nature indicates that the hopanes and steranes migrated into the Pilbara rocks after they were laid down. Others dispute his hypothesis as just one explanation of several possible answers. The Harvard University biochemist Andrew Knoll said in the magazine Science News that studies from South Africa, the only other pristine site with rocks of the same era as the Pilbara, had also detected biomarkers which support the original Pilbara theory.
Jennifer Eigenbrode, an organic geochemist at Nasa Goddard Space Flight Centre, said there were other possibilities for the carbon traces, such as the early organisms absorbing carbon from methane rather than carbon dioxide. Both gases were abundant at the time and the recycling of the carbon within the environment could explain the wider range of carbon-isotope readings of the kind found by Dr Rasmussen's team.
Woodward Fischer, a geochemist at Califronia Institute of Technology, has cautioned that the high-sensitivity microprobes similar to that used for the most recent study of the Pilbara rocks are also susceptible to contamination, with diesel exhausts, fossil fuel emissions and smog - all capable of influencing the presence of hopanes. In the meantime, what had once been seen as a leap forward in proving the origins of life has reverted to a debate between whether it might instead have been a stumble.
jhenzell@thenational.ae