金沙所有登入网址:When did the Earth turn green?
By Rachel Nowak Photosynthesis – the process by which organisms like plants convert light energy into chemical energy – may not have been around quite as long as previously thought. That’s the conclusion of a study of solidified oil that formed around 2.7 billion years ago in the Pilbara region of Western Australia. The study, led by Birger Rasmussen of the Curtin University of Technology in Perth, means the planet had to wait another 550 million years for photosynthesis to get going, and that the oldest known eukaryotic (complex) cells are one billion years younger than previously thought. “A lot of people will revisit their understanding of the late Archaean period in light of these results,” says Woodward Fischer of the California Institute of Technology, Pasadena. Photosynthesis converts carbon dioxide and water into carbohydrates and oxygen. The process is the most likely explanation for “the great oxidation” event 2.4 billion years ago, when oxygen in the atmosphere started to build up, paving the way for the evolution of complex life-forms like animals. The oxygen surge is also considered to be the strongest clue to the timing of the evolution of photosynthesis. But until now it has conflicted with the fossil evidence. Almost a decade ago, Jochen Brocks, then at the University of Sydney, found minute traces of organic molecules that could only have come from photosynthetic cyanobacteria in Pilbara shale (Science, vol 285, 1033). Other organic molecules were indicative of eukaryotic cells. At the time, Brocks’ analysis dated the so-called “molecular fossils” at 2.7 billion years ago. But the new evidence from the Rasmussen team suggests that what Brocks had found was actually molecular contaminants from a more recent era. Brocks is a co-author of the Rasmussen paper. “The existing unambiguous fossil evidence for the timing of photosynthesis now moves to 2.15 billion years ago,” says Rasmussen, referring to fossilised cyanobacteria that have been found in Canada’s Belcher Islands. Rasmussen, Brocks and their colleagues used a relatively new device called a NanoSIMS ion probe to monitor the types of carbon isotopes in solidified oil – the proposed source of Brocks’ organic compounds – in the bits of rock left over from the original study. “The oil had to have formed in the rock, but its isotopic signature was completely different to that of the microbial fossils, so we concluded that the microbial fossils were more recent contaminants,” says Rasmussen. Other paradoxes remain to be solved, however. Since Brocks’ discovery a decade ago, “molecular fossils” of photosynthesis from before 2.4 billion years ago have turned up at other sites. “We suspect that those studies will turn out to be flawed, too,” says Rasmussen. Journal reference: Nature, DOI: 10.1038/nature07381 Evolution – Learn more about the struggle to survive in our comprehensive special report. More on these topics: