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Precocious

作者:苗虹吱    发布时间:2019-03-07 01:08:02    

By Jeff Hecht TRACES of oil extracted from Australian shale have pushed the date for the origin of complex cells back another half a billion years. Compounds in the oil suggest that eukaryotic cells, which make up all life on Earth except for bacteria, had evolved as early as 2.7 billion years ago. Little evidence of the earliest life on the planet remains. The oldest fossils so far found are the vestiges of bacteria-like cells that lived 3.5 billion years ago. Eukaryotes are often much larger and, unlike bacteria, have internal membranes around the chromosomes and other specialised components. However, it’s not until about 2.1 billion years ago that fossil imprints appear in the geological record that are so large that they can only be eukaryotes. In a search for earlier evidence of eukaryotes, a team of researchers in Australia has come across faint chemical traces of complex cell membranes in 2.7 billion-year-old shale. They found them in droplets of oil extracted from rock 700 metres below the surface in the remote Pilbara region of north-western Australia. According to Jochen Brocks, a graduate student at the University of Sydney, the oil contained steranes, molecules with 26 to 30 carbon atoms arranged in four rings. These are produced by the decay of cholesterol and other steroids found in the membranes of eukaryotes, but not bacteria (Science, vol 285, p 1033). “When I saw these signals appearing, I thought `contamination’,” Brocks recalls. However, he found no trace of the steranes in the surrounding volcanic rock, as you would expect if the oil had seeped in after the shale was deposited. Whether the early eukaryotic cells that left the steranes behind resemble modern ones is still unclear, says Andy Knoll, a specialist in early life at Harvard University. Eukaryotes evolved many specialised features, such as mitochondria to produce energy and membrane-bound packets for digesting toxins. “We don’t know if most or only a few of those attributes were present 2.7 billion years ago,” Knoll says. But Brocks speculates that the cells were fairly complex. “My feeling is that these eukaryotes were very similar to those that live today,” he says. They may have been photosynthetic algae, for example, since only a large group of cells, as found in an ocean algal bloom, would have produced sterane in the concentrations he found. Although it’s possible that some of the bacteria-sized fossils dating back to that time were actually early eukaryotes, they probably would not tell us much about what the first eukaryotes were like, says William Schopf of the University of California at Los Angeles. Even some modern algae are no bigger than the largest bacteria, he says,

 

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