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THE DATING GAME
Gustaf Arrhenius holds a sample of Greenland rock containing evidence of 3.8-billion-year-old life.
HOW LONG DID IT TAKE for life to emerge after the formation of Earth? Scripps marine geochemist Gustaf Arrhenius helped narrow the inception window to a mere 800 million-year span. Earth was formed about 4.6 billion years ago. Scientists generally have believed that because of the heavy bombardment of Earth by asteroid-like bodies, it took another billion or more years for the first life forms to appear. Because this life appears to have been biochemically advanced, the very first life forms must be even older, Arrhenius contended in his research during the mid-1990s. This work resulted in a claim that life emerged on Earth at a date of 3.8 billion years. "We have 800 million years to play with," Arrhenius said. "Life must have either emerged on Earth or arrived on Earth in this time." The 49-year veteran of Scripps set off controversy and a flurry of interest in 1996 when he and his graduate student Stephen Mojzsis reported that carbonate rocks obtained by colleagues in western Greenland contained trace amounts of deposits that seemed to have been produced by life even older than previously believed to have existed. At the time, scientists believed these carbonate rocks were of sedimentary origin. Arrhenius had to back away from that initial claim, however. With student Mark van Zuilen and colleague Aivo Lepland, he concluded that the matter they had isolated was formed by hot crustal fluids impregnating the sedimentary rock much later in history. The false alarm led the team to develop verification methods that could prevent similar claims of early-life discoveries. Carbon comes in two stable varieties, or isotopes, bearing different numbers of neutrons in their atoms. Organisms concentrate the "lightest" carbon form with the fewest neutrons. The team is refining methods to distinguish this carbon from similarly light carbon produced by nonbiological sources. |
New analysis of sedimentary rock has found measurable evidence of life dating back 3.8 billion years with no trace of meteorite bombardment or increase in space dust, revalidating Arrhenius's 1996 claim. Behind the revival were the verification methods developed by the Scripps team. Minik Rosing, a researcher from the Geological Museum of Copenhagen, in collaboration with the Scripps team, verified the presence of biologically generated carbon in rock from the same area in Greenland in 1999. This time, the sample truly could be said to bear evidence of life 3.8 billion years old. It was also clearly of sedimentary origin. In Rosing's dull gray-and-black-striped rocks lay an apparent record of early-life forms that either gained energy through photosynthesis on the surface of an ocean devoid of oxygen or lived as bottom dwellers, nurtured by the chemical energy from compounds released by seafloor vents. The remains of those creatures left minute freckles of organically produced carbon, which crystallized into graphite. The lack of impact features in the rock and the absence of interplanetary dust in it led Arrhenius to believe that Earth might not have been as heavily bombarded in the beginning as previously thought.
Graduate Student Mark Van Zuilen
The methods of distinguishing biogenic from nonbiogenic carbon developed by van Zuilen and Lepland have helped support this new evidence and it has now gained widespread acceptance, Arrhenius said. |
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