Evidence for Life on Earth More Than 3850 Million Years Ago

摘要

Some years ago, the remains of what were almost certainly microorganisms were discovered in the 3450-million-year-old War-rawoona sedimentary rocks of northwestern Australia. The evidence for life in these rocks rests on she morphology of the carbonaceous remains and the isotopic composition of the contained carbon. In the late 1970s, the presence of even earlier life on Earth was proposed on the basis of carbonaceous residues in 3800-million-year-old rocks at Isua in western Greenland. Unfortunately, the rocks at Isua have been deformed at such high temperatures and pressures that the original morphology of the contained organic matter was lost. To complicate matters further, the isotopic composition of the bulk carbon probably changed during metamorphism. The evidence for life on Earth as early as 3800 million years ago has therefore been weak and equivocal. New isotopic data for carbon in these and in even older rocks have now been published by Mojzsis et al.: Their results elevate the hints ofa very early origin of life on Earth to a solid suggestion.