Читать книгу Quantum Evolution: Life in the Multiverse - Johnjoe McFadden - Страница 33

The first undisputed evidence of life is in rocks about three and a half billion years old. Large circular structures called stromatolites (from the Greek meaning ‘stony carpet’) are present in fine-grained flint-like sedimentary rock⁷ called chert. Some of these rocks contain curious white concentric rings about a metre across. Nobody knew the origin of these structures until in the 1950s scientists cut sections of the rock and examined them under the microscope. They found rods, spheres and chains of cells, unmistakably the fossilized remains of microbes. The oldest stromatolite microfossils were found by the University of California’s paleobiologist J. William Schopf, in chert outcrops near Chinaman Creek in Western Australia dated to three and a half billion years ago. Schopf found many short rods and filaments in the rock very similar in size and shape to the cells of modern bacteria, including the photosynthetic cyanobacteria of Chapter Two. Some of the fossils were incredibly well preserved, allowing Schopf to discern their internal structure and even recognize cells that appeared to have been undergoing cell division before their lineage was abruptly petrified. Cells of various sizes and shapes were present suggesting that a diverse microbial community was already thriving.⁸

Rocks even more ancient than those of Chinaman Creek are found on Isua in West Greenland. These date back about 3.85 billion years. Nobody has yet found any microfossils in these rocks but this is unsurprising since they have gone through episodes of heating at high pressures that would have destroyed the structure of any ancient organisms. Another way of looking for evidence of past life is to examine the ratio of carbon-12 to carbon-13 isotopes⁹ in sedimentary rock. Plants and bacteria that convert inorganic carbon (as carbon dioxide, carbonate or methane) to biomass discriminate against carbon-13 so their tissues are enriched by the lighter carbon-12 isotope. Rocks that hold the fossilized remains of ancient carbon-fixing plants and microbes preserve this preponderance. Microfossil-bearing rocks including the ancient cherts show the unmistakable signature of biological carbon fixation. Although the Isua rocks contain no fossils they do have grains of graphite within them which could be the roasted remains of ancient microbes. Scientists from the Scripps Institution of Oceanography, University of California, examined the Isua graphite and found enriched levels of carbon-12, indicating that the carbon may possibly be of biological origin. The levels were not as high as in later rocks, possibly due to high temperature re-equilibration of the isotope ratios, but do suggest a biological origin.