by Carl Strang
A number of studies came out last year addressing the conditions of early Earth, and how life might have gotten started. That latter subject certainly is not suffering from a lack of ideas. The opposite in fact is true, and there is a confusing array of possibilities that need to be tested and sorted out. The following notes are from studies or reports of studies that caught my eye.
Clery, Daniel. 2013. Impact theory gets whacked. Science 342:183-185. This news article reviewed the history of the giant-impact moon formation theory and its discussion at a recent meeting. The model, first proposed in 1975 by William Hartman and Donald Davis, has become the consensus understanding of how the moon formed, but there have been problems. Computer simulations have indicated that the impact as understood would leave the moon formed “almost exclusively” from material in the impactor rather than the Earth. Moon rocks have shown clearly, however, that much of the moon is made of Earth material. Several new modifications have been offered. In one the impactor is much smaller than the Mars-sized body of the original model, but the Earth is spinning much faster. In another, the impactor is closer to the Earth in size. Both require some further process to set up the system we see today. The problem could be resolved if the impactor were similar to Earth in composition, i.e. if the inner planets are not as different in composition as comparisons of Earth and Mars suggest. If rocks can be obtained from Venus, and these prove to be Earth-like, a return to the original simpler giant impact model could take place.
Zaleski, Daniel P., et al. Detection of e-cyanomethanimine toward Sagittarius B2(N) in the Green Bank Telescope Primos survey. The Astrophysical Journal, 2013; 765 (1): L10 DOI: 10.1088/2041-8205/765/1/L10 They found a precursor to one of the building blocks of DNA, and another for an amino acid, in an interstellar cloud. The materials apparently assembled on ice particles.
L. M. Longo, J. Lee, M. Blaber. 2013. Simplified protein design biased for prebiotic amino acids yields a foldable, halophilic protein. Proceedings of the National Academy of Sciences 110 (6): 2135 DOI: 10.1073/pnas.1219530110 From a ScienceDaily article. As an alternative to the RNA-first idea, and to the possibility of Earth life first appearing around hydrothermic vents, they have developed evidence supporting the possibility of a protein-first idea in a salt-rich environment. The amino acids thought to have been present at the time prove capable not only of forming chains under such conditions, but also of having the critical quality of folding properly in the presence of the salts.
Wordsworth, Robin, and Raymond Pierrehumbert. 2013. Hydrogen-nitrogen greenhouse warming in Earth’s early atmosphere. Science 339:64-67. There also is an interpretive article in this issue. Collisions between the nitrogen and hydrogen atoms, common in the Archean atmosphere, would have resulted in structural changes, as the atoms in each molecule bounced back and forth, that would give them greenhouse gas properties. The authors argue that this was an important factor keeping water liquid at a time when the sun was dim. As methane-metabolizing microorganisms became abundant that gas would have increased, but the authors argue that this would have cooled the Earth, possibly resulting in ice ages in those early times.
Bryant, David E., et al. 2013. Hydrothermal modification of the Sikhote-Alin iron meteorite under low pH geothermal environments. A plausibly prebiotic route to activated phosphorus on the early Earth. Geochimica et Cosmochimica Acta 109: 90 DOI: 10.1016/j.gca.2012.12.043 Their study focuses on the importance of phosphorus and ATP in cellular energetics, and supports the possibility that phosphorus-bearing meteorites landing in hot acidic pools around volcanos could have been a step between inorganic chemistry and organic life.
University of Washington (2013, July 29). Natural affinities — unrecognized until now — may have set stage for life to ignite. ScienceDaily. Retrieved July 31, 2013, from http://www.sciencedaily.com /releases/2013/07/130729161514.htm This ScienceDaily article describes findings that may shed light on the origin of life. Researchers at the University of Washington have found that certain fatty acids, in combination with the components of RNA, coalesce to form mutually protective structures, the fatty acids in a bag that brings together the nucleic acids, potentially making it easier for the latter to combine into RNA molecules. The nucleic acids reduce the disruptive effects of salt water on the fatty acid bags.
Crowe, Sean A., et al. 2013. Atmospheric oxygenation three billion years ago. Nature 501 (7468): 535 DOI: 10.1038/nature12426 From a ScienceDaily article. Study of fossil soils indicates that oxygen appeared in small amounts in the atmosphere 3 billion years ago, rather than 2.3, suggesting that photosynthesis began that much earlier.
Johnson, J. E., et al. 2013. Manganese-oxidizing photosynthesis before the rise of cyanobacteria. Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1305530110 From a ScienceDaily article. They studied marine deposits in South Africa from the beginning of the Proterozoic, before cyanobacteria had developed chlorophyll-based photosynthesis. They found concentrations of oxidized manganese, which apparently had been formed by earlier microorganisms acquiring electrons from dissolved manganese in a chemical process that could have been the precursor of photosynthesis by cyanobacteria.
B. A. Killingsworth, J. A. Hayles, C. Zhou, H. Bao. 2013. Sedimentary constraints on the duration of the Marinoan Oxygen-17 Depletion (MOSD) event. Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1213154110 Oxygen isotope ratios point to a large and rapid buildup of atmospheric carbon dioxide as being responsible for the end of the Snowball Earth glaciation in the Proterozoic. The ice blocked off potential sinks, especially in the oceans, so the greenhouse gas simply built (the ScienceDaily article describing the study didn’t mention the source of the gas, but volcanos would have continued to be active).
Wacey, David, et al. 2013. Nanoscale analysis of pyritized microfossils reveals differential heterotrophic consumption in the ∼1.9-Ga Gunflint chert. PNAS, DOI: 10.1073/pnas.1221965110 They found evidence of preferential bacterial consumption of cyanobacteria (blue-green algae) by bacteria, the earliest evidence of one kind of organism eating another. The consumers were apparently not so interested in another bacteria species. The process would have released sulfur dioxide, adding a rotten eggs odor to the atmosphere.
Bernhard, Joan M., et al. 2013. Insights into foraminiferal influences on microfabrics of microbialites at Highborne Cay, Bahamas. PNAS, DOI: 10.1073/pnas.1221721110 They conducted experiments that supported the idea that the extinction of most stromatolites 1 billion years ago was caused by foraminifera which appeared at that time and disturbed the layered stromatolite structure (stromatolites were the abundant, layered structures formed by cyanobacteria, responsible for the early oxygenation of the atmosphere). A similar group of structures, called thrombolites, also appeared about this time with a clumped rather than layered structure. These coexist with foraminifera today.
Pennisi, Elizabeth. 2013. Nervous system may have evolved twice. Science 339:391. This news article described two genomic studies of ctenophores (comb jellies, sea animals that superficially resemble jellyfish but belong to another phylum entirely) that point to their evolving nervous systems independently of, and earlier than, all other animals. The authors of one of the studies conclude that these animals may have preceded even sponges, possibly supporting the notion that ctenophores were tied to the Ediacaran biota of the late Proterozoic.