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Watson AJ 《Astrobiology》2008,8(1):175-185
Structurally complex life and intelligence evolved late on Earth; models for the evolution of global temperature suggest that, due to the increasing solar luminosity, the future life span of the (eukaryote) biosphere will be "only" about another billion years, a short time compared to the approximately 4 Ga since life began. A simple stochastic model (Carter, 1983) suggests that this timing might be governed by the necessity to pass a small number, n, of very difficult evolutionary steps, with n < 10 and a best guess of n = 4, in order for intelligent observers like ourselves to evolve. Here I extend the model analysis to derive probability distributions for each step. Past steps should tend to be evenly spaced through Earth's history, and this is consistent with identification of the steps with some of the major transitions in the evolution of life on Earth. A complementary approach, identifying the critical steps with major reorganizations in Earth's biogeochemical cycles, suggests that the Archean-Proterozoic and Proterozoic-Phanerozoic transitions might be identified with critical steps. The success of the model lends support to a "Rare Earth" hypothesis (Ward and Brownlee, 2000): structurally complex life is separated from prokaryotes by several very unlikely steps and, hence, will be much less common than prokaryotes. Intelligence is one further unlikely step, so it is much less common still. 相似文献
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Schiffbauer JD Yin L Bodnar RJ Kaufman AJ Meng F Hu J Shen B Yuan X Bao H Xiao S 《Astrobiology》2007,7(4):684-704
Abundant graphite particles occur in amphibolite-grade quartzite of the Archean-Paleoproterozoic Wutai Metamorphic Complex in the Wutaishan area of North China. Petrographic thin section observations suggest that the graphite particles occur within and between quartzite clasts and are heterogeneous in origin. Using HF maceration techniques, the Wutai graphite particles were extracted for further investigation. Laser Raman spectroscopic analysis of a population of extracted graphite discs indicated that they experienced a maximum metamorphic temperature of 513 +/- 50 degrees C, which is consistent with the metamorphic grade of the host rock and supports their indigenicity. Scanning and transmission electron microscopy revealed that the particles bear morphological features (such as hexagonal sheets of graphite crystals) related to metamorphism and crystal growth, but a small fraction of them (graphite discs) are characterized by a circular morphology, distinct marginal concentric folds, surficial wrinkles, and complex nanostructures. Ion microprobe analysis of individual graphite discs showed that their carbon isotope compositions range from -7.4 per thousand to -35.9 per thousand V-PDB (Vienna Pee Dee Belemnite), with an average of -20.3 per thousand, which is comparable to bulk analysis of extracted carbonaceous material. The range of their size, ultrastructures, and isotopic signatures suggests that the morphology and geochemistry of the Wutai graphite discs were overprinted by metamorphism and their ultimate carbon source probably had diverse origins that included abiotic processes. We considered both biotic and abiotic origins of the carbon source and graphite disc morphologies and cannot falsify the possibility that some circular graphite discs characterized by marginal folds and surficial wrinkles represent deflated, compressed, and subsequently graphitized organic-walled vesicles. Together with reports by other authors of acanthomorphic acritarchs from greenschist-amphibolite-grade metamorphic rocks, this study suggests that it is worthwhile to examine carbonaceous materials preserved in highly metamorphosed rocks for possible evidence of ancient life. 相似文献
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Nicholson WL Ricco AJ Agasid E Beasley C Diaz-Aguado M Ehrenfreund P Friedericks C Ghassemieh S Henschke M Hines JW Kitts C Luzzi E Ly D Mai N Mancinelli R McIntyre M Minelli G Neumann M Parra M Piccini M Rasay RM Ricks R Santos O Schooley A Squires D Timucin L Yost B Young A 《Astrobiology》2011,11(10):951-958
We report the first telemetered spaceflight science results from the orbiting Space Environment Survivability of Living Organisms (SESLO) experiment, executed by one of the two 10?cm cube-format payloads aboard the 5.5?kg Organism/Organic Exposure to Orbital Stresses (O/OREOS) free-flying nanosatellite. The O/OREOS spacecraft was launched successfully to a 72° inclination, 650?km Earth orbit on 19 November 2010. This satellite provides access to the radiation environment of space in relatively weak regions of Earth's protective magnetosphere as it passes close to the north and south magnetic poles; the total dose rate is about 15 times that in the orbit of the International Space Station. The SESLO experiment measures the long-term survival, germination, and growth responses, including metabolic activity, of Bacillus subtilis spores exposed to the microgravity, ionizing radiation, and heavy-ion bombardment of its high-inclination orbit. Six microwells containing wild-type (168) and six more containing radiation-sensitive mutant (WN1087) strains of dried B. subtilis spores were rehydrated with nutrient medium after 14 days in space to allow the spores to germinate and grow. Similarly, the same distribution of organisms in a different set of microwells was rehydrated with nutrient medium after 97 days in space. The nutrient medium included the redox dye Alamar blue, which changes color in response to cellular metabolic activity. Three-color transmitted intensity measurements of all microwells were telemetered to Earth within days of each of the 48?h growth experiments. We report here on the evaluation and interpretation of these spaceflight data in comparison to delayed-synchronous laboratory ground control experiments. 相似文献
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We report on the use of a portable instrument for microbial detection in the Mojave Desert soil and the potential for its use on Mars. The instrument is based on native fluorescence and employs four excitation wavelengths combined with four emission wavelengths. A soil dilution series in which known numbers of Bacillus subtilis spores were added to soil was used to determine the sensitivity of the instrument. We found that the fluorescence of the biological and organic components of the desert soil samples studied can be as strong as the fluorescence of the mineral component of these soils. Using the calibration derived from B. subtilis spores, we estimated that microbial content at our primary sampling site was 10(7) bacteria per gram of soil, a level confirmed by phospholipid fatty acid analysis. At a nearby site, but in a slightly different geological setting, we tested the instrument's ability to map out microbial concentrations in situ. Over a ~50 m diameter circle, soil microbial concentrations determined with the B. subtilis calibration indicate that the concentrations of microorganisms detected varies from 10(4) to 10(7) cells per gram of soil. We conclude that fluorescence is a promising method for detecting soil microbes in noncontact applications in extreme environments on Earth and may have applications on future missions to Mars. 相似文献
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《五年之约》导演:尼古拉斯·斯托勒主演:杰森·西格尔、艾米丽·布朗特故事梗概:汤姆和维莱特是一对相恋已久的恋人,有各自稳定的工作和热闹的大家庭。终于在极其普通的一天,汤姆向维莱特求婚了,而维莱特也答应了他。按照正常情况推断,本应该是好事将近,但一切都因维莱特接到了一个异地大学的工作机会而改变。汤姆决定辞去自己热门餐厅主厨的职位,随未婚妻迁徙异地工作和生活,但他们之间的关系也随着婚期一拖再拖而产生隔阂,最终这对愿为爱情而奔波的情侣是否真的终成眷属了呢? 相似文献
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介绍2种在应用双光束激光多普勒测速仪测量弯管内流速时,修正测量体位置和光束之间夹角的计算方法。一种是Snell折射定律的简化近似法,而另一种是精确的光线追踪法。针对弯管实例,将这2种方法所得的结果相比较,可看出近似法的不足是对三维折射的修正不够充分,然而与在直管中折射效应的测量结果相比较,证明这2种修正方法均是可行的。 相似文献