* Mr. Rupak Bhattacharya-Bsc(cal) Msc(JU) 7/51 Purbapalli, Po-sodepur; Dist 24 Parganas(north), Kol-110,West Bengal, India**Professor Pranab kumar Bhattacharya MD(cal) FIC Path(ind); Now Professor&HOD of Pathology At Calcutta School of Tropical Medicine CR avenue Kol-73 Ex Professor of pathology&HOD RIO,& Institute of Post Graduate Medical Education & Research,244 a AJC Bose Road, Kolkata-20, west Bengal, India **Miss Upasana Bhattacharya- Student, Mahamayatala, Garia, kol-86,only daughter of Prof.PK Bhattacharya***Mr.Ritwik Bhattacharya B.com(cal) Somayak Bhattacharya BHM MSC Student PUSHA,7/51 purbapalli, Po-sodepur Dist 24 parganas(north) , Kolkata-110,West Bengal, India******** Mrs. Dalia Mukherjee BA(hons) Cal, Swamiji Road, South Habra, 24 Parganas(north) West Bengal, India**** Mrs Oaindrila Mukherjee-student ,Swamiji Road, South Habra, 24 Parganas(north), West Bengal, India
An excess of L-amino acids was detected in Murchison and Murray, two meteorites of the carbonaceous chondrite class ,although some discrepancies in the reported results remain to be yet resolved. Cronin et al. (1) originally discarded the evidence for small excesses of L-enantiomers in Murchison as controversial and possibly caused by terrestrial contamination. Later, however, they themselves found an enantiomeric excess of various amino acids that have never been reported, or are of limited occurrence, on Earth (2,3). The detection of a significant 15N enrichment in individual amino acid enantiomers from Murchison, when was compared with their terrestrial counterparts, it confirmed that the source of these amino acids was extraterrestrial and not any terrestrial contamination. Carbonaceous chondrites formed ~4.5 billion years ago (i.e., before the origin of life on Earth). There is still some controversy regarding the actual origin of those meteoritic amino acids (i.e., on the meteorite parent body via Strecker synthesis in liquid water [1,4] or in the interstellar medium followed by incorporation into the parent body [2, 5]. Experiments with interstellar ice analogues have shown that the UV-light–induced synthesis of amino acids was possible under the types of conditions likely to be found in interstellar dust (5, 6). No matter which scenario is the correct one, the finding of an excess of L-amino acids in carbonaceous chondrites strongly suggests that the excess is of extraterrestrial origin and existed in the solar system before the origin of life on Earth.
The experiments further indicated that at least some amino acids do not undergo complete racemization during their residence in space, transit to Earth, atmospheric entry, and surface impact. The -methyl amino acids found to exhibit considerable excess of the L-enantiomer in the Murchison meteorite are reportedly quite resistant to racemization (2). Racemization half-lives of meteoritic -amino acids, the ones used for protein synthesis in contemporary terrestrial organisms, were calculated from models, taking into account the various environments that such an amino acid was exposed to in space (7). In the temperature range between 150 and 300K, the racemization half-lives varied between amino acids by approximately 5 orders of magnitude, with glutamic acid and iso-leucine predicted to retain an enantiomeric excess much longer than phenylalanine, aspartic acid, and alanine. These calculations suggested that the reported D/L value for glutamic acid in Murchison of 0.3 (8) was close to the original value, whereas that of alanine (D/L = 0.5) could correspond to original values in the range of 0.5 to 0.35 (7 ). Note, however, that others did not observe any enantiomeric excess in alanine (3 ). Other experiments suggested that amino acid racemization at high temperatures, as may be encountered during atmospheric entry and surface impacts of space bodies, would be very rapid (9 ). Incorporation into rocks of a size to prevent their being heating all the way through should, however, overcome this problem. The presence of a variety of amino acids in meteorites raises the further question of whether not only the source of enantiomeric excess in terrestrial amino acids but also possibly the provenance of pre-biotic amino acids themselves was extraterrestrial. Meteorites are actually considered unlikely to have made a significant contribution to the total amount of pre-biotic organics (10, ). In contrast, impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5–3.8 billion years ago are thought to have been important sources not just of amino acids but also a variety of prebiotic organic molecules (11, 12). Even greater amounts of organic material are likely to have been accreted from interplanetary dust particles, which are currently contributing ~3.2 x 105 kg year-1 of intact organics. How large a portion of the total inventory of organics on early Earth came from extraterrestrial sources depends on a variety of factors, foremost among them the actual composition of Earth’s early atmosphere and hence the extent of endogenous production. Whereas Miller and Urey assumed a fully reducing early terrestrial atmosphere for their famous experiments, it is now thought that it was non reducing or slightly reducing (12–14). The efficiency of organic synthesis decreases rapidly as a function of the H2/CO2 ratio. It has been calculated that with UV light as the energy source, a yearly production of 2 x 1011 kg organics would have occurred in a reducing atmosphere, whereas only 3 x 108 kg year-1 would be produced in a neutral atmosphere (H2/CO2 = 0.1) (12). Recent experiments suggested that high-energy particles, but not UV light, were able to generate amino acid precursors under mildly reducing conditions (10). The delivery to Earth of large amounts of extraterrestrial carbonaceous compounds, including many of the building blocks of life, might actually fall under a new expanded definition of panspermia (15). Originally, however, the term panspermia referred to the transfer of some form of viable extraterrestrial organism. Theoretically, the transfer of such organisms between planets within our solar system is possible on rocks ejected by large impacts (16). A majority of these ejecta were heated to temperatures that would kill all microbes; however, some remain almost un shocked (17). Further heating during the ascent through the atmosphere of the home planet requires that the ejecta be of a size that prevents heating to 100°C all through, with a diameter of >0.2 m estimated as necessary. Similar heating occurs during the entry into and passage through the atmosphere of the target planet and the landing there. In between, microbes would have to survive thousands of years of travel through space. Space is a very hostile environment in which UV and ionizing radiation, extreme vacuum, and very cold temperatures individually, and even more so in combination, are potentially lethal (10). Theoretical and experimental results indicate, however, that protection from these sterilizing factors may be possible (10). The ability of some bacteria to form spores makes them attractive candidates for extraterrestrial organisms that might have introduced life to Earth (18). Spores represent a dormant state. This offers the advantage of the absence of (detectable) metabolism and high resistance to a variety of physical insults, including those imposed by prolonged space travel. Only a small proportion of spores were found to survive space travel of up to 6 years (i.e., a minute fraction of the actual time they may have to spend in space during transfer between planets [ 18]). A single living organism may be enough to seed life on another planet, however.
Panspermia theories offer the advantage of overcoming the difficulties arising from the shortness of the time interval during which life on Earth must have become established. Life could not have arisen, or would have been destroyed if it did, during the heavy bombardment period that ended about 3.8 Gyr ago. Microfossils and stromatolites indicate that life must have originated more than 3.5 Gyr ago, and evidence of biologically mediated carbon isotope fraction puts the existence of life back even farther, to ~3.8 Gyr ago. This leaves a very narrow window of time for the emergence of terrestrial life and adds some plausibility to scenarios in which a preformed extraterrestrial life form started life on Earth. Ultimately, however, postulating an extraterrestrial origin not just for organic bio molecules but for entire organisms simply shifts the location of the origin of life, without addressing the underlying questions of how life arose and at what point during this process homo chirality became established.
Clearly the questions of life’s origin and the relationship of its emergence to the phenomenon of homochirality are the subject of active investigation. To conclude this review, we are struck by the ‘‘symmetry’’ of some of the possible mechanisms linking these questions and the expressions of these in aspects of biology. Homochirality, a prerequisite of life’s emergence in some scientists’ view, might arise as a consequence of the roles played by cosmology (e.g., by cold dark matter and cold dark energy) and occur at the far edge of galaxies. The conjunction of these (the dark) with our increasing understanding of the processes that control nuclear fusion and supernovas in providing both the building blocks and the energy (the light) to drive life’s processes leads us to conclude with a quote alluding to the symmetry of light and dark. Thus the darkness bear its fruit, and prove itself
Life in other universes- possible with symmetry breaking!
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