Not sure if this was posted before, but anyway.........
https://arxiv.org/ftp/arxiv/papers/0907/0907.3552.pdf
Origin of Life
Abstract
The evolution of life has been a big enigma despite rapid advancements in the fields of biochemistry, astrobiology, and astrophysics in recent years. The answer to this puzzle has been as mind-boggling as the riddle relating to evolution of Universe itself. Despite the fact that panspermia has gained considerable support as a viable explanation for origin of life on the Earth and elsewhere in the Universe, the issue remains far from a tangible solution. This paper examines the various prevailing hypotheses regarding origin of life like abiogenesis, RNA world, iron-sulphur world, and panspermia; and concludes that delivery of life-bearing organic molecules by the comets in the early epoch of the Earth alone possibly was not responsible for kick-starting the process of evolution of life on our planet.
Concluding Remarks :
Given the extraordinary potential of extremophiles to survive in highly inhospitable environments on the Earth, possibility of life on extraterrestrial bodies like Mars, Europa, Titan, Enceladus, Ganymede, Callisto, and Triton in the solar system cannot be ruled out. Also, study of the planetary geology reveals that our solar system could have many worlds with liquid water, the essential ingredient for life. Recent data from NASA’s Mars Exploration Rovers also corroborate the speculation that water has at least intermittently flowed on the red planet in the distant past (Warmflash and Weiss 2005). Moreover, studies based on data from NASA’s Mars Reconnaissance Orbiter have revealed that the red planet once hosted vast lakes, flowing rivers, and a variety of other wet environments that had the potential to support life. It is thus not unreasonable to hypothesize that life existed on Mars long ago, and perhaps continues there even today. Permafrost environments on Mars may also help harbour life in the light of the fact the ancient bacteria on Earth are capable to spring back to life after being in state of prolonged hibernation for nearly half a million years in harsh and frozen conditions. Besides, analyzing data gathered by Cassini spacecraft, scientists have recently confirmed presence of heavy negative ions about 10,000 times the mass of hydrogen in the upper regions of Titan’s atmosphere which is devoid of oxygen and comprises mainly of nitrogen and methane. These particles may act as building blocks for complicated organic molecules - the harbinger for earliest form of life in Titan’s atmosphere (Richardsons et al. 2007). Moreover, Cassini’s radar mapping reveals that Titan is just covered in carbon-bearing material. It is a giant factory of organic chemicals with several hundred lakes and sea of hydrocarbons (methane and ethane). Life may have even got a foothold on the torrid Venus. Though the Venusian surface is too hot (~ 480°C) and under too much atmospheric pressure (90 bars) to be habitable, the planet could still conceivably support sulfur-based microbial life high in its atmosphere as do sulphur-eating ‘chemotrophs’ on the Earth. Keeping in view the great surviving capabilities of extremophiles on the Earth, possibility of traces of life forms being found on billions of unexplored planetary bodies outside our solar system cannot be negated as well. Out of 721 exoplanets detected beyond our solar system till January 2012, the extrasolar planet (Jupiter like gaseous planet) named ‘HD209458B’ located at some 150 light years from the Earth in the constellation of Pegasus is believed to be harbouring water vapour in its atmosphere giving rise to speculation of the presence of life-supporting microbes in its atmosphere based on the analysis of the infrared spectrum in the range of 7.5 - 13.2 µm (Maa et al. 2007). The planetary habitability chart (Fig.8) provides clue where life might exist on extrasolar planets based on study of our solar system and life on Earth. Recent computer simulations of the known extrasolar planetary systems suggest about half of the hitherto known exoplanets could harbour Earth-like world raising possibility of traces of life being found thereon. The author describes at length the possibility of finding traces of life on the habitable planets and moons within the solar system and beyond in his article titled ‘Searching for Life on Habitable Planets and Moons’ published in Journal of Cosmology ( Lal 2010). Despite strong possibility of existence of a large number of extraterrestrial life-systems in the Universe, there remain nagging uncertainties in regard to the timing of commencement of the process of evolution of life on the primitive Earth and elsewhere in the Universe. Though panspermia provides satisfactory explanation to the origin of life on Earth and elsewhere in the Universe, it however, fails to address the long-standing riddle as to when and where precisely life originated first in the Universe, nor does it provide any clue about how transformation from prebiotic matter into primitive bacterial cells was brought about.
Evidence from comets and meteorites as also from the experiments that simulate the conditions on the early Earth suggests that probably a combination of terrestrial and extraterrestrial factors were responsible for kick-starting the process of transforming pre-biotic organic compounds into entities that we call ‘life’ on Earth. There is growing evidence to support the view that emergence of catalytic RNA was a crucial early step in the evolution of life on Earth. How that RNA came into being, however, remains unknown so far. Moreover, the “RNA World” hypothesis does not seem to provide satisfactory explanation to the initiation of mechanism of ‘self-replication’ in organisms in the early history of Earth, which is so crucial to the understanding of the process of evolution of life on our planet and other habitable bodies in the Universe. We do not yet understand the steps leading from abiotic early Earth to the RNA World( Orgel 1995). Experiments involving biologically produced RNA have so far failed to provide concrete proof regarding the RNA world being the pathway between non-life and life.To date, no possible explanation has been advanced as to how primitive self – replicating RNA molecules could have made transition into modern cellular systems that rely heavily on a variety of proteins to process genetic information. The RNA World hypothesis that hints at first living organism having an RNA-based genome appears to be on shaky ground also considering reproductive strategy of viruses whose RNA-based genome requires DNA of a living host to survive and replicate. Moreover, despite development of sophisticated biotechnology tools in the recent years, scientists still have not been successful in transforming inanimate matter into life in the laboratory.
The available scientific knowledge simply fails to provide the viable clues about the process of evolution of life from non-life. Besides, the underlying uniformity of life on the Earth, with all modern organisms sharing the same DNA-based mechanism for genetic transmission, is indicative of the fact that life emerged here only once during the planet’s entire history (Burliinski 2006).It is ironic that the crucial timing hitherto remains unknown to the mankind.
Ever since Oparin and Haldane initiated the modern theory of life’s origin from non-life in 1930s, we have learnt much about how life operates, but almost nothing about how it originates. It is a puzzle whose mystery will perhaps remain unknown to the humanity ad infinitum. ……………………………………………………………
Quite an Interesting section on Panspermia within the paper........