The question of evolution of life on the Earth and elsewhere in the Universe has ever been as challenging as the question of evolution of the Universe itself. Science does not provide authentic explanation regarding the origin of the Universe in the controversial ‘Big Bang’ theory for evolution of the Universe, nor does it provide any satisfactory explanation regarding the origin of life despite considerable advancements in the fields of astrobiology, astrophysics, and genetics in recent years. The ‘Big Bang’ model for evolution of the Universe is not secure enough to serve as a foundation for beliefs about the origin of life, which is exemplified very much by the fact that the most distant galaxies, we can see today, look as rich and fully evolved as our own, even though they are barely 5% of the current age of the Universe, i.e. about 700 million years after the Big Bang (z ~7), as revealed in the Hubble Ultra Deep Field (HUDF) images taken with Hubble’s Advance Camera for Surveys and Near Infrared Camera. Among the several factors leading to beginning of life on this planet, ‘panspermia’ appears to provide the most favored hypothesis for emergence of life on our planet. This paper examines the various prevailing hypotheses regarding origin of life on this planet. It also hints at a very interesting and crucial inference that probably delivery of life-bearing organic molecules by the comets in the early history of Earth alone was not sufficient to provide the requisite trigger mechanism for initiation of life on our planet.

          Early Earth and Beginning of Life: Earth formed as part of the birth of the solar system about 4.6 billion years ago. It was then very different from the world known today. There were no oceans and oxygen in the atmosphere. During the period 4.3 - 3.8 Gya (the Hadean Epoch), Earth is believed to have undergone a period of heavy meteoric bombardment for about 700 million years. It was bombarded by planetoids and other material leftovers from the formation of the solar system. This bombardment, combined with heat from the radioactive breakdown, residual heat, and heat from the pressure of contraction, caused the planet at this stage to be fully molten. Heavier elements sank to the centre while the lighter ones rose to the surface producing Earth’s various layers. The early Earth was lifeless and simply inhospitable with its atmosphere dominated by the surrounding materials from the solar nebula, especially light gases such as hydrogen and helium. The planet is believed to have cooled quickly with formation of solid crust within 150 million years, and formation of clouds in about 200 million years. The subsequent rains gave rise to the oceans within 750 million years of Earth’s formation, making it a habitable planet for the first time in its history. The evidence have been reported of a possible Hadean ocean as early as 4.3 Gya. It may not be out of place to mention here that liquid water is the most essential ingredient to trigger beginning of life. Water provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to emergence of life. Steam escaped from the crust while more gases were released by volcanoes, creating the second atmosphere in Earth’s early history. Life on Earth may have emerged during or shortly after the early heavy bombardment phase, perhaps as early as 3.90 - 3.85 billion years ago, but the precise timing remains uncertain.

          A prebiotic reducing atmosphere comprising carbon dioxide (CO₂), methane (CH₄), carbon monoxide (CO), ammonia (NH₃), free hydrogen (H₂), and water vapor, if present, predicts that building blocks of biopolymers - such as amino acids, sugars, purines, and pyrimidines would be formed in abundance. Recent modeling of the Earth’s early atmosphere however, suggests in contrast, more neutral conditions (e.g. H₂O, N₂, CO₂), thus, precluding the formation of significant concentrations of prebiotic organic compounds. It is generally believed that until 2.4 billion years ago, Earth’s atmosphere was generally devoid of oxygen. Volcanic activity was intense, and without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface. Thus, the early Earth was just one big chemical evolution experiment. Many scientists now accept the notion that ancient meteorites and comets helped jumpstart life on our planet by bringing a significant amount of water, organic molecules and even amino acids to early Earth. Dust particles from comets and meteorites rich in organic compounds, rained down on early Earth. 

REFERENCES

        Origin of life – происхождение жизни, Big Bang theory – теория Большого взрыва, the Universe - Вселенная, Hubble camera – камера телескопа Хаббл, panspermia – панспермия (термин обозначающий распространение жизни во Вселенной), life-bearing organic molecules – пребиотические (рождающие живое) органические молекулы, planetoids - астероид, radioactive breakdown- радиоактивный пробой, residual heat – остаточное тепло, pressure of contraction – давление сжатия, solid crust – твердая кора, biopolymers - биополимеры, amino acids - аминокислоты, purines - пурины, pyrimidines - пиримидины, prebiotic organic compounds – пребиотические органические структуры.