What is life? It is very difficult to answer. Life is generally defined philosophically as well as biologically. Before defining life we need to answer the question of origin of life? There are two lines of thinking about the way life first emerged. If life arose from a biotic process, then it must exist everywhere, wherever chemicals and biological evolution can lead to conditions that are suitable. Origin of life should be an inevitable consequence of laws of physics as inherent in the Schrodinger’s equation. With about 200 billion stars in our galaxy and over 100 billion galaxies in the universe, we must have a large number of hospitable planets on which life could exist, as can be predicted by probability considerations given by the Drake Equation.’
As on earth, it is reasonable to assume that life elsewhere must also be carbon and water based, although other possibilities like silicon- based structures have been discussed. If life exists anywhere else, it is unlikely to have shapes and forms similar to ours. So if we wish to look for life, we ought to define what exactly is meant by life. In brief, life must satisfy the conditions of reproduction and metabolic activity and be capable of change through these processes.
On the other hand, all experiments done hitherto on the Earth show life can only arise from life, Chemists have been able to make complex organic molecules such as proteins, amino acids, DNA, RNA and other complex building blocks of life in the laboratory but no one has been able to synthesize a cell or put together simple structures such as mitochondria or chloroplasts from its constituents. Abiotic synthesis of a cell may, therefore, be the key to the origin of life. Whether life exists elsewhere or not would be determined by the probability of the formation of the first cell from end products of chemical evolution, which clearly must have occurred in many parts of the universe.
Different theories have been propounded to explain the facts about life. One of the most acceptable theory is of Oparin- Haldane Theory which is similar to the modern theory of origin of life. This theory caters both chemical and biological evolution.
Summary of main steps in the origin of life according to Modern theory of Origin of Life.
There other theories which presents and gives several fact about the origin of life. First of all we know from fossils found in deep sea sediment cores that, over the aeons, life have evolved from simple to complex forms. Fossils records on the earth show that life started at about 3.5 billion years ago or even a few hundred million years earlier and the complexity has grown exponentially with time. Secondly, life occurs wherever conditions are favourable for its sustenance. Life has been found in diverse environments like Antarctica and in hydrothermal vents at the bottom of the oceans, where photosynthesis is not possible, due to lack of sunlight. Here, other alternatives path ways such as fixation sulphide, the number of species grows exponentially. Thus starting with single bacteria, a colony as big as the whole earth can grow in a period as short as week. We may therefore infer that, wherever life is found, it will be in abundance.
Meteorites have provided crucial information about prebiotic chemistry. The carbonaceous meteorite which fell at Murchison, Australian 1969 has provided valuable clues abiotic synthesis. This meteorite formed very early, at the beginning of the formation of the solar system and contains besides carbon and hydrated minerals several amino acids. Many of these occurs in racemic proportion and the nitrogen isotopic ratios provide definite evidence that they are not terrestrial contaminants acquired after the fall of meteorite. Urey-Miller synthesis of formation of complex organic molecules in reducing environment containing CO2, CH4, H2O, etc. have shown that it is easy to form these building blocks of life by abiotic processes. However, initiation of biotic processes, if at all it can occur from abiotic processes, is expected to take a long time. We know that earth formed early, within 50 or 60 million years of formation of the solar system by collisional accretion of planetismals. Thus, there was hardly any time available for transition from abiotic processes which is supposed to take a long time.
In this context the hypothesis of Panspermia advanced by Helmholtz, Arrhenius and recently favored by Hoyle and Wickramsinghe is noteworthy. If life seeds are present everywhere in the universe, then life must occur wherever the conditions are conducive, within our planetary system and outside. If we survey various bodies of the solar system we find that Moon, Mars, Europa, Ganymede, Titan and comets in Kuiper belt or Oort’s cloud have frozen water on or below their surface.
Life possibly began in space as shown by recent probes inside comets, a new study by scientists from Cardiff University has said. Professor Chandra Wick-ramasinghe and colleagues at the University’s Centre for Astrobiology have long argued the case for panspermia-the theory that life began inside comets and then spread to habitable planets across the galaxy Now the team has claimed that findings from space probes sent to investigate passing comets have revealed how the first organisms could have formed. They said ‘The 2005 Deep Impact’ mission to Comet Tempel discovered a mixture of organic and clay particles inside the comet.
One theory for the origins of life proposes that clay particles acted as a catalyst, converting simple organic molecules into more complex structures. Incidentally, the 2004 ‘Stardust Mission’ to Comet Wild 2 found a range of complex hydrocarbon molecules potential building blocks for life.
Wickramasinghe said radioactive elements could keep water in liquid form in comet interiors for millions of years making them potentially ideal incubators’ for early life. He said billions of comets in our solar system and across the galaxy contain far more clay than the early Earth did. As such, the odds of life starting on Earth rather than inside a comet stood at one trillion trillion (10 to the power of 24) to against one, he said.
“The findings of the comet missions, which surprised many, strengthen the argument for panspermia. We now have a mechanism for how it could have happened. All the necessary elements like clay, organic molecules and water are there. The longer time scale and the greater mass of comets make it overwhelmingly more likely that life began in space than on earth,” said Wickramasinghe. Inspite of absence of any evidence so far for origin of life, the subject remains extremely fascinating. It is hoped that with improved technology, the coming century may provide an answer to the question.