The Origin of Life

WHEN DARWIN PRESENTED a rational approach to explain the origin and evolution of the diverse species of animals, a question naturally arose: Where is the beginning of this process? If life begets life, and organisms evolve through the process of natural selection, then how did the first organism come into being? The problem is, no doubt, difficult. There is no witness; neither is there any fossil record of the first living form; none of its characteristics is directly known. Therefore, scientists took the logical approach to the problem. Since the process of evolution proceeds from the simple to the complex, therefore if we go back in time, we should logically expect to find simpler organisms. Fossil evidence also supported this assumption: the earlier a fossil is, the simpler is its anatomy. If one extrapolates this logic further backward in time, one comes to the conclusion that the first organism was the simplest possible living form. The quest then is: What could be the simplest manifestation of life? We may get a clue if we look around us, at the biological diversity that adorn the face of our planet. For instance, the structural makeup of a unicellular organism is far simpler than that of a multi-cellular one. Among the various single-celled organisms, some have cell-walls, others don't. Obviously the latter is simpler, of which a typical example is the virus — which is simply a large molecule. Like many other chemical substances, it can be crystallized and preserved — and in such a situation there is absolutely no sign of life in it. However, when it enters the cell of a living host, it replicates itself using the ingredients within the host — whereby it shows characteristics of life. Thus, in case of molecules, the ability to replicate by collecting the ingredients from the surrounding may be viewed as a manifestation of life. And such replicating molecules may be considered as the simplest embodiment of life. Therefore, scientists believe that on the face of our planet, molecules capable of replication might have constituted the earliest lifeform. What was this molecule like? The clue, again, is supplied by the biological world of today. Though there are diverse kinds of species with very many differences between them, is there any aspect that is shared by all life forms? Indeed, there is. If we look for the similarities, we would definitely come across a point of basic unity — which must have been present in the earliest life-form also. This point of commonness is that all life forms are made of molecules with a basic structural element involving the carbon chain. One important attribute of the element carbon is that it can form long chains linking with other carbon atoms virtually limitlessly, producing complex molecules. In fact, the existence of life is dependent on this special property of carbon which is called catenation. So it is a reasonable assumption that the molecule constituting the first life form also had a long chain of carbon atoms. Can we guess precisely what had been the shape of the carbon-chain? The answer to even this question comes from the similarities amongst the living organisms today. We may note that the various types of proteins which constitute the body-parts of all living organisms are made of the basic building block called amino acids. The amino acids are considered to be the fundamental constituent of all life forms. So we can safely assume that amino acids played a decisive role in the anatomy of the first