Every organism is the offspring of previously existing organisms. The evolution of life is a continuous process. Life cannot spring from non-life, as many proponents of "spontaneous generation" believed before the middle of the nineteenth century. Yet it seems that spontaneous generation must have happened at least once; that is what we mean by the "origin of life". Kelvin and other late 19th century luminaries maintained that the origin of life could not have happened by means accessible to scientific knowledge and reason, precisely because it would have entailed spontaneous generation. Such was the influence of these luminaries that the topic did not receive serious scientific attention until well into the 20th century.
The genetic code is more or less universal: all proteins in all organisms are made from the same amino acids, and all amino acids in proteins have the same "handedness" - as do most other biological molecules. Also, all organisms have certain key metabolic pathways in common. This suggests that every organism extant today, and all organisms that lived in the knowable past, can be traced to a single common ancestor31. This ultimate ancestor might not have been the first organism - there might have had predecessors that went extinct - but it is hard to deny that it (and any such predecessors) arose from non-living matter, that it was a product of spontaneous generation. If spontaneous generation happened when life began then obviously it was possible at that time. But very shortly afterwards it ceased to be possible, otherwise we would not be able to trace all organisms to a single common ancestor.
Several attempts have been made to evade this inference. Some authors have suggested that life began elsewhere in the galaxy (presumably within the solar system) and was transported to earth by meteorite or comet, presumably in the form of spores. However, such spores would have had to survive the conditions of interplanetary space, battered by cosmic radiation,
31 An indefinitely large number of amino acids could, chemically speaking, be incorporated into proteins, but only twenty actually are. The genetic code, the correlation between each amino acid and the DNA/RNA base triplet that encodes it, is constant over all organisms known today - there are minor exceptions only in mitochondrial DNA - and there is no convincing chemical reason why this should be so. Life that arose independently from non-living matter would almost certainly have chemically different proteins and a different genetic code, assuming that it used proteins and nucleic acids at all. Moreover, amino acids and other biological molecules exist in two or more mirror-image forms (isomers). These forms are geometrically different but chemically identical, yet only one form - left-handed amino acids, for instance - is used by organisms. The most likely inference is that the choices of amino acids and their nucleic acid correlations were established (by chance) when life began, and have remained fixed ever since. In other words, all extant life has descended from the same ancestor.
for many millenia; and the body transporting them would have reached a very high (sterilising) temperature when it finally accelerated through the atmosphere to the Earth's surface. So this is an unlikely scenario. Even if it were true, it would merely shift the problem of the origin of life to another world, even less well understood than the prebiotic Earth. Therefore, the extraterrestrial origin hypothesis does not answer the question of how life began; it tries to dodge it, and by a rather implausible argument.
Another attempt to evade the problem assumes the steady-state rather than the "big bang" theory of cosmology. If the cosmos has always existed, i.e. had no beginning, then it is possible to suppose that life too has always existed. Therefore, the question of its origin becomes void. This is the position famously adopted by Fred Hoyle and his colleagues. It has found little support during the past few decades because the steady-state theory of cosmology is now almost universally rejected; too much evidence favours the "big bang" alternative. But if Hoyle were correct, we would have to infer that "life", being in effect as eternal and omnipresent as the cosmos itself, is somehow written into the laws of physics. In what sense could this be so? This question is, in effect, the origin-of-life problem in disguise.
Such speculations have minority followings; most people tacitly accept that the "spontaneous generation" problem is real. To explain how life began on Earth from lifeless matter is a huge challenge. To explain why spontaneous generation subsequently became impossible, and has remained impossible ever since, may be equally difficult.
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