Extraterrestrial theory on the origin of life suggests that life originated elsewhere and not on earth. Its proponents argue that if life originated on earth, then it should have existed on other planets under similar conditions. In their view, life forms entered the earth from other heavenly bodies such as the meteorites and comets. They base their evidence on the recent discovery of a trail of the amino acid called glycine, which was found on dust particles collected from a fallen comet. An isotopic analysis done on the dust material indicated that there had been vigorous cometary impacts long after the earth coalesced, but before the commencement of life forms. Scientists suggest that the complex organic materials essential for life may have been formed from dust grains surrounding the Sun before the formation of the earth. The discovery of glycine supports their argument that the basic building blocks of life forms existed on other heavenly bodies and not necessarily on earth alone. The Viking Mars Lander report, which associated the emission of gas from heated comet soil, has been criticized on grounds that it lacks corroborative evidence and fails to provide clear scientific proof since the comet dust used for the study is said to have been contaminated.
The heterotrophic theory on the origin of life suggests that life began several billion years ago because of the evolution of various elements. Under this theory, the pre-existing elements in the atmosphere were mainly Hydrogen, Water, and Methane, as opposed to the current atmosphere where the dominant elements in the atmosphere are Nitrogen and Oxygen (Oparin, 1957). It suggests that these conservatives may have come from loose molecules and living systems. It is argued that the macromolecules were formed because of the association between particles that precipitated out in the form of water droplets. With time, the macromolecules formed protocells that derived enzymes from the surrounding medium for survival. The resultant cells were called heterotrophs. This theory is challenged on the basis that the conservatives could not have arisen or existed out of each other. The fact is that they must have existed before, making the evolutionary point of view difficult to defend.
The autotrophic theory, on the other hand, borrows its reasoning from the heterotrophic concept. It suggests that carbon dioxide gas released by heterotrophs during respiration was used by autotrophs in photosynthesis. The autotrophs released Oxygen to the atmosphere as a by-product of anaerobic respiration. It is suggested that the high levels of Oxygen may have intoxicated the heterotrophs causing some of them to die. The surviving photosynthetic heterotrophs and the autotrophs evolved and mutated with time to form the present life forms. A carbonaceous chondrite study that was conducted on a fallen meteorite revealed that it contained elements that, under this theory, formed the basis of the existing forms of life. A recent study conducted by researchers suggests that indeed the first forms of simple cells were autotrophs. Proponents of this theory associate the effect of heat from the sun with the photosynthetic characteristics of autotrophs. Their study indicates that autotrophs occupy the hottest parts of the modern world, which they observe to be similar to the niche held in the early times of life formation. In their view, the autotrophs formed the first forms of life and not the heterotrophs.
Reference
Oparin, I. (1957). The Origin of Life on the Earth. New York: Academic Press.