It is true that the chemoautotrophic organisms are able to live on simple inorganic foods that were, in all probability, available to living things under early conditions in the development of the earth. However, it does not necessarily follow that chemoautotrophic forms are the only ones that could have existed in the beginning. It seems even more reasonable to assume that early living forms developed a pigment like chlorophyll that enabled primordial bacteria to utilize the sun's energy in synthesizing organic matter. Such photosynthetic pigments are found in purple or green bacteria. These photoautotrophic forms could have existed on the simple foods available when life began as readily as could chemoautotrophic forms.
In either case, it is necessary to assume that living protoplasm, with its complex enzymatic systems, existed before primordial bacteria, which utilized inorganic materials as food. In other words, complex proteins had to be in existence before either chemoautotrophic or photoautotrophic bacteria of the types now found on the earth could exist.
Even if it is granted that photoautotrophic living things were primordial, it must also be granted that when the existence of such organisms is postulated we are not starting with the beginning of life itself. So little is known about the possibility of living proteins (protoplasm) developing out of inorganic compounds that speculation regarding this development has brought but very little information that is factual.
In the present edition of Bergey's Manual, the classification used has been rearranged on the assumption that the photoautotrophic bacteria extant today presumably are the living organisms that are most nearly like the primordial types of bacteria.
In support of this thought it should be kept in mind that the earliest living forms must necessarily have been free-living forms, not saprophytes nor parasites. This being the case, forms such as viruses that are very tiny in size and therefore necessarily of a simple structure ought not to be regarded as primitive just because of a comparatively simple structure. The viruses are adapted to life within living protoplasm, and they represent an extreme degree of specialization to a parasitic existence. They are known as organisms that invade the living cells of higher plants and animals, including man. The latter are the living things that were latest in development in geological time. Viruses could not have existed before their host plants and animals were developed.
The term "viruses" ought not to be used for the hypothetical, very tiny, free-living primordial organisms that must have existed before primordial bacteria. Some investigators feel that such organisms may still exist in some as yet unrecognized form.
It is not surprising that a great development has taken place in outline classifications since bacteriologists first tried to develop such classifications to express the possible relationships of the organisms with which they have worked. While O. F. Mueller (Animalcula infusoria et Marina. Hauniae. 1786) and C. G. Ehrenberg (Die Infusionsthierchen als vollkommende Organismen. Leipzig, 1838) made
