Abiogenesis

Abiogenesis, in its most general sense, is the generation of life from non-living matter. The term is primarily used in the context of biology and the origin of life.

Some confusion exists on this topic, because early concepts of abiogenesis were later proven to be incorrect. These early concepts (referred to here as "Aristotelian abiogenesis" for clarity) held that living organisms could be "born" out of decaying organic substances, et cetera, which we now know does not occur.

The modern definition of abiogenesis is concerned with the formation of the simplest forms of life from primordial chemicals. This is a significantly different thing from the concept of Aristotelian abiogenesis, which postulated the formation of complex organisms. Different hypotheses for modern abiogenetic processes are currently under debate; see, for example, RNA world hypothesis, proteinoid, Miller experiment.

History of Abiogenesis Hypotheses

Aristotelian abiogenesis, also known as spontaneous generation, (and, in older texts, Generatio acquivoca, Generatio primaria, archegenesis and archebiosis), was the theory according to which fully formed living organisms sometimes arise from not-living matter. Aristotle explicitly taught this form of abiogenesis, and laid it down as an observed fact that some animals spring from putrid matter, that plant lice arise from the dew which falls on plants, that fleas are developed from putrid matter, and so forth. Alexander Ross, in commenting on Sir Thomas Browne's doubt as to "whether mice may be bred by putrefaction," gives a clear statement of the common opinion on abiogenesis held until about two centuries ago. Ross wrote:

"So may he (Sir Thomas Browne) doubt whether in cheese and timber worms are generated; or if beetles and wasps in cows' dung; or if butterflies, locusts, grasshoppers, shell-fish, snails, eels, and such like, be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by formative power disposed. To question this is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants."

The first step in the scientific refutation of the theory of Aristotelian abiogenesis was taken by the Italian Francesco Redi, who, in 1668, proved that no maggots were bred in meat on which flies were prevented by wire screens from laying their eggs. From the 17th century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, abiogenesis did not occur, but that omne vivum e vivo, every living thing came from a pre-existing living thing.

The discovery of the microscope carried the refutation further. In 1683 Antoni van Leeuwenhoek discovered bacteria, and it was soon found that however carefully organic matter might be protected by screens, or by being placed in stoppered receptacles, putrefaction set in, and was invariably accompanied by the appearance of myriads of bacteria and other low organisms. As knowledge of microscopic forms of life increased, so the apparent possibilities of abiogenesis increased, and it became a tempting hypothesis that whilst the higher forms of life arose only by generation from their kind, there was a perpetual abiogenetic fount by which the first steps in the evolution of living organisms continued to arise, under suitable conditions, from inorganic matter.

It was due chiefly to Louis Pasteur that the occurrence of abiogenesis in the microscopic world was disproved as much as its occurrence in the macroscopic world. If organic matter were first sterilized and then prevented from contamination from without, putrefaction did not occur, and the matter remained free from microbes. The nature of sterilization, and the difficulties in securing it, as well as the extreme delicacy of the manipulations necessary, made it possible for a very long time to be doubtful as to the application of the phrase omne vivum e vivo to the microscopic world, and there still remain a few belated supporters of abiogenesis. Subjection to the temperature of boiling water for, say, half an hour seemed an efficient mode of sterilization, until it was discovered that the spores of bacteria are so involved in heat-resisting membranes, that only prolonged exposure to dry, baking heat can be recognized as an efficient process of sterilization. Moreover, the presence of bacteria, or their spores, is so universal that only extreme precautions guard against a re-infection of the sterilized material. It was thus concluded definitely that all known living organisms arise only from pre-existing living organisms.

Modern concepts of abiogenesis

Even as Aristotelian abiogenesis was being disproven, many scientists, such as T. H. Huxley, continued to postulate a "primordial archebiosis", in which the living organisms observed in the present world had originally arisen in a series of stages from non-living matter. Such scientists pointed out that the disproof of Aristotelian abiogenesis applied only to "known existing organisms", not to unknown forms of life or proto-life which may have existed under the vastly different conditions of the early Earth.

Recent experiments have shown that under simulated conditions resembling those thought to have existed shortly after Earth first accreted, many of the basic organic molecules that form the building blocks of modern life are able to spontaneously form. Simple organic molecules are of course a far cry from a fully functional self-replicating life form, but in an environment with no pre-existing life these molecules could accumulate and provide a rich environment for chemical evolution. The spontaneous formation of complex polymers from abiotically generated monomers under these conditions is straightforward.

There are many different hypotheses regarding the path that might have been taken from simple organic molecules to modern cells and metabolism. The RNA world hypothesis, for example, suggests that short RNA molecules could have spontaneously formed that would then catalyze their own continuing replication. Early cell membranes could have formed spontaneously from proteinoids, protein-like molecules that are produced when amino acid solutions are heated. Other possibilities include systems of chemical reactions taking place within clay substrates or on the surface of pyrite rocks. None of these various hypotheses have strong evidence behind them at this time, however. Many of them can be simulated and tested in the lab, but a lack of undisturbed sedimentary rock from that early in Earth's history leaves few opportunities to determine what may have actually happened in practice.

A theory put forward by Thomas Gold has life first developing not on the surface of the earth, but several kilometers below the surface. We now know that microbial life is plentiful up to five kilometers below the earth's surface in the form of archaea. We also know that archaea are older prokaryotes than the bacteria, which live on the surface including the oceans. Discovery of microbial life below the surface of another body in our solar system would lend significant credence to this theory.

An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially. Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds and ultraviolet light. The rain of cometary material on the early Earth could have brought significant quantities of complex organic molecules, and it is possible that primitive life itself may have formed in space and been brought to the surface along with it. A related hypothesis holds that life may have formed first on early Mars, and been transported to Earth when crustal material was blasted off of Mars by asteroid and comet impacts to later fall to Earth's surface. Both of these hypotheses are even more difficult to find evidence for, and may have to wait for samples to be taken from comets and Mars for study.