Introduction

Many theories have been advanced as explanations for the origin and history of life on Earth. The most popular with the majority of evolutionary theorists is the primordial soup theory, which states that self-replicating entities, the precursors to life as we know it, arose spontaneously out of the chemical environment of the early Earth. This theory argues that the chance reactions taking place at high rates in the chemical mixture of the early atmosphere eventually gave rise to molecules with the property of replication.

The Primordial Soup Theory

The exact environment of the early atmosphere on Earth is not known, but some chemicals have been hypothesized: carbon dioxide, ammonia, water, and methane. When gases such as these are placed in a flask and zapped with electricity, amino acids and other organic molecules are formed. (Stanley Miller pioneered these experiments; however, the atmosphere is now thought to be less favorable to the formation of organic molecules than once believed.) These results imply that energy stimulation of these molecules, possibly in the form of lightning or ultraviolet radiation, or even pressure waves, can under certain conditions form complex organic substances. At some point, after many repetitions of the energy stimulation, a molecule was formed by chance that had a very unique property: it could replicate itself using various component molecules in its environment. For example, a molecule whose chemical properties allowed it to cause other molecules to react, thus forming two of the original molecule, could quickly set up a population of such self-replicating molecules. (This idea of the generation of life from nonliving chemicals is called abiogenesis.)

At first, the new replicator would have free run of the chemical resources in the primordial soup. The replicators would probably reproduce freely under such conditions because they would have a monopoly on the available resources. In addition, the sheer numbers of copies being made, added to the fact that the replicator would be very primitive and without editing mechanisms, would result in numerous copying errors. These errors are mutations that will later be used in the development of natural selection.

After a while, the replicators would have used up many of the available chemicals in the soup (especially in localized areas), and they came into direct competition with each other for the use of the remaining resources. This set up a situation where natural selection could begin to operate on the population. Those replicators that were better at reproducing, either because they reproduced faster, out-competed others for resources, used fewer chemicals, etc., came to dominate the soup. Natural selection favored those that possessed traits allowing them to out-replicate the others.

After a long time, the resources of the area would become extremely scarce. Selection pressure would come into existence strongly favoring those replicators that could synthesize at least some of their own chemicals needed for reproduction. Once a certain chemical became scarce, selection pressure would favor those replicators that produced the chemical themselves.

Eventually, it seems reasonable to theorize that replicators might begin manufacturing enough chemicals that they would want to keep all the chemicals close by. One way to do this is to manufacture a container (made of chemicals that the replicator can already manufacture). This was the precursor to modern cells.

Other replicators would have evolved alternative strategies for dealing with the intense competition for resources. They may have developed ways of manufacturing chemicals that broke down other replicators into their component structures, thus providing raw materials from the destruction of competitors. Still others may have evolved new chemicals with new and improved properties and thus out-competed the other replicators.

Cooperative Replicators and the Development of Vehicles

Over time, replicators probably began to cooperate among themselves, with multiple replicators existing together in an aggregate, each producing a different chemical or performing a different function. These "replicator teams" quickly came to incorporate the container idea mentioned above, for the dual reason of keeping chemicals in one area and holding the replicator team together. In time, under the influence of continual competition and selection, the replicators would come to develop more and more organized, advanced structures. These structures would most likely be devoted to manufacture of chemicals, storage, repair, etc. Cell organelles were born.

With the birth of the proto-cell, the first vehicle would arise. With organelles, replicators would become more and more specialized while developing greater and greater complexity in their biochemical pathways. The end result of all this developmental activity is the ancestral bacterium.

The Development of Eukaryotes

Bacteria, the most primitive of modern-day cells, lack a nucleus to direct and regulate cell function and are thus called prokaryotes. This is consistent with the model of their evolution: various replicators coming together and cooperating, but without the guidance of a central system. However, to form eukaryotic cells (cells with nuclei), a guidance system is needed.

This guidance system will most likely arise from the replicators themselves eventually. The replicators, increasingly specialized, will be too numerous in a complex cell to simply float about, and they will concentrate in one region. This region may develop a membrane for protection, and the nucleus is born.

Further development of the eukaryotic cell will take place when some of them begin to ingest other cells as sources of chemicals. At some point, the other cells might not have been broken down by the proto-eukaryote's digestive chemicals. These ingested cells might then provide extra energy to the larger cell, thus establishing an endosymbiotic relationship. This is believed to have taken place in the incorporation of mitochondria, chloroplasts, and even cilia into eukaryotic cells.

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