Prebiotic Evolution

Introduction

Although the idea is hard to accept now, it used to be widely believed that living organisms could immediately form from non-living matter. This reasoning was called "spontaneous generation". It was used to explain the sudden appearance of organisms in neglected places- Why do maggots appear where meat is left to rot? This theory was soon discredited, through several experiments that isolated different samples of such incidents, and discovered that such organisms originated from larvae. In other words, life reproduces from life.

The idea that life could only form from life caused many disturbances, particularly among the proponents of the theory of evolution. The theory of evolution supports the idea that spontaneous changes led to more diverse and complex organisms. This would mean that as one gets closer to the source of life, organisms become simpler and finally becomes non-organic matter, the simpler form of matter. In short, the theory of evolution predicts that life "generated spontaneously" from non-organic matter. The differences between the findings of the experiments and the theory of evolution were not solved until the Russian scientist, A.I. Oparin proposed the prebiotic theory of evolution.

Oparin proposed that life formed in a series of steps from non-organic matter. This theory is known as the theory of prebiotic evolution. Prebiotic evolution holds that life originated gradually from interaction between different chemicals in the Earth's atmosphere. These interactions led to the first amino acids, which then formed more complex molecules such as proteins and DNA. These molecules, in turn, combined to perform different functions and created the first proto-cell. One of the reasons that Oparin's theory remains sound, is that as a hypothesis, it could be proved wrong. This led to the first experiment that laid the foundation for Oparin's theory.

Miller's Experiment

Oparin suggested that several chemicals in interaction with the environment led to the building blocks of life. It was left to Stanley Miller and Harold Urey to simulate these hypothetical interactions. The early Earth had a chaotic environment with energetic processes, including lightning, volcanic heat and intense radiation. To simulate these conditions in the laboratory, Miller and Urey used a spark-discharging apparatus. They provided the early atmosphere gases: methane, hydrogen, ammonia and left them to the high-energy spark discharges.

Later, Miller observed that upon condensation, water had changed colour, and a tar-like substance had formed in the flask. Miller examined the two substances and found several different amino acids, the building blocks of proteins. These results generated great excitement among biologists, and many such experiments were conducted. This list of discovered amino acids grew with new results.

The apparatus that Miller and Urey used to simulate the early stages of life on earth.

The Primordial Soup

Miller's experiment and several others were used to propose the idea of a "primordial soup", a situation were amino acids collected and combined to form more complex molecules. However, for such a situation to occur, there must perfect orientation between amino acids, so that they combine among themselves rather than react with other elements. If the primordial soup did occur then there had to be many combinations of amino acids to ensure that a fraction of these molecules would combine "properly" with the right activation energy. Given the heavy odds against the formation of amino acids (explained later), the chances of a primordial soup seems very slim. To account for this, Oparin and several others suggested that matter has a tendency for self-organization which guides itself to the formation of organic matter.

Limitations

The theory that life arose from non-organic matter through various stages has serious limitations, most which result from our limited understanding of the past and the assumptions in Oparin's theory. These limitations include-

1) Lack or presence of oxygen

2) Amino acid formation

3) Destructive energy

Could life have survived with or without oxygen?

The amount of oxygen in the early atmosphere is widely disputed. Certain biologists claim that if oxygen was present in the atmosphere, then life would not be able to form, as evidence has shown that oxygen was poisonous to early forms of life. On the other hand, others point to the existence of oxides in minerals, that could not have formed without the presence of oxygen. They claim that geologic evidence shows that the composition of the early atmosphere included oxygen.

Miller's experiment explained, if not completely, the formation of amino acids. However, it did not go beyond that. Amino acids are instrumental in producing proteins, which in turn carry out specific functions of a cell. No present cells exist without proteins. The formation of proteins is a much more complex process than that of amino acids, and involves a larger range of variables. So far, such a process has not been successfully reproduced in the laboratory, and therefore brings up more questions about the prebiotic theory.

Moreover, if the primordial soup did develop, it had to be sufficiently protected from the destructive energetic processes on the early Earth.

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