Life

The many theories for the origin of life are hotly debated. Some claim that it is impossible to life to originate from unliving, inorganic chemicals. Several scientific theories, however, claim the opposite. These theories are very complex, but the basics of one of these will be explained briefly: abiogenesis.

Abiogenesis

Abiogenesis is the proposal that life emerged from non-life; that the first living cells were formed from inorganic, unliving particles. Abiogenesis differs from other theories regarding the spontaneous generation of life in two ways.
First, instead of assuming that a complete and complex organism was spontaneously generated all at once, it proposes that the initial life forms were much simpler than modern cells and may have grown in complexity over time.

Scientists assume that there are three main phases involved in the origin of life; the creation of monomers, the creation of polymers and the creation of living cells. Monomers are small molecules or atoms with the potential of chemically binding with other monomers of the same type to form a polymer, a chain of these simple molecules.
Living cells are formed of several 'building blocks' of polymers and monomers, such as amino acids, nucleotides and fatty acids.

According to experiments by Stanley Miller, monomers may have formed spontaneously in an environment such as that on the early Earth. Assuming a reducing atmosphere with no free oxygen and an energy source, monomers would form and accumulate over time. This 'primordial soup' may have even been bolstered by meteorites containing these organic precursors.

Further Reading: Glycine Discovered on a Comet.

This article explains how NASA's Stardust spacecraft discovered glycine, a fundamental building block of life, on the Wild 2 comet.

Now that the origin of monomers has been accounted for, the next stage of complexity can be reached. Polymerisation is the stringing together of monomers to form more complex molecules. This can include peptides, proteins and nucleic acids. Joining amino acids together could form peptides which catalyse (accelerate) other chemical reactions thought to be essential to the origin of life. Most scientists studying abiogenesis theorise that protein catalysis occured much later, since peptides would be formed randomly and have no method of reproduction. RNA is now the main focus of most scientists.

RNA is a string of four different nucleotides, linked together via their sugar and phosphate groups. The nitrogenous base of each nucleotide is able to form hydrogen bonds with the bases of other nucleotides in the string. Depending on the string of nucleotides, three-dimensional shapes can be formed. This is significant because the formation of shapes is important to biological catalysis. There is evidence to support RNA as a catalyst. Futhermore, the nucleotide sequence of RNA could theoretically be replicated, allowing RNA to function as an information storage molecule.

Both RNA's ability to function as a catalyst and information storage molecule with the potential of self-replication make it an important molecule in the formation of life. Many scientists believe that there was once an 'RNA world'; this idea presents a base for the formation of more complex life forms.

DNA, perhaps the most important molecule involved in the creation of life, is another problem. In order to form DNA, various proteins are needed. However, in order to form these proteins, information provided by DNA is needed. RNA provides a solution to this issue because of its function as a self-replicator: both the imformation (in the form of a template) and the catalytic ability to synthesise an RNA molecule can be found on the RNA molecule itself. The basic parts of DNA, deoxyribose and thymine, are not exclusively synthesised by DNA-specific functions. They can occur as a result of modification to the pre-existing RNA synthesis pathways, suggesting that DNA could arise as a consequence of metabolic modifications to RNA.

Once the RNA world was formed, the origin of cellular life could begin. First, the RNA world generated the process of information-guided protein synthesis similar to that which occurs in modern life forms. Second, these biochemical reactions were encapsulated by a membrane. Finally, the information storage ability of RNA was transferred to DNA in an easily retrievable manner.[1]

An example of natural selection.

An example of natural selection.

Image © Elembis.

Evolution

Evolution is a widely accepted theory explaining how all of the lifeforms on Earth originate from common ancestors. While evolution has many mechanics, it is essentially change via descent. As a lifeform reproduces, its genetic information is passed down to the offspring, including any mutations that occur. Beneficial mutations may aid the survival of a creature, allowing it to survive longer and pass the mutation to its offspring. Over time, these mutations will accumulate and result in an entirely different lifeform. This mechanic is called 'natural selection'.

If an organism's offspring produces a beneficial mutation, it may gain an advantage over the non-mutated organisms, eventually resulting in their extinction. This process could allow the gradual shift from one organism to another as mutations accumulate and are passed on to offspring. An example of this can be seen in the image to the right.[2]

Species are forced to adapt to their environment or face extinction. Organisms that can survive, possibly as a result of a mutation, will reproduce and eventually evolve to suit their habitat. An example of this can be seen in bacteria today: the soil bacterium Sphingobium, has evolved a metabolism capable of dissolving the synthetic (man made) pesticide pentachlorophenol.

Bibliography.