Origin of life on Earth

Whether as a result of conditions like those simulated by the Miller-Urey experiments, or as a result of deposition from comets, it is possible that the early atmosphere was enhanced in these prebiotic organic molecules. So the question is how we go from these simple organic molecules to the life that is around us now. Evidence suggests that there was a gap of almost 1 Gyr between the formation of the Earth and the appearance of the first multicell organisms.

At the molecular and cellular level, a fundamental tenet of life is the ability to replicate itself. That replication allows for the development of multicell organisms. It also allows for generations of organisms. Imperfect replications are called mutations. Mutations can have no effect, provide a change for the better (more able to survive), or provide a change for the worse (less able to survive). According to Darwin's theory of evolution, the increased survival rate of the beneficial mutations means that the better trait is passed on to future generations.

At the molecular level, for most life as we know it, the replication is carried out by a very large molecule, deoxyribose nucleic acid (DNA). It is shaped like a ladder twisted into a double helix structure. This structure was first worked out in 1953 by James Crick (England) and William Watson (USA), and they won the Nobel Prize in Chemistry for their work. There are four possible parts, called nucleotides, for each position of the ladder. The nucleotides are made of subunits, called base, sugar and phosphate. In assembling a chain of nucleotides, sugars link to phosphates, so they alternate in the chain. The bases dangle off to the side. Each base can only have a particular base partner on the other side of its ladder rung. These partners are called base pairs. The sequence of bases determines the genetic information that is carried in the DNA, and governs the replication.

There is another nucleic acid, ribonucleic acid (RNA) . The current purpose of RNA is to facilitate the replication of the DNA by transferring information from the DNA to proteins. RNA consists of chains of up to a few thousand nucleotides. Each nucleotide has a phosphate, ribose (which is a five-carbon sugar), and one of four bases (adenine, guanine, cytosine and utracil). All four bases have a flat ringlike structure. By comparison, the sugar in DNA is deoxyribose (ribose without an oxygen), and one of the bases, utracil, is replaced by thymine (utracil to which a methyl group, CH3, has been added). So you see there is a very close relationship between DNA and RNA. It is possible that historically RNA played a role in the development of DNA.

So, the question is, how do you go from a pre-biotic soup to the complex DNA in less than 1 Gyr? One suggestion is that, once the simple amino acids formed, they would have had so many chances to react over half a billion years, that it was possible to form DNA molecules by random chance. One argument in favor of this is that since there was no life, there were no predators. Therefore, once formed, simple amino acids could stay for hundreds of millions of years without being destroyed. However, realistic calculations show that the likelihood of random formation is very small.

Most organic chemists working on this problem have decided that it is more likely that the DNA we have today is the end product of a series of well defined steps, building up more complex classes of molecules. While each of these steps may have taken some time, none was as improbable as the direct formation of DNA, and even the sequence of events is much more likely than the direct formation. Of course, there is no agreement on what those steps were. This is in part due to the fact that the initial conditions are not well known. Also, the current state is so far removed from the initial conditions that there are many equally plausible ways of getting here. We will just briefly note some of the more prevalent ideas.

It is generally agreed by those looking at the possible large steps that would lead from simple amino acids to DNA, that RNA is an important intermediary. In fact, one might be tempted to note that amino acids are the constituents of proteins, and proteins are the constituents of RNA, so the amino acids could have formed into proteins and the proteins formed into RNA. However, those studying how RNA works today have noted that RNA is a catalyst for the synthesis of proteins. This means that the RNA would have had to form first.

The formation of RNA without making proteins first is quite difficult (given the complexity of RNA). Chemists working on the problem have focused on the idea of finding enzymes that might serve as catalysts for this process. Remember, a catalyst is something that helps promote some reaction but is not changed in the process. Some catalytic reactions have been proposed which may have created RNA on a time scale of less than a year. Once the RNA formed, it could begin the process of replication. Furthermore, one by-product of such a process was the formation of certain proteins.

Once the chemicals are available, the development of life requires the formation of cells. Cells are the basis of all life we know now, and one of the questions that is still being addressed is when the cells first developed. In the first RNA that developed, replications that directly produced surviving molecules were favored. With the development of cells, a replication (and some variation) could be favored because it produced something which could help the cell survive. There are two different views on when cell walls began to appear. One is early in the process, and the other is late, about 3.8 Gyr ago. Different processes for the formation of RNA favor one or the other picture.

Cell walls are made of a lipid bilayer (double molecular level). It is about 5 X 10~7 cm thick. The molecules that make up these typically have two ends, one that attracts water and the other that attracts fat. Membranes grow by adding more material to a pre-existing membrane.

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