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Molecular Fossils

What is a molecular fossil ?

Until relatively recently, technology has not been capable of analyzing the molecular structures of fossils. The field of molecular fossil study has only been accessible to scientists within the last few years. Thus, today, only very little is known about the molecular fossil record. Basically, a molecular fossil is just the preservation of organic material from dead organisms that have since been transformed into fossils or been mostly decayed. This type of fossils, though, is very delicate, since it is based on the very molecular structures that made up the organism. It is therefore very susceptible to decay, as chemical reactions break apart the bonds that hold the molecule together. However, traces of organic molecules are still found, and with the advance of technology, much more will be uncovered in the years ahead.

There are four main groups of organic molecules that are preserved in the fossil record, and these include the following: nucleic acids (which include deoxyribonucleic acids (DNA) and ribonucleic acids (RNA)), proteins, carbohydrates, and lipids. Other organic molecules often exist that could be better preserved than these that are unique to specific types of organisms, but here, we will mainly focus on the organic molecules just mentioned, which are common among all (or at least most) organisms.

Nucleic acids could probably tell us the most about the dead organism that left them behind, but they are also the most fragile. DNA and RNA have bonds that are very reactive and easily hydrolyzed. Degradation of these molecules begin soon after a cell dies, so there is very little chance of preservation. Even so, as of late, the technique of polymerase chain reaction (PCR) can be used to analyze these molecules. It is a technique in which specific sequences in the DNA or RNA are amplified for analysis. It has also been used to fit together separate fragments of sequences that overlap to form a more complete sequence. Think of this as a jigsaw puzzle, where you often have extra pieces. As you put together different parts of the jigsaw to form the picture, you notice that some parts overlap, so removing the overlapping pieces from one portion allows you to form a more complete picture with the two pieces. This is an example of how advancing technology is improving the study of molecular fossils. Nucleic acids have been found mostly in Miocene plants.

Proteins are most abundant in animals, but the twenty amino acids that make them up can be found in any species. Proteins often form very complex three-dimensional shapes that fold back on themselves to allow for hydrogen bonding and cross-linking between adjacent atoms. Thus, it is a more rigid compound than DNA or RNA. Still, to be better preserved, it should be encased in biomineral crystalline structures, where it can be isolated from decaying chemical reactions.

Carbohydrates consist of monosaccharides and polysaccharides. Monosaccharides are the more simpler of the two compounds, which makes polysaccharides the more clarnopollencomplex. Polysaccharides include starch and cellulose, found mainly in plants. Other carbohydrates include glycogen, chitin, and other simple sugars (which can be found in all organisms). They, too, are more chemically resistant to decay than nucleic acids. However, they are also more prone to other organisms taking them up for use (specifically by microorganisms), causing biological decay, or "recycling" back into the ecosphere. They, like proteins, are easily degradable to water soluble components. Most carbohydrates are from the Cenozoic, though there have been evidence for carbohydrates dating from the Cambrian and Precambrian.

Lipids are the most resistant to decay of the four main groups of organic molecules. They are also highly insoluble in water. All cells produce this type of organic compound to be used in their membranes and in energy storage. These type of organic molecules are found in kerogens (explained below).

What are the conditions for molecular fossilization ?

Since all these molecules are susceptible to some degree of decay, they would be best be preserved in very isolated areas. Different molecules have different requirements for preservation, but almost all would welcome as little contact with air and microorganisms. Temperature and oxygen levels are also factors. The best places to possibly find these type of fossils are (in increasing order of abundance) fossil shells and bones, organic-rich muds and shales, coal, and kerogens. Kerogens are described as "poorly characterized, highly heterogeneous, amorphous, insoluble polymeric material found in the geological record". Kerogens, in fact, carry just over 80% of the organic molecules found in the fossil record.

What can we learn from molecular fossils ?

Since very little is still known about the molecular fossil record, very little can be told from them. Mainly, some evolutionary relationships can be cleared by molecular analysis of fossils. However, contamination of specimens are still a large problem in today's analyses. Contamination can come from just touching the sample (finger grease), the organic material of their containers (especially plastic), airborne microorganisms, and chemical reactions. In fact, contamination could be as extreme as initial exposure to the air. Thus, special precautions are imperative to molecular fossil research, but again, as technology advances, more will be learned from these organic remains.


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