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It can be noticed that this concept is modern, having been developed after the invention of the microscope and the discovery of the genetic code.

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To reach that point, many modifications took place throughout history, as will be seen later. It is in that specific context that life will be approached in this essay. But in a panorama with so many possibilities, why will we limit ourselves to discuss only the common kind of life that we know? Are there other kinds, here or in other planets? How can we recognize them? The restriction is not because we deny the possibility of the existence of other life paradigms, but because that is the only one that makes a scientific approach possible, because it presents observable data and theoretical models.

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Countless attempts to formulate a general concept of life were and are still being made, but none of them presented significant advantages to understand the life we know, nor did it predict the existence of still unknown forms that can be observed. A General Theory of Life has not been developed yet and that limits our capacity to understand it.

History of cell membrane theory

Only the discovery of other life specimens independent from the one we know on Earth could lead us to broaden the conceptual horizons. In the final item, we will show projects that aim at discovering life outside the Earth and we will discuss its feasibility and its potential contribution to the understanding of life in the scientific realm.

Evolution is the process of change in organisms throughout time, in such a way that those that exist today are different from the initial ones. Although there is a continuity chain throughout time, it is hard to infer the properties of the first organisms based on the current ones. All the genetic phyla body architectures existent today appeared in the so-called "Cambrian explosion" that occurred around that time.

It is characterized by the appearance of the multicellular organisms. In the Pre-Cambrian geological era older than M. Fossils of microorganisms were traced back to a past as distant as 3. They are found in rocky aggregates that are still inhabited by bacterial colonies, the so-called stromatolites, 1 such as those from Apex, a formation in Western Australia. They present eleven different kinds of fossils, showing how the cells divided and multiplied themselves although there is some disagreement whether they are real fossils.

Their shapes are not distinguishable from the current photosynthetic algae cyanobacteria that infest several environments on Earth. Even being primitive for current life, those fossils are from organisms so complex that they cannot have been the first life forms.

Microbiologists and molecular biologists claim that cyanobacteria were one of the last great bacteria groups to appear. How can we take our studies even further back in time? It is very hard to find rocks that are older than 3. The rocks from the surface are forced to plunge by the plate tectonics, and they are cooked under pressure deep beneath the surface. The older a rock, the rarer it is. Therefore, there is no hope of finding fossils much older than 3. Biological evolution is a surprising and unexpected fact when we have in mind that the genetic code works to make an exact copy of itself.


The double helix structure is an extra guarantee of fidelity, providing two copies of each genetic information. If there were only forces that maintain the identity, biological diversity would not exist. However, there are processes that lead to imperfections in reproduction. Those are random "mistakes", natural in any copying process due to environmental radioactivity, cosmic rays coming from space or chemical agents.

They generate copy-molecules that are different from the original ones in such a way that, when the molecule participates in the reproduction, the resultant organism will present in general small differences from its ancestor. If it adapts to the environmental conditions, it will survive and might leave descendants, increasing the biological diversity. There is no trend to produce more complex or more "perfect" organisms, as many people believe.

The most complex ones do not seem to be more advantageous from the survival point of view as compared to simpler ones. If that was true, there would be far more complex organisms than simple ones, unlike what can be observed in nature. That idea of evolution as improvement is preached by the creationists, according to whom nature follows an intelligent design. They apply it not only to the biological evolution, but also to all natural phenomena.

It is simply translated in the belief that the natural forces would not be able to create "order" and "beauty" if they were not guided by an intelligence that is external to matter itself. Adopting that point of view means giving science the role of merely finding out what that plan subjacent to nature is, which has already been predetermined forever. Such determinism was abandoned by Quantum Mechanics almost a century ago.

Behind its huge variety of shapes, colors and sizes, the current organisms have very similar characteristics that can be used as important parameters to understand their origin. All organisms have a high percentage of water, which favors the hypothesis that their origin occurred in an aqueous environment. Its atomic composition is also admirably simple. They are among the five most abundant in the Universe, leaving out only helium, which does not form chemical bonds. The biochemistry of life is made up of combinations of those atoms, forming water H 2 O , methane CH 4 , ammonia NH 3 , carbon dioxide CO 2 , sugars, proteins, fatty acids and others.

Although many proteins have metallic elements and require certain ions to operate, the most abundant elements are, by far, the ones mentioned above. The fact that life is made up of the most widely found atoms in nature indicates that it is simply an expression of the opportunity and not an exceptionality, a miracle, that could be done with arbitrary materials, including rare ones. Although bacteria, whales, palm trees and elephants are so different from one another in shape, chemically speaking they are extremely similar.

The nucleotides and aminoacids used by the living organisms are few and almost the same. The joining of those monomers in large chains constitutes the biopolymers: the nucleic acids RNA and DNA and proteins. They are responsible for the observable biological diversity. In the current living organisms, DNA carries the code to assemble proteins that are responsible for several functions.

Besides the material composition, the form of energy processing metabolism is also very similar in all living organisms, occurring by means of a few intimately related processes.

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Figure 4 indicates that ALL living organisms are relatives and have a common origin. A powerful way to diagnose the relationship among the living organisms is through genomic analysis 3 of the 16S ribosomal RNA, which cannot be applied to fossils, since they have lost their cellular content.

Danielli, J. F. (James Frederic) [WorldCat Identities]

The ribosomes are molecular complexes within the cells that participate in the production of proteins. Those protein factories are composed of many kinds of ribonucleic acid RNA. Organisms belonging to a biological group that share a recent history have similar RNAr, and the more distant their relationship is, the more they differ. The comparison of 16S RNAr between two groups originated from a common evolutional branch allows us to assess how many changes took place since separation.

Therefore, we can build a tree in which the length of the branch is proportional to the number of changes occurred, called the universal phylogenetic tree.

Membrane Research: Classic Origins and Current Concepts

One can notice that, although plants and animals are the most familiar forms of life for us, they make up only two of the twenty branches of the tree of life. Besides those two, only fungi have members observable without a microscope. Most of life is invisible to the naked eye. All known organisms belong to one of the three domains: B acteria or Eubacteria , Archaea or Archaebacteria and E ukarya or Eukaryotes.

All branches connect to a single one in a region somewhere between Bacteria and Archea. That branch was supposedly the one of the last common ancestor called progenote. The phylogenetic tree is in conformity with the study of the fossils, which demonstrates that the oldest ones belonged to Bacteria and Archea domains, and the most recent ones belonged to Eucarya domain.

It could be read as a time sequence, in which the present time would be in the tip of the branches and the past towards the connection with another branch.

However, the division times of the branches cannot be measured accurately, since the mutation rates are not constant in time.