Life

(A) Homeostasis (stability and balance) – living things have to maintain a stable environment inside their bodies in order to function properly. For example, they must keep the concentration of salts and minerals within a certain range and the more complex animals must also keep their body temperature close to an ideal temperature. When these conditions vary the organism will experience thirst, fever, hunger or other sensations, and if the environment is not restored to its ideal range the organism will die. Homeostasis is based on the principle of stability and balance. Not too hot, not too cold, not too salty, not too watery, not too acidic, not too basic, etc. In order to maintain homeostasis the different parts (or organs) of the organism must communicate with each other and respond to the other parts. This communication between organs creates feedback loops and regulatory networks which are the basis for homeostasis.

(B) Organization (complexity and hierarchy) – living things are often called organisms because they are composed of smaller parts which are in turn composed of even smaller parts. Since living organisms perform many tasks, these tasks are assigned to specialized units. These units can be arranged into larger units which perform even more complex tasks. For example, animals are composed of organs, which are made up of tissues, which are made of cells, which are divided into compartments (the nucleus, cytoplasmic space and mitochondria) which are similar to the simpler bacterial cells, which contain organelles, which are made up of proteins and other complex molecules. This is based on the principles of complexity and hierarchy.

Crystals – they grow and consume minerals in solution.

Fire – grows, consumes fuel and reproduces.

Clouds – grow, absorbs energy and humidity from the environment (eats), and reproduce (clouds cause the formation of other clouds).

Computer viruses – they reproduce, move and consume energy and disc space (they eat) and a few even adapt.

However, these examples are not considered to be living organisms for the two following reasons. (A) First of all, these objects do not share all the traits of living organisms. Most living organisms posses all the defining traits listed above. (B) Secondly, living organisms do not just act like they are alive, they are alive. They contain life. The objects listed above might resemble living creatures in some ways but they do not posses life. A later section discusses a formal definition of life.

Some plants move very little – For example, large woody trees are basically fixed into a rigid position. However, even trees often have parts which move slowly, like their leaves and seed pods. Also, sap and nutrients move through their internal vascular structure. Furthermore, these trees had more motility when they were young and growing.

Some organisms cannot reproduce – Old and injured animals (like old people, for example) cannot reproduce, and some organisms are born without the ability to reproduce. This is true of unusual crosses like the mule. Mules cannot produce more mules. However, they themselves are the product of reproduction, they are produced by crossing a horse and a donkey. Furthermore, the cells inside these animals are constantly reproducing themselves and that is what keeps these animals alive.

Some organisms can exist in a state of suspended animation where they no longer have to eat – This is true of seeds, for example. However, one could say that seeds are not actually alive in the full sense of the word, but are only potentially alive. They become alive when the seed absorbs water and the tiny plant inside the seed begins to absorb nutrients and energy stored in the seed.

Many animals quit growing when they reach a pre-determined size – However, all these animals had a period of significant growth early in life. Furthermore, even many mature animals can still grow if they over eat, but this growth is unhealthy.

The traits which characterize living organisms are most strongly expressed in young organism. As an organism ages and approaches death the expression of the traits which characterize living organisms begins to diminish.

DNA and RNA are very similar except that the sugar in DNA is missing an oxygen atom and one of the nucleotides is different in RNA compared to DNA. Nevertheless, these small differences causes DNA to take the form of a stiff double stranded double helix while RNA is a flexible single stranded chain. This flexibility allows RNA to combine with protein to form complex nano-robotic multimolecular structures which are involved in transcription, translation and replication of genetic information. However, the DNA double helix is much more stable and easier to copy accurately than RNA.

1. Materialism – this is one of the oldest view of life and holds that life is produced by the proper mixture of elements. This is the view held by Empedocles.

2. Hylomorphism – this view was developed by Aristotle around 322 BC and holds that life is produced by the existence of a proper form or design. It is this form or structural design which produces life.

3. Vitalism – This view was developed in its modern form by Stahl in the 17th century. It held that life is in essence immaterial. Some popular descriptions of vitalism see life as an immaterial fluid, a force or an energy field. The force in the Star Wars double movie multi trilogy appears to be based loosely on the concept of vitalism. In the Star Trek series they have technology which is able to detect life signs. This also assumes that life is some sort of substance that can be detected by instruments. Even though these are works of fiction, it shows that many people tend to define life using a vitalistic point of view.

Most modern biologists would state that these ancient theories have been discredited, at least in their original form, although they each might hold an element of truth. Scientific progress in the last century has allowed us to understand more accurately the structure and function of the cell, the smallest unit of life. In spite of this, there is no general agreement about the definition of life. However, there is one definition which seems to explain many aspects of life.

A possible definition of life – Life has something to do with the integration of information and energy. In other words, energized information is an important, if not the essential, component of life.

Another definition of life – Life is the control of energy and matter by information.

This definition has the advantage of being based on two things that are measurable. We can measure the amount of energy an organism consumes, and we can measure the amount of information contained in the genetic material of the cells of the organism. It also implies a definition for living organisms.

A living organism is a physical object which integrates information and energy.

This allows us to recognize other forms of life which might not be chemical. For example, when can we consider a robot to be alive? If a robot is alive, is this life like that contained in a plant, in an animal or in a human being? This is important to determine because we confer very few rights to plants, a few basic rights to higher animals but recognize that humans have fundamental unalienable rights. This also leads us to the possibility tha organisms are alive at different levels. For example, the body of a person who is brain dead still has a measure of life. At the other end of life, at the beginning, is there a difference between the life contained by a fertilized human egg or blastula and a third trimester fetus? We will examine this briefly in the next section.

The Greek term for life is ‘bios.’ This is the level that uses DNA, RNA, ribosomes and other protein macromolecules to process information. This is the DNA-RNA-protein form of life.

Some organisms have a nervous system where specialized thread-like cells transmit information quickly using electrical signals. We call these organisms ‘animals’ and call non-animals ‘plants’, or ‘plant-like’ organisms. Animals are able to process and use information much more quickly and efficiently than plants. Furthermore, nerve based information systems have a great ability to acquire vast quantities of information from their environment fairly quickly, something which DNA-RNA-protein life is not able to do. Ancients called this ability to process and use information ‘anima’ and called the system which processes this information a ‘soul’.

There is one animal which is able to process and communicate abstract information using symbolic language. This animal is the human being, which, of course, includes you and me. We are not sure how or why we are able to process information this way. However, it distinguishes us from all other living physical organisms. We call the processing of abstract information ‘thought’ or ‘thinking’ and the ability to think is called ‘sentience.’ As far as we know, humans are the only physical living organisms that can think. Some people call whatever it is that allows us to think the ‘spirit’.

We saw that life is the control of matter and energy by information or the integration of information and energy by complex physical objects. In order for life to exist there must exist information, there must exist molecular machinery which allows this information to be stored and used and the informational blueprint for making, maintaining and running these complex molecules must be stored in that information system. We saw that the smallest organisms in existence today consist of a cell, DNA, RNA, ribosomes and a host of other complex protein molecules involved in controlling translation, transcription, replication, metabolism, and in maintaining the structure of the cell and homeostasis. All these things must be in place and functioning properly for life to exist.

Even if all we want the organism to do is to replicate there needs to be a complex system in place. Replication is dependent on translation because replication is performed by complex proteins. Therefore, there needs to be a system in place to perform translation and transcription in order to create these proteins. The information to create all these proteins must be stored in DNA or a precursor. In all known life forms the structure responsible for protein synthesis is the ribosome. The amount of information contained in the ribosome alone in modern bacteria is the equivalent of 30,331 base pairs. This is 11997 for the large subunit proteins, 9384 for small subunit proteins, 7470 in rRNA and 1480 in tRNA. The RNA polymerase, which produces messenger RNA from DNA templates, contains 6 protein subunits which have the information content of 12,333 base pairs. These are just two of many protein assemblages involved in the most basic functions of life. This does not include molecular machinery necessary for metabolism and homeostasis. How this complex system first came into existence is one of the fundamental questions of our reality.

Even though this view is held widely, it has many difficulties. The first difficulty is that the synthesis of the nucleic bases, especially the ones based on pyrimidine is quite difficult and the product is relatively unstable. For example, the nucleoside cytosine has a half-life in isolation of 19 days at 100 °C (212 °F) and 17,000 years in freezing water. Furthermore, each nucleotide base has two sites which are used for the phosphate bond. In order to form chains of RNA the hydroxyl (OH) oxygen at the third carbon must bind to the phosphate bound to the oxygen at the fifth carbon atom. This is called 3' to 5' linkage. However, solutions of nucleosides and activated phosphorus will just as easily form 5' to 5' and 3' to 3' linkages. Furthermore, linkages can occur in the exocyclic amino group of the two ring nucleic bases adenine, cytosine, and guanine. The ribose sugar in RNA also has a hydroxyl oxygen at the second carbon, 2', which can form a phosphate bond. This is one of the main differences with DNA because the sugar deoxyribose is missing that particular oxygen which eliminates one source of binding error. Consequently, the nucleotide has three or four binding sites but only two are used, and they must be used in the correct order. Furthermore, so far, the mechanisms postulated for the prebiotic formation of d-ribose produce a mixture of many sugars, including l-ribose and no mechanism for extracting pure d-ribose has been described. These are simply a few examples of the problems faced by the production of RNA by natural means. Many other steps are involved and most of them face significant problems.

Not only is it difficult to synthesize RNA, once formed it is also much more unstable than DNA because of the hydroxyl group at the 2' carbon. This presence of this oxygen has two consequences. First of all, it can bind to the phosphate bound to the oxygen at the 3' carbon. This causes the bond with the 5' carbon of the following nucleotide to be broken. This ruptures the chain at that point. Furthermore, the 2' hydroxyl group makes the RNA molecule much more flexible. This flexibility is useful in incorporating RNA into the complex protein structures found in the ribosomes. However, in solution this allows the strand to loop and when the two strands cross it brings free binding sites into close proximity to phosphate bonds allowing the chain to break and re-link at the crossing point. This obviously scrambles the information found in the chain. This makes RNA fairly unstable even in laboratory conditions. The reactivity which makes it a better enzyme than DNA also makes it much more unstable than DNA and basically unsuitable as a storage deposit for information.

Another problem with the RNA world is that even though RNA shares functions with both DNA and protein in that it can store information and also serve as a catalyst, it is not very good at either. We saw that RNA is not stable, and copying RNA is much more difficult than copying DNA because it does not form a stable double helix. A second RNA strand must be formed from the first and then that strand must be copied again. Furthermore, the catalytic properties of RNA are limited. The utility of RNA lies in serving as an intermediary in the transfer of information between DNA and proteins. This is the role which it plays in all known living organisms. An organism composed purely of RNA would be fragile and inefficient even in theory.

A final problem with the RNA world theory is that even with all the technology and information which we have at our disposal, nobody has ever been able to create an RNA organism which can replicate in a soup of nucleotides even under ideal controlled laboratory conditions. This is true even when we have a known solution to the problem of transcritption, translation and replication in the form of modern living organisms. If organisms composed of RNA were possible, one would think that we could reverse engineer them from what we have now. However, this has not occurred. If someone could create such a self replicating system it would go a long way towards showing that the RNA world is a likely precursor to the modern form of life.

Because of difficulties with the RNA world, some have postulated that other types of nucleic acid chains were the precursors to the present system. Peptide nucleic acid (PAN), threose nucleic acid (TNA) and glycol nucleic acid (GNA) have been artificially synthesized and have been suggested as possible precursors. However, their prebiotic synthesis faces many of the same objections as those which apply to the prebiotic synthesis of RNA.

Panspermia

Some have speculated that RNA was indeed the precursor but that the origin of life occurred on another planet that was more favorable to the origination of life than the early earth. Early life-forms in the form of bacteria or spores were then accidentally or deliberately transported to Earth. Accidental transportation may have taken the form of a large meteoritic impact which scattered bacteria containing debris throughout the galaxy to be deposited on Earth by meteorites. This theory is purely speculative since no alien life forms have been isolated from meteorites. Claims to have found bacteria inside meteorites have yielded slightly different strains of common terrestrial bacteria making it likely that the bacteria were contaminants from Earth.

The creation of artificial, simple, stable, self replicating chemical systems which are able to store information and reproduce would lend much weight to the natural formation of life. However, every year that passes without such a feat makes it more likely that a self replicating system simpler than the DNA-RNA-protein system which currently exists is simply not possible. The complexity of this system makes it highly unlikely that it arose by chance and points to a supernatural origin for the system. If the origin of life was the result of supernatural intervention this leaves open the possibility that other aspects of the story of life are also the result of the intervention of this supernatural intelligence.