Another paper I wrote back in 2001:
What does it mean for something to be alive? The question has haunted everyone from ancient philosophers to modern scientists. Most people would consider a bacterium to be alive, while at the same time do not consider a rock to be alive. As the organisms become smaller, the distinction blurs. A virus is a piece of nucleic acid inside of a protein coat. Is a virus alive? Even smaller and simpler than viruses are prions, abnormally folded proteins that catalyze the misfolding of similar peptides. Prions are the cause of bovine spongiform encepalopathy (Mad Cow Disease) and its human analog Croitzfelt-Jacobson disease. They are only single molecules, yet they replicate and spread with enough force to strike fear in the heart of hamburger eaters worldwide.
Although there is much confusion and blurring of living and non-living matter, a thing may generally be said to be alive if it has all of the following characteristics:
Localized Zones of Low Entropy- The entropy of a closed system increases in all processes, but a region of low entropy (high order) can be created by increasing disorder in the surroundings. Living beings have a definite border and take in energy to increase the order of molecules within this border.
Metabolism- Living beings utilize metabolic processes that take in energy in various forms (chemical, solar, etc.) and use that energy to create the entropy gradient. They may also excrete unusable substances.
Ability to Replicate Autonomously– Although an individual organism need not be able to replicate autonomously to be considered alive, members of the species must be able to do so. Although it can steal energy or molecules from other organisms, the replication must occur by the metabolism of the parent or daughter.
Internalized Information Storage System- The organism must contain some system for storing the information necessary to carry out the above tasks and there must be some way for the information to be transferred to the child organism after replication.
Although viruses and prions contain localized low entropy zones with a definite border, internalized information storage systems, and the ability to replicate, they fail in two critical ways. They have no metabolic processes and their replication is not autonomous- they require the replication machinery of other cells to make copies of themselves. Is the earth a single living organism? Unfortunately, the earth cannot autonomously make copies of itself, so it would not of itself be considered to be living under this definition. Crystals act as a template to induce the formation of more crystals, and contain definitive low entropy zones, but they cannot be considered alive because they have no metabolism.
There are many characteristics of life that some lifeforms have that others do not. The organisms can be made up of many smaller units (multi-cellular), have consciousness or intelligence, or even have the ability to alter their own internalized information storage system. However, none of these characteristics are necessary to posses for an organism to be classified as alive.
Humans will create new forms of life in the new future- in fact we are already doing this in a certain sense through evolutionary pressure. When patients do no take all of their antibiotics, we contribute to the formation of new strains of drug resistant microbes. In the same way our destruction of habitat introduces pressures on organisms to evolve into different species. However, we are not technically creating (from scratch) any new life forms. We have deciphered the basic chemistry of life however, and at the same time we can synthesize ever-longer pieces of nucleic acid in the laboratory. Bacteria have been found with as few as 5000 genes, so the number required for life has to be less than that. As our knowledge of bioinformatics increases we will be able to “create” larger life forms by creating their DNA and then injecting that DNA into cells of similar creatures and letting them develop inside of a surrogate mother. This raises many ethical questions, however. Another way that humans might create other forms of life is mechanical and electrical- computer systems will soon begin to rival the processing power of larger and more complex creatures, overtaking the human brain (at least in raw information storage and processing ability) sometime in the next century. Whether they will be able to outsmart humans or ever be self-aware is another question entirely.
The main problem with our analysis of life is that we have only one example- life on earth. We really do not know if life exists anywhere else or what it will be like. However, we can be reasonably sure that it will have to follow the same laws of physics that we follow. If we find life elsewhere in the solar system, we can also assume that it must conform to the characteristics listed above. Since NMR spectra reveal a plethora of organic compounds that are common in earth life and that form naturally in space, we can guess that extra-terrestrial life will be somewhat similar to earth life chemically. It will probably have DNA or RNA as an information storage molecule, and it will probably use some chains of peptides to carry out catalytic and structural functions. The “bacteria” may even be shaped somewhat like earth bacteria. However, macroscopically the organisms will probably be quite different. If the chemistry of other worlds uses other solvents besides water, however, the chemistry is likely to be very different. The same thing would be true of non-carbon based life forms, which might use silicon or have no central base element.
Whether or not we will ever find life that, like us, can ponder these questions is unknown, but if we do then they will provide a wealth of information to help us answer many of them.