Many bacteria and all viruses possess less complexity than required for an organism normally defined as “living,” and for this reason must live as parasites which require the existence of complex cells in order to reproduce. For this reason Trefil noted that the question of where viruses come from is an “enduring mystery” in evolution. Viruses usually are much smaller than parasitic bacteria and are not considered alive because they must rely on their host even more than bacteria do. Viruses consist primarily of a coat of proteins surrounding DNA or RNA that contains a handful of genes, and
since they do not reproduce in the normal way, its hard to see how they could have gotten started. One theory: they are parasites who, over a long period of time, have lost the ability to reproduce independently… Viruses are among the smallest of “living” things. A typical virus, like the one that causes ordinary influenza, may be no more than a thousand atoms across. This is in comparison with cells which may be hundreds or even thousands of times that size. Its small size is one reason that it is so easy for a virus to spread from one host to another--its hard to filter out anything that small (Trefil, 1992, p. 91).
In order to reproduce, a virus's genes must invade a living cell and take control of its much larger DNA. A bacterium is 400 times greater in size than the smallest known virus, while a typical human cell averages 200 times larger than the smallest known bacterium. The QB virus is only 24 nanometers long, contains only 3 genes and is almost 20 times smaller than Escherichia coli, billions of which inhabit the human intestines. E. coli is 1,000 nanometers long compared to a typical human cell that is about 10,000 nanometers long (1 nanometer equals 1 billionth of a meter, or about 1/25-millionths of an inch) and contains an estimated 100,000 genes. Researchers have detected microbes in human and bovine blood that are only 2-millionths of an inch in diameter, but these organisms cannot live on their own because they need more than simple inorganic, or common inorganic molecules to survive.
Since parasites lack many of the genes (and other biological machinery) required to survive on their own, in order to grow and reproduce they must obtain the nutrients and other services they require from the organisms that serve as their hosts. Independent free-living creatures such as people, mice and roses are far more complex than organisms like parasites and viruses that are dependent on these complex free-living organisms. Abiogenesis theory requires that the first life forms consisted of free-living autotrophs (i.e. organisms that are able to manufacture their own food) since the complex life forms needed to sustain heterotrophs (organisms that cannot manufacture their own food) did not exist until later.
The simplest microorganisms, Chlamydia and Rickettsea, are the smallest living things known, but also are both parasites and thus too simple to be the first life. Only a few hundred atoms across, they are smaller than the largest virus and have about half as much DNA as do other species of bacteria. Although they are about as small as possible and still be living, these two forms of life still possess the millions of atomic parts necessary to carry out the biochemical functions required for life, yet they still are too simple to live on their own and thus must use the cellular machinery of a host in order to live (Trefil, 1992, p. 28). Many of the smaller bacteria are not free living, but are parasite like viruses that can live only with the help of more complex organisms (Galtier et al., 1999).
The gap between non-life and the simplest cell is illustrated by what is believed to be the organism with the smallest known genome of any free living organism Mycoplasma genitalium (Fraser et al., 1995). M. genitalium is 200 nanometers long and contains only 482 genes or over 0.5 million base pairs which compares to 4,253 genes for E. coli (about 4,720,000 nucleotide base pairs), with each gene producing an enormously complex protein machine (Fraser et al., 1995). M. genitalium also must live off other life because they are too simple to live on their own. They invade reproductive tract cells and live as parasites on organelles that are far larger and more complicated but which must first exist for the survival of parasitic organisms to be possible.
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