Size Limits of Very Small Microorganisms:
This concerns the Minimum Size of the "simplest" life of any sort
A 1998 symposium held by the National Academy of SciencesM.20 asked the question: how small can a living cell be—any sort of life anywhere in the universe, not just life on earth? The approach used was to estimate the smallest possible number of genes needed to carry on the minimum cell activities, and then go on from that estimate to determine the smallest physical cell size that could package it.
The answer is: not simple at all; in fact the simplest conceivable life form is itself very complex—not just "life as we know it" but any sort of "alien" life.
This is relevant to the very first living species on Earth, because it addresses the question of how "simple" can that life be? The issue that precipitated the symposium was the NASA claim to have found "Martian fossils". Presumably these "fossils" would represent life forms that could exist under conditions similar to the first living species on Earth.
Size of Existing Species on Earth. The following figure shows the number of DNA base pairsM.21 and physical size of the various living species.
Viruses have very small genomes but they are not capable of independent living (they depend on a host cell to provide metabolism) so the smallest size would be that of the smallest bacterium, which (at the time of the symposium) was Mycoplasma genitalium, a small bacterium of the urinary tract, with a DNA of about 580,000 base pairs encoding 520 genesM.22. However genitalium is not capable of independent existence (as the first living species must be) because it relies on food supplied by its environment. But the symposium concluded that this was close to the minimum possible size.
Still the question arises: how small could the DNA of a living species possibly be, and still be able to metabolize and reproduce? Perhaps all species today are much larger than the minimum size possible. The following table compares species according to physical size. Note the size of the Mars "fossils".
The consensus of the symposium was that "Free-living organisms require a minimum of 250 to 450 proteins along with the genes and ribosomes necessary for their synthesis. A sphere capable of holding this minimal molecular complement would be 250 to 300 nm in diameter." This is far larger than the alleged Martian fossils.
A statement of the minimum genome size varied among the participants. One participant suggested 320,000 bp coding for 256 proteins (p.43), but without asserting that this size could be free-living. A "cell that synthesizes all of its cellular material from CO2 requires... closer to 750 genes." For comparison the symposium estimated that the smallest actual modern autotrophM.23 has about 1500 genesM.24. Using 1000 bp as the size of an average gene, the minimum genome size for an autotrophy must be at least 750,000 bp. Such a bacterium must include DNA coding to manufacture the nucleotides and amino acids, because these building-blocks of life do not occur naturally in significant amounts. Even this size assumes the availability of fixed nitrogen.
The conclusion has great implications about the basic complexity of the first living species. In short, it is so complex, that the likelihood of its arrival by undirected natural processes is vanishingly small.
The Essential Chemicals of Life. Could some other radically different forms of life exist somewhere in the vast universe? Several prominent authors have asked this question. The answer, in short, is "No!" All conceivable life must be based on liquid water, carbon chains, amino acids -- all of which are built up of the essential elements H, C, O, and N. Thus the conclusions of the symposium appear to apply to any living matter, anywhere in the universe.