Created: September 9, 2015
Revised:Synopsis Sept 2017  

The Creation Narrative of Science and the Bible
 

Dr. David C. Bossard

Dr. David C. Bossard
Biographical Information


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.

genome 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.

Protein Inventory for E. Coli and M. genitalium*

 
Functional Class of Genes
# of Genes 

E. coli
M. genitalium 
Regulation 178 7
Structure
237 17
Phages and other inserted elements
87 0
Transportation and binding 427 34
Energy Metabolism
243 31
DNA Replication, etc. 115 32
Transcription 55 12
Translation 191 101
Intermediary metabolism 658 37
Other cell processes 188 21
Other enzymes and identifiable genes 277 27
Unknown 1,632 152
Total 4,288 471
* Size Limits, p.18


[*fn]M.20 The report of this  symposium was published by the National Academy of Sciences in 2000, titled Size Limits of Very Small Microorganisms.

[*fn]M.21 The base pair (bp) count i s a rough measure of the number of genes required for the species, the average gene requiring about 1000 bp.

[*fn]M.22 See The Minimal Genome Project for a list of the smallest bacteria. In 2002, a smaller bacterium was discovered, but it too is incapable of independent existence. Note from the figure that the genome size only weakly correlates with the complexity of the species: for example, the human DNA is only of middling size when compared with other mammals, and many plants have dna that is orders of magnitude larger than the human DNA. See Claire M. Fraser et. al. The Minimal Gene Complement of Mycoplasma genitalium,  Science Vol. 270, 20 October 1995.

[*fn]M.23 An autotroph is a plant that gets its nourishment from inorganic sources. However, no plant is a strict autotroph in this sense, because no eukaryote is able to fix nitrogen. The only reliable source of fixed nitrogen (until the 20th century discovery of the Haber process) is organic waste, derived from the bodies or waste products of earlier life.

[*fn]M.24 Ibid, pp 77-78. Among modern bacteria on earth, the smallest known autotroph requires over 1 million bp.

M.25 [*fn]M.25 Table from Size Limits, op. cit., Peter B. Moore, "A Biophysical Chemist's Thoughts on Cell Size." For current sizes of these genomes see Wikipedia or The Minimum Genome Project (op. cit.)

M.26 [*fn]M.26 The question of regulation involves much more than a count of the regulatory genes. In sexual reproduction the egg carries with it an ongoing regulatory environment derived from the mother. This is why, for example, the mitochondrial DNA comes from the mother: it is passed on from the contents of the egg, and not from the sexually-formed DNA. At the moment the egg is fertilized, this regulatory environment has already determined the initial expression of the embryonic DNA. This is the miracle implied in the Biblical phrase "according to his kind".

M.27 [*fn]M.27 Lord Alfred Russel Wallace (8 January 1823 – 7 November 1913), one of Charles Darwin's early and enthusiastic collaborators, wrote the seminal book, Man's Place in the Universe (1903) on this subject. He followed this in 1911 by The World of Life: a manifestation of creative power, directive mind and ultimate purpose. In 1913 Lawrence J. Henderson (June 3, 1878, Lynn, Massachusetts – February 10, 1942 wrote The Fitness of the Environment: An inquiry into the Biological Significance of the Properties of Matter. A recent follow-on to Henderson's book is A. E. Needham, The Uniqueness of Biological Materials (1965), which has extensive discussions of the most essential elements and molecules required for life. A more recent update to Henderson's book is Michael J. Denton, Nature’s Destiny, op. cit.

M.28 [*fn]M.28 Note for M.28

M.29 [*fn]M.29 Note for M.29


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