Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization
© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm
7.6 Early Biological Systems
Thus far we have concentrated on the parallel development of polymeric amino acids (proteins) and polymeric nucleotides (DNA). We’ve seen that Dr. Sidney Fox’s proteinoid microspheres exhibit many properties which are strikingly similar to those displayed by contemporary living cells. We’ve also seen that Dr. Leslie Orgel has succeeded in demonstrating accurate, if erratic, replication in primitive polynucleotides. And yet, despite these remarkable achievements, the great final question remains untackled: How and when did the first living organism arise?*
The answer is as unsatisfying as it is precise: No one knows. The arguments on this score smack of the "chicken-or-the-egg" controversy. It is unknown at present if proteins and protocells came first, to be followed later by replicative nucleic acids, or whether the nucleic acids were first, and from them the cells later spawned.
It has been fairly clearly demonstrated that life as we know it could not have arisen if either one or the other was wholly absent.521 Organisms lacking nucleic acids would have no means of achieving genetic continuity and evolutionary progression, while organisms without proteins would find themselves severely limited in their ability to utilize the chemicals in their environment. Some manner of coevolution seems to be indicated.
One theory holds that nucleic acids evolved some kind of boundary layer, a proteinous skin to protect themselves from their surroundings -- the so-called "naked gene" theory. When this invention inhibited or prevented reproduction, the parent nucleic acid molecule became extinct. When the new boundary layer served to protect the DNA without interfering with replication, these were the "protobionts" which survived.
There is some experimental evidence to support the view that polynucleotides might be able to influence protein synthesis directly.1431,2255 To do this, they must cause a selective linear organization of amino acids, and must facilitate amino acid polymerization.1444 Unfortunately, other studies have shown that the interaction between polynucleotides with individual (monomeric) amino acids is relatively weak.1248
More convincing, perhaps, is the idea that cells were first. Self-assembly in molecular structures has been known for many decades, and experimental evidence to date favors the easy synthesis of proteins in comparison to polynucleotides.1634 Sidney Fox has remarked that the sequence:
protoprotein —> protocell —> nucleic-acid-coded contemporary cell
is the most valid evolutionary sequence because it proceeds from the simple to the more complex.1625
The primitive protocell, as modeled by the proteinoid microspheres, could have exhibited many of the properties customarily regarded as belonging only to "living" things. Under Fox’s theory, the cell would have developed nucleic acids to serve its ends, rather than the other way around.
One final piece of evidence seems to argue for the primacy of cells. In 1974, Dr. Fox and his colleagues published some experimental findings on micro-spheres which seem to imply that the proteinoid protocell can do everything optimistically predicted for it. The abstract of the paper reads, in part:
Proteinoid microspheres of appropriate sorts promote the conversion of ATP to adenine dinucleotide and adenine trinucleotide. When viewed in a context with the origin and properties of proteinoid microspheres, these results model the origin from a protocell of a more contemporary type of cell able to synthesize its own polyamino acids and polynucleotides.1435 (emphasis added)
We’ve seen that scientists have discovered a relatively smooth chain of synthesis from the stuff of stars to the stuff of life. On the basis of pre biotic experiments performed to date, it is probable that most of the organic molecules of life with a molecular weight less than 1000 spontaneously appeared in significant quantities during the early years of our world. While a number of problems remain, most indications are that the origin and development of life on Earth had a certain inevitability about it.
From the simplest compounds present when our planet first congealed about 4.6 eons ago, to the first viable protobiont some half a billion years later, the patterns of development and the upward march of complexity seem unavoidable. Only the most general conditions must be needed for carbon-based life to arise: A body of water, a primitive reducing atmosphere, some source of energy, and lots of time. Life, in Soviet Academician Oparin’s own words, is "an obligatory result of the general growth of the universe."2297
Even now we humans just begin to suspect the truth: The universe is not ours alone to keep.
* There are countless side issues that cry out to be discussed at this point, but which unfortunately can be given only a passing nod. First of all, there is the absolutely fascinating question of the genetic code. As is well-known, genetic information is written on the DNA strand in short, three-nucleotide "words" called codons. By properly reading these encoded blueprints, a cell can construct exactly the right protein molecules.
For instance, a series of three adenine nucleotides in a codon tells the cellular machinery to use one molecule of an amino acid called lysine at that location. Three guanines in a row means that a molecule of the amino acid glycine should be used. One by one, the codons tell which amino acid to use and in what order, and proteins are built up in precisely the right way.
What is the origin of this marvelous code?1064,1444,2383 Is a three-nucleotide codon somehow optimal,1777 or would four have been more evolutionarily efficient?1064,1066 Why not the simplicity of only two? And what determined the rules of the coding itself? Three guanines mean glycine to a virus, a dandelion, or a human. Is the code somehow efficiency-maximized or error-minimized?1065 (It appears to be!1066,2378)
What is the origin of chromosomes,2301 and the true purpose of genes?2322 These are important questions for xenobiologists to be asking, because the universality of our genetic mechanisms will determine the limits of variation that can be expected in alien biochemistries.
For xenologists, of course, there are far more fundamental issues that must be raised. For example, why must genetic information be stored digitally in a linear sequence of monomer units? Could not some form of analog system serve? What of the possibility of genetic systems whose information was stored, replicated and transcribed in a planar fashion rather than linearly?1777
Dr. Francis Crick has pointed out that on Earth, DNA is used for "replication" and proteins are used solely for "expression" or "action." Is it possible, he asks, "to devise a system in which one molecule does both jobs, or are there perhaps strong arguments, from systems analysis, which might suggest that to divide the job into two gives a great advantage?"22 Others have echoed this idea.521
Similarly, Michael Arbib of the University of Massachusetts at Amherst questions "whether it is necessary for any lifeform to have a genotype distinct from a phenotype; in other words, whether we have to have a program to direct growth and change, or whether in fact the organism might be able to reproduce itself as a whole."85 Crick seems to agree,, suggesting that it might be possible to "design a system which was based on the inheritance of acquired characteristics."22 (At least one science fiction story has been written along these lines.2216) Arbib also wonders: "One might imagine some planet whose beings reproduce by xerography with no gene required!"85 The possibility of inheritance without genes has been suggested before,1178 although in a different context. (A general review of replication was published in 2004 by Freitas amd Merkle.)
And we must take care not to be guilty of "nucleic acid chauvinism." We are familiar with only one molecular replicating system, but there is no reason why others should not be possible. Gordon Allen writes: "Life on other planets need not be based on nucleic acids or proteins if their catalytic functions can be otherwise provided."1591
Dr. Alexander Rich at MIT also suspects that the functions of Earthly nucleic acid are not unique. Rich believes that "other molecules could be used to form other polymers which could be used as information carriers for living systems." Later, he elaborates:
I think it would be amusing to make a chemical system of complementary polymers based on monomers that are not nucleic acid derivatives, simply to demonstrate that it can be done. In about ten years’ time, I think we will have a well-developed field of synthetic polymeric information carriers that will give us a great deal of insight into our own terrestrial system. That another system is possible might have relevance, if not to biology on this planet then perhaps to another.1587,1632
Clearly, a search should be made for non-nucleic acid self-replicating molecules. Exotic systems based on silicon, boron, or nitrogen-phosphorus chemistries are possible: Specialists in these fields expect an abundance of compounds comparable to that of carbon chemistry.1777 But we must not anticipate the subject matter of the next chapter.
Last updated on 6 December 2008