The Origin of Life
By Walter Bradley
Introduction
Antony Flew, a British philosophy professor and leading champion of atheism
for more than half a century, changed his mind and became a deist at the age of
81. In a telephone interview with ABC News (12/9/2004), Flew indicated that a
"super-intelligence is the only good explanation for the origin of life and the
complexity of nature." Nicholas Wade in the New York Times (6/13/2000)
summarized the current state of the affairs regarding the origin of life as
follows: "The chemistry of the first life is a nightmare to explain. No one has
yet developed a plausible explanation to show how the earliest chemicals of
life—thought to be RNA—might have constructed themselves from the inorganic
chemicals likely to have been around on early earth. The spontaneous assembly
of a small RNA molecule on the primitive earth 'would have been a near miracle'
two experts on the subject helpfully declared last year." What is it about the
origin of life that has so confounded scientists and persuaded atheists to
become deists or theist? Why is the origin of life considered one of the
"great, unsolved mysteries of science" (Discover 1993)?
The minimal functional requirements for a living system include processing
energy, storing information, and replicating. Lila Gatlin captures the essence
of the problem by noting that life may be defined operationally as an
informational processing system that has the ability to store and process
information that is essential for its own reproduction. These biological
operations are made possible by very complex molecules such as DNA, RNA, and
protein. In this essay, I would like explore the "miracle of the origin of
life" by providing on overview of the molecular complexity that is essential to
life and by indicating why it is so difficult for unguided natural laws,
sometimes characterized as chance and necessity, to ever adequately account for
the origin of these remarkable molecules of life.
Information and the Molecules of Life
Protein, RNA, and DNA are all long polymer chains. The "mer" in "polymer"
means building block and "poly" means many. The protein molecule is a polymer
typically composed of 100 to 300 smaller molecular building blocks (or mers)
called amino acids. There are 20 distinct types of amino acid building blocks
in protein, with five shown schematically in Figure 1. These amino acids
chemically react to form long polymer chains, which subsequently folds up into
a three dimensional structures, as seen in Figure 2. It is this distinctive
structure that allows various proteins to serve as catalysts, making chemical
reactions in living systems go a million times more rapidly.
Figure 1. Five schematics of various amino acids
Figure 2. A polymer chain folded into a 3-D protein
structure
The sequencing of the twenty different kinds of amino acids is what
determines the three dimensional structure. Only a very, very small fraction of
the possible sequences of amino acids give three dimensional structures that
have any biological utility. In fact it has been predicted theoretically and
confirmed experimentally that the probability of getting the correct sequence
of amino acids for a protein such as cytochrome C is approximately 1 in 1060.
How then are proteins ever successfully assembled from amino acids in living
cells?
The DNA and RNA molecules are the key to getting the remarkable sequences of
amino acids in proteins that provide critical biological functions in living
cells. The DNA is encoded with information that can be used to sequence the
amino acids in various proteins for a given organism. The m-RNA molecule
receives this encoded information from the DNA and then serves as a template to
get exactly the right sequencing of amino acids to give over 300 distinct
functional proteins. We may think of the DNA as the "computer brain" for each
cell, controlling the sequencing of amino acids in 300 or more distinctive
proteins, which in turn control the necessary chemistry of life in the
cell. To make a DNA molecule with the right encoded information for
E-coli bacteria would require 4,600,000 instructions for the chemist, or the
equivalent of 800 pages of information. So while this solves the problem of the
origin of the necessary information to sequence (or encode) various proteins,
it does not solve the mystery of the origin of this huge amount of information,
but merely transfers it back to the DNA (or possibly RNA in the first living
system). The origin of the large amount of information in DNA that is expressed
in the amazing molecular complexity essential for life is the central enigma of
the origin of life.
Making DNA, RNA and Protein under Prebiotic Conditions
DNA molecules reproduce themselves (with the help of proteins) and, assisted
by RNA, encode the various amino acid sequences in proteins that make
possible the efficient uses of energy in living systems. Thus, DNA, RNA and
protein provide the necessary functions of life: namely, information storage,
replication, and efficient utilization of energy. But how were the first DNA,
RNA and protein produced? Origin of life research for more than 50 years has
tried to answer this question? What have we learned?
Origin of life research began in the 1950s with the attempt to chemically
synthesize the basic molecular building blocks for protein and DNA, including
various amino acids, bases, and sugars. The early success of Miller and Urey in
making these molecular building blocks, ostensibly under early earth
conditions, was seriously undercut in the 1980s when it was determined that the
early earth's atmosphere was never rich in methane, ammonia. or hydrogen, the
chemical gases used in their experiments. One cannot produce more than
minuscule yields of amino acids and ribose sugar when one uses a plausible
prebiotic chemistry. Today the origin of these essential building blocks of
life remains a mystery.
A second problem is that the building blocks on the prebiotic earth would
have been surrounded by many other chemical reagents that react with the
building blocks much more quickly than they react with each other. Unless such
destructive cross reactions could somehow be avoided, the emergence of DNA, RNA
or protein would be impossible.
A third problem is the assembly of the building blocks into the polymer
chains. For example, amino acids can be joined (in chemical reactions) in a
variety of ways, but only one type of joining of adjacent amino acid molecules
(i.e., chemical bonds called peptide) give a polymer chain that has the
function of a protein, as seen in Figure 3. In a similar way, 3-5
phosphodiester linkages are needed but 2-5 linkages dominate in the
polymerization of polynucleotides, which is a primary step in the formation of
DNA and RNA.
Figure 3. Peptide bonds forming to join amino acids in a polymer
chain
A fourth challenge results from the fact that amino acids and sugars come in
right-handed or left-handed versions (structures that are identical except that
they are mirror images, as seen in Figure 4). All amino acids chemically react
equally with each version equally rapidly, but living systems have only L-amino
acids and D-sugars. How could we possibly get 100 or more amino acids that are
all Ls from a mixture of equal concentrations of Ls and Ds? This problem has
been studied extensively but the explanation remains elusive.
Figure 4. Left-handed and right-handed amino acids - mirror
images
Beyond the problems of producing the building blocks under plausible
prebiotic conditions, avoiding fatal cross chemical reactions and getting the
building blocks assembled, getting on L amino acids or D sugars, the most
challenging problem in the origin of life scenario is how to get the correct
sequencing of amino acids in proteins and the correct sequencing of bases in
DNA to give information that can provide biological function. As previously
noted, the information encoded on the DNA of E-coli bacteria is the equivalent
of 800 pages of information. While it is sometime argued that this can happen
with some kind of chemical selection over time, no selection is possible on
molecular systems that do not yet have the capacity to replicate with
occasional mistakes and provide functions that give selective advantage.
Functional DNA, RNA or protein might be able to incrementally improve with
replication mistakes acted upon by selection, but this is meaningless in
molecules that are not yet sufficiently complex as to provide at least minimal
function. It is the molecular version of the old problem of which came first,
the chicken or the egg.
Summary
Michael Behe has argued that there are irreducibly complex hurdles that an
evolutionary process driven by natural selection cannot overcome —for
example, the concurrent development of a multiple component system that
provides no selective advantage until each of the components has developed to a
rather advanced level and can function together as a system. The origin
of life would seem to be the quiescent example of an irreducibly complex hurdle
in the meta-narrative of the origin and development of living systems. The
necessary information, which expresses itself as molecular complexity, simply
cannot be developed by chance and necessity but requires an intelligent cause,
an intelligent designer, a Creator God.
BIOSKETCH: Walter Bradley, formerly Professor and Head of
the Department of Mechanical Engineering at Texas A&M University, is
Distinguished Professor of Engineering at Baylor University. He received his
Ph.D. in Material Science from the University of Texas (Austin). In addition to
publishing over 150 technical articles in refereed journals and conference
proceedings on material science and engineering, he has co-authored several
seminal works on the origin of life, including an article in Debating
Design: From Darwin to DNA (edited by William Dembski and Michael Ruse)
and the book The Mystery of Life's Origin, originally published by
Philosophical Library. To this day, Mystery remains the best-selling
advanced level text on the origin of life.