Thanks for your post. I enclose the following interesting article:
September-October 1995
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The Beginnings of Life on Earth
by Christian de Duve
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Keywords
RNA world, thioester world, protometabolism, prebiotic evolution,
extraterrestrial molecules, origin of life
Abstract
The chemical evolution leading to cellular life on earth almost four billion
years ago likely passed through a stage where RNA alone performed all of the
functions of the modern macromolecules RNA, DNA and protein. However the
so-called RNA world was itself too complex to evolve directly from organic
molecules found on the prebiotic earth. More likely, the RNA world emerged
from and was supported by a primitive sort of metabolism fueled by the bonds
in sulfur-containing compounds called thioesters.
Text and Illustrations
Advanced forms of life existed on earth at least 3.55 billion years ago. In
rocks of that age, fossilized imprints have been found of bacteria that look
uncannily like cyanobacteria, the most highly evolved photosynthetic
organisms present in the world today. Carbon deposits enriched in the
lighter carbon-12 isotope over the heavier carbon-13 isotope--a sign of
biological carbon assimilation--attest to an even older age. On the other
hand, it is believed that our young planet, still in the throes of volcanic
eruptions and battered by falling comets and asteroids, remained
inhospitable to life for about half a billion years after its birth,
together with the rest of the solar system, some 4.55 billion years ago.
This leaves a window of perhaps 200-300 million years for the appearance of
life on earth.
This duration was once considered too short for the emergence of something
as complex as a living cell. Hence suggestions were made that germs of life
may have come to earth from outer space with cometary dust or even, as
proposed by Francis Crick of DNA double-helix fame, on a spaceship sent out
by some distant civilization. No evidence in support of these proposals has
yet been obtained. Meanwhile the reason for making them has largely
disappeared. It is now generally agreed that if life arose spontaneously by
natural processes--a necessary assumption if we wish to remain within the
realm of science--it must have arisen fairly quickly, more in a matter of
millennia or centuries, perhaps even less, than in millions of years. Even
if life came from elsewhere, we would still have to account for its first
development. Thus we might as well assume that life started on earth.
How this momentous event happened is still highly conjectural, though no
longer purely speculative. The clues come from the earth, from outer space,
from laboratory experiments, and, especially, from life itself. The history
of life on earth is written in the cells and molecules of existing
organisms. Thanks to the advances of cell biology, biochemistry and
molecular biology, scientists are becoming increasingly adept at reading the
text.
An important rule in this exercise is to reconstruct the earliest events in
life's history without assuming they proceeded with the benefit of
foresight. Every step must be accounted for in terms of antecedent and
concomitant events. Each must stand on its own and cannot be viewed as a
preparation for things to come. Any hint of teleology must be avoided.
Building Blocks
The early chemists invented the term "organic" chemistry to designate the
part of chemistry that deals with compounds made by living organisms. The
synthesis of urea by Friedrich Wöhler in 1828 is usually hailed as the first
proof that a special "vital force" is not needed for organic syntheses.
Lingering traces of a vitalistic mystique nevertheless long remained
associated with organic chemistry, seen as a special kind of life-dependent
chemistry that only human ingenuity could equate. The final demystification
of organic chemistry has been achieved by the exploration of outer space.
Spectroscopic analysis of incoming radiation has revealed that the cosmic
spaces are permeated by an extremely tenuous cloud of microscopic particles,
called interstellar dust, containing a variety of combinations of carbon,
hydrogen, oxygen, nitrogen and, sometimes, sulfur or silicon. These are
mostly highly reactive free radicals and small molecules that would hardly
remain intact under conditions on earth, but would interact to form more
stable, typical organic compounds, many of them similar to substances found
in living organisms. That such processes indeed take place is demonstrated
by the presence of amino acids and other biologically significant compounds
on celestial bodies--for example, the meteorite that fell in 1969 in
Murchison, Australia, Comet Halley (which could be analyzed during its
recent passage by means of instruments carried on a spacecraft), and
Saturn's satellite Titan, the seas of which are believed to be made of
hydrocarbons.
It is widely agreed that these compounds are not products of life, but form
spontaneously by banal chemical reactions. Organic chemistry is nothing but
carbon chemistry. It just happens to be enormously richer than the chemistry
of other elements--and thus able to support life--because of the unique
associative properties of the carbon atom. In all likelihood the first
building blocks of life arose as do all natural chemical
compounds--spontaneously, according to the rules of thermodynamics.
The first hints that this might be so came from the laboratory, before
evidence for it was found in space, through the historic experiments of
Stanley Miller, now recalled in science textbooks. In the early 1950s,
Miller was a graduate student in the University of Chicago laboratory of
Harold Urey, the discoverer of heavy hydrogen and an authority on planet
formation. He undertook experiments designed to find out how
lightning--reproduced by repeated electric discharges--might have affected
the primitive earth atmosphere, which Urey believed to be a mixture of
hydrogen, methane, ammonia and water vapor. The result exceeded Miller's
wildest hopes and propelled him instantly into the firmament of celebrities.
In just a few days, more than 15 percent of the methane carbon subjected to
electrical discharges in the laboratory had been converted to a variety of
amino acids, the building blocks of proteins, and other potential biological
constituents. Although the primitive atmosphere is no longer believed to be
as rich in hydrogen as once thought by Urey, the discovery that the
Murchison meteorite contains the same amino acids obtained by Miller, and
even in the same relative proportions, suggests strongly that his results
are relevant.
Miller's discovery has sparked the birth of a new chemical discipline,
abiotic chemistry, which aims to reproduce in the laboratory the chemical
events that initiated the emergence of life on earth some four billion years
ago. Besides amino acids and other organic acids, experiments in abiotic
chemistry have yielded sugars, as well as purine and pyrimidine bases, some
of which are components of the nucleic acids DNA and RNA, and other
biologically significant substances, although often under more contrived
conditions and in lower yields than one would expect for a prebiotic
process. How far in the direction of biochemical complexity the rough
processes studied by abiotic chemistry may lead is not yet clear. But it
seems very likely that the first building blocks of nascent life were
provided by amino acids and other small organic molecules such as are known
to form readily in the laboratory and on celestial bodies. To what extent
these substances arose on earth or were brought in by the falling comets and
asteroids that contributed to the final accretion of our planet is still
being debated.
The RNA World
Whatever the earliest events on the road to the first living cell, it is
clear that at some point some of the large biological molecules found in
modern cells must have emerged. Considerable debate in origin-of-life
studies has revolved around which of the fundamental macromolecules came
first--the original chicken-or-egg question.
The modern cell employs four major classes of biological molecules--nucleic
acids, proteins, carbohydrates and fats. The debate over the earliest
biological molecules, however, has centered mainly on the nucleic acids, DNA
and RNA, and the proteins. At one time or another, one of these molecular
classes has seemed a likely starting point, but which? To answer that, we
must look at the functions performed by each of these in existing organisms.
The proteins are the main structural and functional agents in the cell.
Structural proteins serve to build all sorts of components inside the cell
and around it. Catalytic proteins, or enzymes, carry out the thousands of
chemical reactions that take place in any given cell, among them the
synthesis of all other biological constituents (including DNA and RNA), the
breakdown of foodstuffs and the retrieval and consumption of energy.
Regulatory proteins command the numerous interactions that govern the
expression and replication of genes, the performance of enzymes, the
interplay between cells and their environment, and many other
manifestations. Through the action of proteins, cells and the organisms they
form arise, develop, function and evolve in a manner prescribed by their
genes, as modulated by their surroundings.
The one thing proteins cannot do is replicate themselves. To be sure, they
can, and do, facilitate the formation of bonds between their constituent
amino acids. But they cannot do this without the information contained
within the nucleic acids, DNA and RNA. In all modern organisms, DNA serves
as the storage site of genetic information. The DNA contains, in encrypted
form, the instructions for the manufacture of proteins. More specifically,
encoded within DNA is the exact order in which amino acids, selected at each
step from 20 distinct varieties, should be strung together to form all of
the organism's proteins. In general, each gene contains the instructions for
one protein.
DNA itself is formed by the linear assembly of a large number of units
called nucleotides. There are four different kinds of nucleotides,
designated by the initials of their constituent bases: A (adenine), G
(guanine), C (cytosine) and T (thymine). The sequence of nucleotides
determines the information content of the molecules, as does the sequence of
letters in words.
Within all cells, DNA molecules are formed from two strands of DNA that
spiral around each other in a formation called a double helix. The two
strands are held together by bonds between the bases of each strand. Bonding
is quite specific, so that A always bonds with T, and G is always partnered
with C on the opposite DNA strand. This complementarity is crucial for
faithful replication of the DNA strands prior to cell division.
During DNA replication, the DNA strands are separated, and each strand
serves as a template for the replication of its complementary strand.
Wherever A appears on the template, a T is added to the nascent strand. Or,
if T is on the template, then A is added to the growing strand. The same is
true for G and C pairs. In the characteristic double-helical structure of
DNA, the two strands carry the same information in complementary versions,
as do the positive and negative of the same photograph. Upon replication,
the positive strand serves as template for the assembly of a new negative
and the negative strand for that of a new positive, yielding two identical
duplexes.
In order for DNA to fulfill its primary role of directing the construction
of proteins, an intermediate molecule must be made. DNA does not directly
participate in protein synthesis. That is the function of its very close
chemical relative RNA.
Expression of DNA begins when an RNA molecule is constructed bearing the
information for a gene contained on the DNA molecule. RNA, like DNA, is made
up of nucleotides, but U (uracil) takes the place of T. Construction of the
RNA molecule follows the same rules as DNA replication. The RNA copy, called
a transcript, is a complementary copy of the DNA, with U (instead of T)
inserted wherever A appears on the DNA template.
Most RNA transcripts, often after some modification, provide the information
for the assembly of proteins. The sequence of nucleotides along the coding
RNA, aptly called messenger RNA, specifies the sequence of amino acids in
the corresponding protein molecule--three successive nucleotides (called a
codon) in the RNA specify one amino acid to be used in the protein. The
process is known as translation, and the correspondences between codons and
amino acids define the genetic code.
Not all RNA molecules are messengers, however. Some of the RNAs participate
in protein synthesis in other ways. Some actually make up the cellular
machinery that constructs proteins. These are called ribosomal RNAs, and
they may include the actual catalyst that joins amino acids by peptide
bonds, according to the work of Harry Noller at the University of California
at Santa Cruz. Other RNAs, called transfer RNAs, ferry the appropriate amino
acids to the ribosome. As cell biology has progressed, even more functions
for RNA have been discovered. For example, some RNA molecules participate in
DNA replication, while others help process messenger RNAs.
Scientists considering the origins of biological molecules confronted a
profound difficulty. In the modern cell, each of these molecules is
dependent on the other two for either its manufacture or its function. DNA,
for example, is merely a blueprint, and cannot perform a single catalytic
function, nor can it replicate on its own. Proteins, on the other hand,
perform most of the catalytic functions, but cannot be manufactured without
the specifications encoded in DNA. One possible scenario for life's origins
would have to include the possibility that two kinds of molecules evolved
together, one informational and one catalytic. But this scenario is
extremely complicated and highly unlikely.
The other possibility is that one of these molecules could itself perform
multiple functions. Theorists considering this possibility started to look
seriously at RNA. For one thing, the molecule's ubiquity in modern cells
suggests that it is a very ancient molecule. It also appears to be highly
adaptable, participating in all of the processes relating to information
processing within the cell. For a while, the only thing RNA did not seem
capable of doing was catalyzing chemical reactions.
That view changed when in the late 1970s, Sydney Altman at Yale University
and Thomas Cech at the University of Colorado at Boulder independently
discovered RNA molecules that in fact could catalytically excise portions of
themselves or of other RNA molecules. The chicken-or-egg conundrum of the
origin of life seemed to fall away. It now appeared theoretically possible
that an RNA molecule could have existed that naturally contained the
sequence information for its reproduction through reciprocal base pairing
and could also catalyze the synthesis of more like RNA strands.
In 1986, Harvard chemist Walter Gilbert coined the term "RNA world" to
designate a hypothetical stage in the development of life in which "RNA
molecules and cofactors [were] a sufficient set of enzymes to carry out all
the chemical reactions necessary for the first cellular structures." Today
it is almost a matter of dogma that the evolution of life did include a
phase where RNA was the predominant biological macromolecule.
Origin and Evolution of the RNA World
As certain as many people are that the RNA world was a crucial phase in
life's evolution, it cannot have been the first. Some form of abiotic
chemistry must have existed before RNA came on the scene. For the purpose of
this discussion, I shall call that earlier phase "protometabolism" to
designate the set of unknown chemical reactions that generated the RNA world
and sustained it throughout its existence (as opposed to metabolism--the set
of reactions, catalyzed by protein enzymes, that support all living
organisms today). By definition, protometabolism (which could have developed
with time) was in charge until metabolism took over. Several stages may be
distinguished in this transition.
In the first stage, a pathway had to develop that took raw organic material
and turned it into RNA. The first building blocks of life had to be
converted into the constituents of nucleotides, from which the nucleotides
themselves had to be formed. From there, the nucleotides had to be strung
together to produce the first RNA molecules. Efforts to reproduce these
events in the laboratory have been only partly successful so far, which is
understandable in view of the complexity of the chemistry involved. On the
other hand, it is also surprising since these must have been sturdy
reactions to sustain the RNA world for a long time. Contrary to what is
sometimes intimated, the idea of a few RNA molecules coming together by some
chance combination of circumstances and henceforth being reproduced and
amplified by replication simply is not tenable. There could be no
replication without a robust chemical underpinning continuing to provide the
necessary materials and energy.
The development of RNA replication must have been the second stage in the
evolution of the RNA world. The problem is not as simple as might appear at
first glance. Attempts at engineering--with considerably more foresight and
technical support than the prebiotic world could have enjoyed--an RNA
molecule capable of catalyzing RNA replication have failed so far.
With the advent of RNA replication, Darwinian evolution was possible for the
first time. Because of the inevitable copying mistakes, a number of variants
of the original template molecules were formed. Some of these variants were
replicated faster than others or proved more stable, thereby progressively
crowding out less advantaged molecules. Eventually, a single molecular
species, combining replicatability and stability in optimal fashion under
prevailing conditions, became dominant. This, at the molecular level, is
exactly the mechanism postulated by Darwin for the evolution of organisms:
fortuitous variation, competition, selection and amplification of the
fittest entity. The scenario is not just a theoretical construct. It has
been reenacted many times in the laboratory with the help of a viral
replicating enzyme, first in 1967 by the late American biochemist Sol
Spiegelman of Columbia University.
An intriguing possibility is that replication was itself a product of
molecular selection. It seems very unlikely that protometabolism produced
just the four bases found in RNA, A, U, G and C, ready by some remarkable
coincidence to engage in pairing and allow replication. Chemistry does not
have this kind of foresight. In all likelihood, the four bases arose
together with a number of other substances similarly constructed of one or
more rings containing carbon and nitrogen. According to the present
inventory, such substances could have included other members of the purine
family (which includes A and G), pyrimidines (which include U, T and C),
nicotinamide and flavin, both of which actually engage in nucleotide-like
combinations, and pterines, among other compounds. The first nucleic
acid-like molecules probably contained an assortment of these compounds.
Molecules rich in A, U, G and C then were progressively selected and
amplified, once some rudimentary template-dependent synthetic mechanism
allowing base pairing arose. RNA, as it exists today, may thus have been the
first product of molecular selection.
A third stage in the evolution of the RNA world was the development of
RNA-dependent protein synthesis. Most likely, the chemical machinery
appeared first, as yet uninformed by genetic messages, as a result of
interactions among certain RNA molecules, the precursors of future transfer,
ribosomal and messenger RNAs, and amino acids. Selection of the RNA
molecules involved could conceivably be explained on the basis of molecular
advantages, as just outlined. But for further evolution to take place,
something more was needed. RNA molecules no longer had to be selected solely
on the basis of what they were, but of what they did; that is, exerting some
catalytic activity, most prominently making proteins. This implies that RNA
molecules capable of participating in protein synthesis enjoyed a selective
advantage, not because they were themselves easier to replicate or more
stable, but because the proteins they were making favored their replication
by some kind of indirect feedback loop.
This stage signals the limit of what could have happened in an unstructured
soup. To evolve further, the system had to be partitioned into a large
number of competing primitive cells, or protocells, capable of growing and
of multiplying by division. This partitioning could have happened earlier.
Nobody knows. But it could not have happened later. This condition implies
that protometabolism also produced the materials needed for the assembly of
the membranes surrounding the protocells. In today's world, these materials
are complex proteins and fatty lipid molecules. They were probably simpler
in the RNA world, though more elaborate than the undifferentiated "goo" or
"scum" that is sometimes suggested.
Once the chemical machinery for protein synthesis was installed, information
could enter the system, via interactions among certain RNA components of the
machinery--the future messenger RNAs--and other, amino acid-carrying RNA
molecules--the future transfer RNAs. Translation and the genetic code
progressively developed concurrently during this stage, which presumably was
driven by Darwinian competition among protocells endowed with different
variants of the RNA molecules involved. Any RNA mutation that made the
structures of useful proteins more closely dependent on the structures of
replicatable RNAs, thereby increasing the replicatability of the useful
proteins themselves, conferred some evolutionary advantage on the protocell
concerned, which was allowed to compete more effectively for available
resources and to grow and multiply faster than the others.
The RNA world entered the last stage in its evolution when translation had
become sufficiently accurate to unambiguously link the sequences of
individual proteins with the sequences of individual RNA genes. This is the
situation that exists today (with DNA carrying the primary genetic
information), except that present-day systems are enormously more accurate
and elaborate than the first systems must have been. Most likely, the first
RNA genes were very short, no longer than 70 to 100 nucleotides (the modern
gene runs several thousand nucleotides), with the corresponding proteins
(more like protein fragments, called peptides) containing no more than 20 to
30 amino acids.
It is during this stage that protein enzymes must have made their first
appearance, emerging one by one as a result of some RNA gene mutation and
endowing the mutant protocell with the ability to carry out a new chemical
reaction or to improve an existing reaction. The improvements would enable
the protocell to grow and multiply more efficiently than other protocells in
which the mutations had not appeared. This type of Darwinian selection must
have taken place a great many times in succession to allow enzyme-dependent
metabolism to progressively replace protometabolism.
The appearance of DNA signaled a further refinement in the cell's
information-processing system, although the date of this development cannot
be fixed precisely. It is not even clear whether DNA appeared during the RNA
world or later. Certainly, as the genetic systems became more complex, there
were greater advantages to storing the genetic information in a separate
molecule. The chemical mutations required to derive DNA from RNA are fairly
trivial. And it is conceivable that an RNA-replicating enzyme could have
been co-opted to transfer information from RNA to DNA. If this happened
during the RNA world, it probably did so near the end, after most of the
RNA-dependent machineries had been installed.
What can we conclude from this scenario, which, though purely hypothetical,
depicts in logical succession the events that must have taken place if we
accept the RNA-world hypothesis? And what, if anything, can we infer about
the protometabolism that must have preceded it? I can see three properties.
First, protometabolism involved a stable set of reactions capable not only
of generating the RNA world, but also of sustaining it for the obviously
long time it took for the development of RNA replication, protein synthesis
and translation, as well as the inauguration of enzymes and metabolism.
Second, protometabolism involved a complex set of reactions capable of
building RNA molecules and their constituents, proteins, membrane components
and possibly a variety of coenzymes, often mentioned as parts of the
catalytic armamentarium of the RNA world.
Finally, protometabolism must have been congruent with present-day
metabolism; that is, it must have followed pathways similar to those of
present-day metabolism, even if it did not use exactly the same materials or
reactions. Many abiotic-chemistry experts disagree with this view, which,
however, I see as enforced by the sequential manner in which the enzyme
catalysts of metabolism must have arisen and been adopted. In order to be
useful and confer a selective advantage to the mutant protocell involved,
each new enzyme must have found one or more substances on which to act and
an outlet for its product or products. In other words, the reaction it
catalyzed must have fitted into the protometabolic network. To be sure, as
more enzymes were added and started to build their own network, new pathways
could have developed, but only as extensions of what was initially a
congruent network.
The Thioester World
It may well be, then, that clues to the nature of that early protometabolism
exist within modern metabolism. Several proposals of this kind have been
made. Mine centers around the bond between sulfur and a carbon-containing
entity called an acyl group, which yields a compound called a thioester. I
view the thioester bond as primeval in the development of life. Let me first
briefly state my reasons.
A thioester forms when a thiol (whose general form is written as an organic
group, R, bonded with sulfur and hydrogen, hence R-SH) joins with a
carboxylic acid (R'-COOH). A molecule of water (H2O) is released in the
process, and what remains is a thioester: R-S-CO-R'. The appeal in this bond
is that, first, its ingredients are likely components of the prebiotic soup.
Amino acids and other carboxylic acids are the most conspicuous substances
found both in Miller's flasks and in meteorites. On the other hand, thiols
may be expected to arise readily in the kind of volcanic setting, rich in
hydrogen sulfide (H2S), likely to have been found on the prebiotic earth.
Joining these constituents into thioesters would have required energy. There
are several possible mechanisms for this, which I shall address later. For
the time being, let us assume thioesters were present. What could they have
done?
The thioester bond is what biochemists call a high-energy bond, equivalent
to the phosphate bonds in adenosine triphosphate (ATP), which is the main
supplier of energy in all living organisms. It consists of adenosine
monophosphate (AMP)--actually one of the four nucleotides of which RNA is
made--to which two phosphate groups are attached. Splitting either of these
two phosphate bonds in ATP generates energy, which fuels the vast majority
of biological energy-requiring phenomena. In turn, ATP must be regenerated
for work to continue.
It is revealing that thioesters are obligatory intermediates in several key
processes in which ATP is either used or regenerated. Thioesters are
involved in the synthesis of all esters, including those found in complex
lipids. They also participate in the synthesis of a number of other cellular
components, including peptides, fatty acids, sterols, terpenes, porphyrins
and others. In addition, thioesters are formed as key intermediates in
several particularly ancient processes that result in the assembly of ATP.
In both these instances, the thioester is closer than ATP to the process
that uses or yields energy. In other words, thioesters could have actually
played the role of ATP in a thioester world initially devoid of ATP.
Eventually, their thioesters could have served to usher in ATP through its
ability to support the formation of bonds between phosphate groups.
Among the substances that form from thioesters in present-day organisms are
a number of bacterial peptides made of as many as 10 or more amino acids.
This was discovered by the late German-American biochemist Fritz Lipmann,
the "father of bioenergetics," toward the end of the 1960s. But even before
that, Theodor Wieland of Germany had found in 1951 that peptides form
spontaneously from the thioesters of amino acids in aqueous solution.
The same reaction could be expected to happen in a thioester world, where
amino acids were present in the form of thioesters. Among the resulting
peptides and analogous multi-unit macromolecules, which I like to call
multimers to emphasize their chemical heterogeneity, a number of molecules
could have been structurally and functionally similar to the small catalytic
proteins that inaugurated metabolism. I therefore suggest that multimers
derived from thioesters provided the first enzyme-like catalysts for
protometabolism.
The thioester world thus represents a hypothetical early stage in the
development of life that could have provided the energetic and catalytic
framework of the protometabolic set of primitive chemical reactions that led
from the first building blocks of life to the RNA world and subsequently
sustained the RNA world until metabolism took over.
This hypothesis implies that thioesters could form spontaneously on the
prebiotic earth. Assembly from thiols and acids could have occurred,
although in very low yield, in a hot, acidic medium. They could also have
formed in the absence of water, for example, in the atmosphere. Perhaps a
more likely possibility is that thioesters formed, as they do in the present
world, by reactions coupled to some energy-yielding process. The American
chemist Arthur Weber, formerly of the Salk Institute, now at the NASA Ames
Research Center in California, has described several simple mechanisms of
this sort that could have operated under primitive-earth conditions.
So far, these ideas are highly speculative, being supported largely by the
need for congruence between protometabolism and metabolism, by the key--and
probably ancient--roles played by thioesters in present-day metabolism, and
by the likely presence of thioesters on the prebiotic earth. But some
experimental evidence has been obtained that supports the thioester-world
model.
I have already mentioned the work of Wieland, Lipmann and Weber. Recently,
highly suggestive evidence has come from the laboratory of Miller, where
researchers have obtained under plausible prebiotic conditions the three
molecules--cysteamine, b-alanine and pantoic acid--that make up a natural
substance known as pantetheine. They have also observed the ready formation
of this compound from its three building blocks under prebiotic conditions.
It so happens that pantetheine is the most important biological thiol, a
catalytic participant in a vast majority of the reactions involving
thioester bonds.
A Cosmic Imperative
I have tried here to review some of the facts and ideas that are being
considered to account for the early stages in the spontaneous emergence of
life on earth. How much of the hypothetical mechanisms considered will stand
the test of time is not known. But one affirmation can safely be made,
regardless of the actual nature of the processes that generated life. These
processes must have been highly deterministic. In other words, these
processes were inevitable under the conditions that existed on the prebiotic
earth. Furthermore, these processes are bound to occur similarly wherever
and whenever similar conditions obtain. This must be so because the
processes are chemical and are therefore ruled by the deterministic laws
that govern chemical reactions and make them reproducible.
It also seems likely that life would arise anywhere similar conditions are
found because many successive steps are involved. A single, freak, highly
improbable event can conceivably happen. Many highly improbable
events--drawing a winning lottery number or the distribution of playing
cards in a hand of bridge--happen all the time. But a string of improbable
events--the same lottery number being drawn twice, or the same bridge hand
being dealt twice in a row--does not happen naturally.
All of which leads me to conclude that life is an obligatory manifestation
of matter, bound to arise where conditions are appropriate. Unfortunately,
available technology does not allow us to find out how many sites offer
appropriate conditions in our galaxy, let alone in the universe. According
to most experts who have considered the problem--notably, in relation with
the Search for Extraterrestrial Intelligence project--there should be plenty
of such sites, perhaps as many as one million per galaxy. If these experts
are right, and if I am correct, there must be about as many foci of life in
the universe. Life is a cosmic imperative. The universe is awash with life.
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©American Scientist 1995