Scenarios for the origin of life
Few problems in the history of science have raised such a tremendous interest
and vivid controversial discussions as those related to the origin of life. Indeed, in
this field modern science has gradually clarified the details of an extremely
complex picture, still puzzling for both researchers and the great public, and
refuelling centennial philosophical debates among the disciples of various
theories.
Astrophysics in connection to elementary particle physics present a novel image
of the Universe, arising from a primordial break in symmetry some 15 billions of
years ago, known as the Big Bang. This early hot ball of fire cooled gradually,
condensing into nuclei and later into atoms. The long range interaction known as
gravity led to the formation of galaxies of stars, where successive cycles of
thermonuclear reactions led to accumulation of heavier elements indispensible
for life. These elements were disposed in star dust by explosions of supernovae,
triggered by the gravitational collapse when the primary nuclear fuel was
insufficient to maintain a steady state, and recondensed in second order stars,
like our own Sun, and in surrounding planetary systems.
The timescale of events related to the appearance and evolution of life on Earth
has been synthesized by Carl Sagan in a metaphoric view - the cosmic year. If all
the events, starting with the Big Bang till now, would be compressed along the
duration of a single year, some important landmarks would be placed as follows:
- somewhere in April: appearance of our Galaxy, the Milky Way
- September 9: formation of the Solar system
- September 22: formation of Earth
- beginning of October: the first assemblies of living matter
- October 25: the first prokariotic cell (i.e. lacking a nucleus)
- end of November: the first eukariotic cell
- December 17: appearance of invertebrates
- December 21: the first terrestrial animals
- December 24: the dinosaurs ~ they survived only for 4 days
- December 29: the first flower
- December 31 at 10:30: the first human beings
23: the silex axe
23:59: the first signs of civilization in Europe
- the last ten seconds: the written history of mankind
second 53: the Trojan war
second 56: the Roman Empire
second 58: the Crusades
second 59: the Renaissance
In the Middle Age a valued theory of life was that of the vital force, a force that
impregnates the living matter, in contrast to the non-living, resting matter; but the
origin and properties of this subtle force remained misterious. Some speculative
biologists (e.g. Spallanzani in Italy) built the theory of spontaneous generation:
life appears spontaneously from non-living matter in appropriate conditions. The
experiments proving such theories, like the appearance of microorganisms in
resting water, were later correctly reinterpreted by Louis Pasteur as
contaminations from the external environment. With the development of
microscopy, a new theory, known as the cell theory, emerged. It was synthetized
by Robert Hooke in the principle: Omni celula e celula. (Every cell derives from
another cell). In the late 1800s, the cell theory was completed with the statement
that the cell represents the basic structural and functional unit of living
organisms, therefore defining the properties of a cell is equivalent to defining the
properties of life.
Also during the XIXth century, a fertile period for the development of natural
sciences, the structure of chemical compounds within living organisms became
gradually unveiled, and some of them were artificially synthesized in the
laboratory. The first success in this field was the synthesis of ureea by Wöhler in
1827, marking the birth of organic chemistry. Relying on such type of knowledge,
the famous biologist Ernest Haeckel pretended that the first precursors of life, the
proteins, were formed by simple interactions of inorganic molecules in the
primordial sea, in appropriate conditions. Today this phenomenon no longer
occurs because the temperature and chemical environment have changed. A
bright demonstration of this theory was produced one century later. Within an
outstanding experiment, published in Science in 1953, the American chemist
Stanley Miller proved that within a reaction bottle filled with methane and
ammonia, exposed to high temperatures (150 °C), ultraviolet radiation and
electric discharges, the elementary bricks of biologic macromolecules
(aminoacids and nucleotides) form spontaneously. But the opponents of
spontaneous generation of life were still not convinced, claiming that generation
of the building bricks is one fact, and their intelligent combination in
macromolecular self-replicating assemblies is a completely different issue, which
could by no means occur by chance alone, reflecting the essence of divine
creation.
The microscopic study of fossils led during the first half of the XXth century to the
theory of coacervates, sustained by Pflüger, Haldane, and Oparin. Coacervates
are primitive living systems consisting in assemblies of macromolecules that
perform redox reactions, using the chemical energy of various molecular
substrates. In the years 1960, Fox showed that by dry heating of a mixture of
aminoacids at ~ 150 °C, primitive proteinoid compounds with typical  and
atypical peptide bonds form spontaneously. These proteinoids in water form
microspheres with catalytic activities (glucose decomposition, esterases,
peroxidases), including synthetic activities: peptide synthesis from aminoacids
and ATP, and polynucleotide syntesis. When deposited on lecithin membranes,
these microspheres generated a transmembrane potential (-20  -70 mV) and
transient phenomena resembling action potentials, which were until then
attributed only to evolved specialized cells.
More recent theories rely on the self-catalytic properties of nucleic acids. Thus it
is conceivable that a primitive self-replicating RNA (called ribozyme) could
accomplish both roles: genetic information carrier and enzymatic effector.
However, spontaneous errors in replication impose severe restrictions on the
maximal length of such a primordial macromolecule – below 100 nucleotides.
Thus, before the coupling between nucleic acids and specialized proteins, that
would detect and correct replication errors, transmission of a significant amount
of genetic information would be prohibited. Modern theorists tried to depict, using
computer simulations, the development of nucleic acid – protein coupling leading
to the appearance of the genetic code, a universal feature of living beings on
Earth. A well-known contribution is the hypercycle theory of Manfred Eigen: selfreproductive autocatalytic cycles that became gradually interconnected by dual
autocatalytic links, with possibilities of specific cooperative interconnections.
Proteins functioned at the beginning just as enzyme cofactors, but, with the
development of the genetic code, ended by acting as the main catalytic effectors.
Another famous theory is Stuart Kauffman’s autocatalytic set theory. It is based
on mathematical graph theory and states that a sufficiently complex system of
polymers, where the synthesis of chemical bonds is thermodynamically favorable
and where at least a few of them have some more or less specific catalytic
activity, will result in an increase in the maximum length of polymers. Thus many
new polymers will arise, and even more catalytic links. From graph theory it
follows that as the ratio between the elements of a system and their connections
grows in favor of the latter, the system’s inner links become more and more
interconnected via cycles.
An Israeli biochemist of Romanian origin, Doron Lancet, shifts the point of
emphasis from RNA or protein chemistry to phospholipid assemblies. In the socalled lipid scenario of life appearance, he claims that small phospholipid
assemblies show the essential features of living beings: self-replication,
compositional diversity and segregation via selection of a limited repertoire of
monomers, achieving a kind of primitive “genetic” information.
Fig. 1 shows two different outcomes of phospholipid micelles, according to the
ratio between N, the average number of monomers within an assembly, and NG,
the total number of monomer types (i.e. the repertoire). In large random-formed
assemblies (N >> NG) each type of phospholipid is present in multiple copies,
therefore all assemblies are virtually identical and selection is impossible. On the
other hand, in small assemblies (N >> NG) there is a wide diversity, each
assembly featuring a distinct composition (e.g., for an assembly of N = 100
molecules and a repertoire NG = 1000, there are 10100 possible assemblies). In
these conditions, fragmentation (fission) leads to a loss of replication capability,
the two fragments being very different. In order to reach an intermediate regime,
where both diversity and fidelity of replication are ensured, small assemblies
should undergo a process of molecular repertoire restriction. As shown in the
insert of Fig. 1, this is possible by occurrence of organized mutual catalysis
networks, according to Kauffman’s theory. Repertoire restriction is not possible at
equilibrium (insert B), but far from equilibrium (insert C). Thus, such assemblies
will reach a critical point, called the Morowitz barrier, where self-replication via
fission occurs, leading to natural selection.
Other authors stress the importance of weak electromagnetic fields in the
development of living systems, recognizing their role in molecular recognition and
orientation of biochemical reactions. Such stable microscopic electromagnetic
fields, contributed by polar macromolecular compounds, biomembranes, etc.,
may have helped to the generation of a genetic code, offering a kind of primitive
heredity, and may still be important for morphogenetic processes like
development of the cytoskeleton or evolution of the embryo.
Supplementary lectures
Eigen M, Schuster P: The Hypercycle, Springer Verlag, Berlin, 1979.
Fox SW, Nakashima T: The assembly and properties of protobiological structures,
Biosystems 12:155-166, 1980.
Jerman I: Electromagnetic origin of life, Electro- and magnetobiology 17:401-413, 1998.
Kauffman SA: The Origins of Order, Oxford University Press, New York, 1993.
Miller SL: Science 117:528-529, 1953.
Miller SL, Orgel LE: The Origins of Life on the Earth, Prentice Hall, Englewood Cliffs, NJ,
1974.
Morowitz HJ: Beginnings of Cellular Life, Yale University Press, London, UK, 1992.
Nicolis G, Prigogine I: Self-organization in Nonequilibrium Systems – From Dissipative
Structures to Order through Fluctuations, John Wiley & Sons, Toronto, Canada, 1977.
Oparin AI, Gladilin KL: Evolution of self-assemblies of proteins, Biosystems 12:133-145,
1980.
Segré D, Lancet D: Composing life, EMBO Reports 1:217-222, 2000.