Post n.4 (English)
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it.wikipedia.org
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From
ancient time and until 1800, man believed in
spontaneous generation. That is it
was thought that life originated itself spontaneously from unliving matter.
Preferred places for spontaneous generation were, in particular, mud and
organic matter in putrefaction. It was the research of Francesco Redi and of
Lazzaro Spallanzani which introduced doubt regarding this belief. Louis Pasteur
gave the last blow by demonstrating experimentally, around 1860, that in the
actual physic-chemical conditions of our planet life can generate only from
life.
If we
exclude spontaneous generation, how did life have its origin on our planet?
For more
than half a century, after Pasteur, there was not much interest in the problem
of the origin of life. The first theory on the origin of life from unanimated
matter, as largely illustrated in preceding articles, it can be dated around
1930 as the work of Oparin and Haldane, and it is known as: the theory of the
prebiotic broth. In it one hypotheses the formation of substances fundamental for
the origin of life in the atmosphere and their gathering in a primitive ocean, primordial
or prebiotic broth, where would have formed more complex molecules and from
these in the end life would have had its origin.
The now
famous experiment by Miller, seemed to be a confirmation of the theory
Oparin-Haldane on the whole and hence also of the prebiotic broth which, also
today, remains the most accredited theory. Unfortunately, after sixty years, in
spite of the contribution of many researchers, the theory of the prebiotic broth
has a failing balance.
But if
research has given a negative conclusion, how could life have had its origin in
the prebiotic broth?
After
having analysed if the first to appear were the proteins
or the nucleic acids, Paul Davies (Da dove viene
la vita, 2000) writes: «All the theories have in common the same idea: once
that life had been born in whatever form, the rest came almost by itself, because
Darwin’s evolution has been able to make its way. Hence it is natural that
scientists try to appeal to Darwinism starting from the very first phase of the
history of life: with his entry into the scene enormous progress is possible
sustained only by the driving force of chance and selection. Unfortunately, though,
so that Darwin’s evolution can begin, it is necessary to have a minimum level
of complexity. How was this initial complexity attained? Put against the wall, the
greater part of scientists wrings their hands and murmur the magic word: chance».
The word
“chance” has however different meanings and it contains for this reason a trap,
which we shall clear further. In the context of the origin of life, the
meanings which are given to the word chance are essentially two.
1 ) Chance as an event fortuitous highly
improbable, almost a miracle; it can happen or not happen and if it happens it
will be unique in its species. Let us imagine a person who, stressed by
work, decides to go on holidays and buys the last ticket available for an
island of the Caribbean. Here, compelled by the waves to promenade at distance
from the shore, he goes down in a hole and finds a treasure bidden by pirates.
It has a chance of the sort ever happened? It can happen or not happen but if it
happens it will be a fortuitous event, with the same characteristics it will
never happen again.
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giornalistico.it
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2) The second meaning is a question of
probability, hence mathematical, early or late it will happen and also more
than once. It is sufficient to remember that the probability to win the pools
for one single player is very small, but if the players are millions, almost
every week there will be at least one winner.
But when
was born this idea of chance? And then life could really have had its origin by
chance?
To
introduce the concept of casuality in the origin of life, seems to have been
Leonard Thompson Troland, American physicist and psycho physiologist, as Iris
Fly reports in her essay (The origin of life on the earth, 2002): «It was
thought that the gene formed itself in an
improvised manner a consequence of a “first event” fortuitous , highly
improbable. Troland attended to this situation, affirming that in the great
extension of time during which life could have emerged even a highly improbable
event could have become possible. All that was requested was a reproduction of
one single molecule self-catalytic. (Troland,
1914, 105), A fortunate collision of molecules or a “fortunate chance”, to take
the expression often used in what follows». Also Haldane thought that life had
a casual origin, but with a different approach. In “Origin of life (Bernal,
Haldane, Pirie, Pringle) he affirms: «I have been reproved for having
introduced “chance”. By “chance” we intend the partial impossibility to predict an
event.[…] on principle the larger part of processes are not unforeseeable,
however it is not possible to predict some particular phenomena without
affirming that, a particular event, has a certain probability of realizing
itself in a definite time».
From 1953,
the years of Miller’s experiment, until 1970 were years of intense research and
strong enthusiasm, the chance was almost forgotten. But with the failure of the
experimental research on the prebiotic broth, the chance revives like a Phoenix.
It was
principally J. Monod ( The chance and necessity) not only to reintroduce the chance
in the origin of life, but also to make it a philosophy of existence. According
to Monod the origin of life in the earth was a unique and unrepeatable event
.«The universe was not going to give birth to life, and neither the biosphere
would give birth to man. Our number comes out on the roulette: why then should
we not perceive the exceptional nature of our condition […]».
Criticism
and consensus of this vision are not lacking. In 1980, the origin of life as a
fortuitous event was definitely archived by the English astronomer Fred Hoyle,
with a metaphor now become classic: «"[…] The spontaneous appearance of an
unicellular organism from a casual combination of chemical compounds, is just
as probable as the creation of a Boeing 707 as the result of a tornado which
crosses a pile of useless metal[…]». Even if there is who attempts again a
solution ad hoc : given a sufficient
long time, even events that seem miraculous become possible; but no-one
believes any more in an event unique and unrepeatable.
Yet the probabilistic
sense of the word “chance” remain.
Between
1950 and 1980, after the discovery of the structure of DNA
by Crick and Watson, the complexity of living systems was understood. The DNA
has formed by tens of thousands of genes and every one of these codifies for a
protein. What is then the probability that, in the prebiotic era at least, one
protein or a gene of biological relevance has
formed? In the actual organisms, a protein (enzyme)
is made up of about 300 residues different from the 20 aminoacids. Since every amino acid is codified by a triplet
of bases (three nucleotides), the gene
which corresponds will be made up of 900 nucleotides. With these data the
number of proteins possible corresponds to 20300, and the number of
corresponding genes 4900, which translated respectively 10390
and 10540. With these numbers the probability, in the prebiotic era,
of the formation of important macromolecules of biological interest, is zero. Even if we reduce the genes, in
the prebiotic era, to 300 nucleotides, the number of genes possible still
results enormous: 10180. Someone proposed to reduce the number of
nucleotides to 75; the number of genes would then be 1044, a number
still too great. In the end, these simple calculations are enough to
demonstrate that the probability that in the prebiotic broth, proteins or functional genes had their origin,
is practically inexistent. We are now in the position to abolish the trap of
the word “chance” and we do it with the words of Mario Ageno, (Le radici della
biologia, 1986): «Event of little or very little probability, practically
realize a continual passage from the second to the first category». Hence, when
the probability of an event is very little, a probabilistic event becomes thus
too fortuitous, almost a miracle. In fact, who still insists with the
calculation of probability to justify the casual origin of life, only imagines
constructing a Boeing with a tornado.
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makingthemodernworld.org.uk
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Around 1980
the theory of the “RNA World” begins, that is it was thought that life had its
origin through the synthesis of an RNA molecule
reproducing itself and this, evolving, had learnt to synthesize proteins.
Manfred
Eigen is a sustainer of the RNA World, he excludes chance: «The genic patrimony that
today one meets in living organisms could not have produced itself by chance as though
throwing dice». (Gradini verso la vita, 1992), and he explains how this process
could have made the first steps towards the origin of life. He goes from the
idea that in the prebiotic world must have existed an ambiance in which were
present a population of at least 1010 auto reproducing molecules of
nucleic acid also of small dimension,
(100-300 nucleotides). These, through reproducing also in the presence of
rudimental enzymes, have participated in a process of functional optimization,
of selection, to the point of attaining dimensions such as to give origin to
functional proteins. To sustain his ideas, Eigen develops a mathematical
simulation of a process auto-catalytic (hypercycle).
But how
were the first molecules capable of reproducing themselves have their origin?
Eigen tries
to give an answer of a deterministic type: in the prebiotic world must have existed
amino acids, ribose and deoxiribose,
nucleobases. For the origin of amino acid, he indicates Miller’s experiment,
whereas for the origin of the other compounds, he gives vague answers and
insufficient, quoting also experiments in ambiance without water. For the molecular asymmetry, which one meets in all organisms,
he does not give satisfactory answers if not to recall phenomena which have
already been confirmed as without influence. He imagines, furthermore, that in
the prebiotic broth would be present polypeptides
of the type proteinoid of Fox with weak catalytic abilities and also stereospecific (hence privileging the dominion of
chemical laws). Some of these must have been capable to synthesize molecules of RNA,
even though, as they are rudimentary catalysts, the time requested for such
synthesis must have been long. In the end the Ribozymes
could have given a contribution of their own.
As already mentioned
in preceding articles, also in the RNA World, all the big problems of prebiotic
chemistry are left in silence.
And so one
can only conclude that on the origin of life nothing is known. Chance though is not
involved, or is it?
At the
conclusion of the chapter were she treats the RNA World, (work already quoted)
Iris Fly affirms: «In conclusion, more general comment is opportune.
Traditionally the genetic conception was associated with the assumption that the
synthesis, highly improbable, of a single sequence reproducing itself was
sufficient to the origin of life. […] The role assigned to the contingent
events in the actual scenes of the “RNA World” is still considerable. Kavanarioti
for example speaks of processes of diversification, active inside the
populations of the first catalysts which would have led “not by selection but
by chance” to the emergence of better catalysts».
But what
type of chance? One wants to simulate a throwing of dice, waiting in reality for
a tornado?
Who made
the effort of giving a more detailed explanation to the appearance of the first molecules
reproducing itself is C. De Duve in a new essay (Alle origini della vita,
2008). The structure of the essay is very interesting. He discerns above all 7
peculiarities, but those which interest us are principally the 1st
“deterministic necessity”: «According to these interpretations, things could not
have gone differently, seeing the chemical-physical conditions which existed».
And the 5th “frozen change”: «Here the singleness is given by the
decision imposed by pure chance between two or more possibilities which are not
the same any more once one has taken a decision, and has hence taken a way
without return». Going from some of the actual knowledge on the biochemistry of
life and analysing the data given by prebiotic chemistry, Christian De Duve
tries a strong conjectural effort in the tentative to give a deterministic
answer to the appearance of an RNA World, and afterwards to the origin of life.
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utenti.quipo.it
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He considers
plausible, like Eigen, the presence, in the prebiotic broth, of sugars,
nucleobases, phosphate and he poses the problem of the synthesis of the
nucleotides: «These facts suggest the existence of an ancient cauldron in which
sugar interacted with pyrophosphoric compounds
and with nucleobases to produce a variety of the sugar-phosphate, nucleosides and derivatives more complex, among which
the ATP and its homologous. The substances which
ended making an important metabolic function, in particular for the synthesis
of RNA, emerged for this mixture by selection, as I have already suggested. The
identification of the conditions in which these reactions could have taken
place with only the help of catalysts (multimers) at hand in the
prebiotic era, pose one of the problem most difficult for future researches».
The problem
of the separation of the functional nucleotides presents itself immediately:
«The mixture could have contained other bases with appropriately complementary structures.
If things were really so, was chance responsible for their refusal, or also a bidden
factor? According to a recent theoretical study, the choice of the nucleobases
could not have been purely accidental, but it could have been the result of a
process of selection, influenced by the resistance to error».
And for
Ribose: «The choice of ribose poses similar questions. The choice of this sugar
rather than the others was fortuitous or due to some molecular property which
advantaged the molecules containing ribose with respect to the others?».
And on the chirality: «The choice of chirality could have been as
we have seen, the consequence of a frozen accident or, alternatively, of some
physical tendency».
And on the
synthesis of RNA: «If we think attentively on the problem, the possibility that
the RNA is the product of a fortuitous chance, gifted with the capacity of
reproducing itself can be refused because it poses requests absolutely implausible
to chance, at the same time suffocating the research. Unless one wants to invoke
the similarly suffocating hypothesis of the intelligent design, we have to
resort to the opinion that the prebiotic conditions were such as to advantage
the synthesis of the reproduction of the RNA molecules through simple chemical
processes. At first sight, however, also this hypothesis seems to impose a too
great effort on credibility».
Unfortunately,
of these simple chemical processes there is no trace and that which remains is,
in the end, only the effort on our credibility.
And hence
one can only conclude that on the origin of life nothing is known. Chance though
is not relevant, or perhaps it is?
The larger
part of scientists is conscious that life could not have had its origin by
chance, but its place has been taken by ideas most hypothetic, often not very credible,
impossible to verify experimentally and also with too many frozen accidents.
But what
remain? Or the intelligent design or chance.
Well, what
now? Better pull up the blanket and cover oneself under the theory of Darwin.
And so chance,
like the Phoenix, becomes a myth, for many scientists another fortress against
the fundamentalism, and all are waiting for a tornado which can construct a
Boeing.
To know more:
Prebiotic chemistry and origin of life
www.ibs.it
www.lampidistampa.it
ISBN 978-88-488-1152-1
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