ON THE QUESTION EVOLUTION VLADIMIR
OF THE ORIGIN AND
OF THE GENETIC SYSTEM N O V A K and V L A S T I M I L
LIEBL
Prebiology Group, Institute of Microbiology, Czechoslovak Academy of Sciences, Praha, Czechostovaka
In the present paper we have attempted a Darwinian approach to the question of the origin of heredity and its mechanisms in connection with the problem of the origin of life. Our conclusions are based on Oparin's coacervate theory of evolutionary abiogenesis, on the sociability theory propounded by one of ourselves (Nov~ik, 1967), and on our ideas of the 'coacervate in coacervate' hypothesis, discussed at the first ISSOL meeting in Varna (Nov~tk and Liebl, 1971). We have further tried to utilize data on the structure and behavior of contemporaneous subcellular organisms (viruses, etc.) and analogies with the evolution of organisms at phylogenetically higher levels (from unicellular organisms to human society) and experimental evidence obtained from various model systems of individual components of living matter and similar substances. According to our view the sequence of the bases in the primordial nucleic acid molecules did not originate as a sudden leap due to chance, but was the outcome of regular and progressive evolution at a given level ofchemogenesis that occurred everywhere the necessary chemicals and environmental conditions prevail. Even if t h e primary abiogenetic proteins originated without a nucleic acid matrix, they presumably had a regular, non-fortuitous base., although they were no doubt far less constant than recent, biogenic proteins. Primary nucleic acid molecules undoubtedly originated where these proteins accumulated in the presence of further necessary materials and other conducive conditions. Both these syntheses, that is to say of proteins and nucleic acids, had to be the most probable chemical reactions under the given conditions, determined by the molecular structure of the relevant substances, although at present we know nothing about their thermodynamic balance. It is feasible that the stimulus for replication of this abiogenic NA molecule was the only step needed to start the actual evolution of life. It was no doubt enzymatic action of one or perhaps several primary proteins, a 'presynthetase' which probably had enzymatic action far less effective and much slower than the enzymatic activities of present-day proteins. From that moment onward, natural selection came into play by preserving those replicating molecules which were more stable and were more readily reproduced in the given environment. The sequence of their bases thus became a primary genetic code, which afterwards underwent successive changes as a result of individual mutations owing to further selection. The original abiogenic proteins in which NA-replication first began, preexisted in the form of coacervates. These became the means of accumulation of the low molecular weight substances needed for the synthesis. The replicating nucleic acid molecules spread in these primary coacervates, irrespective of whether they accrued in the form of drops or as continuous layers, and produced their own specific protein as deterOr#/ins qf L(fe 6 (I975) 269 271. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordrecht-Holland
270
VLADIM~R
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Fig. 1. Six gradual stages of the evolution from the self replicating molecule of nucleic acid to the first primitive cell. (1)- a primary coacervate of protein and low molecular weight substances. (2) in connecti0n with protein molecules individual single helix nucleic acid molecules originate. (3) - origin of the double helix DNA molecules. (4) - around each double helix DNA molecule a secondary coacervate originates9 (5) - the secondary hereditary coacervate divides with each DNA molecule replication being composed of the products of its metabolic activity: transcribed RNA, proteins and low molecular weight substances9 (6) - on the surface of the secondary coacervate a surface membrane originates which is later transformed into the cell-membrane of a primitive bacterial cell.
mined by their base sequence 9 T h e new protein remained in contact with the reproducing N A molecules o n the basis of the same physicochemical forces as the original, p r i m a r y coacervate, thereby forming new, secondary coacervates in the p r i m a r y ones, i.e. coacervates in coacervates. As distinct f r o m the p r i m a r y coacervates, the structure and other features of such secondary coacervates were wholly determined by the sequence of bases of the c o r r e s p o n d i n g N A molecule, i.e. b y its genetic code. A l t h o u g h we do not k n o w the structure of the relevant N A molecule, it m a y be presumed that it was originally monohelical. It was p r o b a b l y c o m p o s e d of a m u c h shorter chain o f bases than the present ones k n o w n ; it m a y have resembled recent t r a n s f e r - R N A as suggested by O h n o (1970)9 It was likely to have been o f a D N A type which developed the characteristic double-helix structure only at later stage, however. Its replication was p r o b a b l y catalyzed by the abiogenic protein of the p r i m a r y coacervate which had thus the function o f a presynthetas. T h e protein molecules p r o d u c e d (transcribed) m i g h t n o t have been m u c h different f r o m the pre-existing protein or proteins, but they were not, of course, identical with them. It was these hereditary 'coacervates in coacervate' which underwent further evolution t h r o u g h successive mutations of the genetic code of their D N A molecules. They
ON THE QUESTION OF THE ORIGIN AND EVOLUTIONOF THE GENETIC SYSTEM
271
constitute the necessary intermediate step in evolution from the primary abiogenic coacervates to primary unicellular organisms (with surface membranes) at the level of the phylogenetically most primitive recent bacteria. The structure of various recent viruses and related organisms, from tobacco mosaic virus, via influenza and papatachi virus bodies, bacteriophages, and mycoplasms to L-forms of bacteria and to ricketsiae, shows us the stages through which this evolution probably passed. As evolution progressed, the structure of the genetic code gradually changed and improved. Single strand-DNA molecule was converted to double-strand, messenger RNA was differentiated and in time transfer RNA and ribosomes developed (see Ohno, 1970; Novfik and Liebl, 1971). That was only possible after the formation of a semipermeable membrane on the surface of the secondary coacervate and this was in turn determined genetically by mutations of the DNA molecule. Further differentiation and the more complicated evolution of the future cytoplasm took place within the membrane. Here, various tertiary coacervates, or supercoacervates, originated and formed the starting point for the development of cell organelles, such as ribosomes, chromosomes, the Golgi apparatus, different types of tubules and vesicles, etc. Short as it is, we believe that this brief note shows that all the pieces in the picture of the gradual origin of living organisms on our planet are, bit by bit, falling into place and that it will not be long before science is able to reproduce any of these processes under laboratory conditions. References Crick, P. H. C. : 1968, J. Mol. Biol. 38, 367. Nowik, V. J. A.: 1967, Zurn. Obgd. Biol. 28, 387. Novfik, V. J. A. and LieN, V. : 1971, International Symposium on Origin of Life and Evolutionary Biochemistry, Abstracts, p. 14, Varna, Bulharsko. Ohno, S. : 1970, Evolution by Gene Duplication, Berlin and New York. Orgel, L. E.: 1968, d. Mol. Biol. 38, 381. Woese, C. R. : 1967, The Genetic Code, New York. Woese, C. R. : 1972, Exobiology, N.Holland Publ. Co., A m s t e r d a m - L o n d o n , pp. 301-339.
OF THE ORIGIN AND
OF THE GENETIC SYSTEM N O V A K and V L A S T I M I L
LIEBL
Prebiology Group, Institute of Microbiology, Czechoslovak Academy of Sciences, Praha, Czechostovaka
In the present paper we have attempted a Darwinian approach to the question of the origin of heredity and its mechanisms in connection with the problem of the origin of life. Our conclusions are based on Oparin's coacervate theory of evolutionary abiogenesis, on the sociability theory propounded by one of ourselves (Nov~ik, 1967), and on our ideas of the 'coacervate in coacervate' hypothesis, discussed at the first ISSOL meeting in Varna (Nov~tk and Liebl, 1971). We have further tried to utilize data on the structure and behavior of contemporaneous subcellular organisms (viruses, etc.) and analogies with the evolution of organisms at phylogenetically higher levels (from unicellular organisms to human society) and experimental evidence obtained from various model systems of individual components of living matter and similar substances. According to our view the sequence of the bases in the primordial nucleic acid molecules did not originate as a sudden leap due to chance, but was the outcome of regular and progressive evolution at a given level ofchemogenesis that occurred everywhere the necessary chemicals and environmental conditions prevail. Even if t h e primary abiogenetic proteins originated without a nucleic acid matrix, they presumably had a regular, non-fortuitous base., although they were no doubt far less constant than recent, biogenic proteins. Primary nucleic acid molecules undoubtedly originated where these proteins accumulated in the presence of further necessary materials and other conducive conditions. Both these syntheses, that is to say of proteins and nucleic acids, had to be the most probable chemical reactions under the given conditions, determined by the molecular structure of the relevant substances, although at present we know nothing about their thermodynamic balance. It is feasible that the stimulus for replication of this abiogenic NA molecule was the only step needed to start the actual evolution of life. It was no doubt enzymatic action of one or perhaps several primary proteins, a 'presynthetase' which probably had enzymatic action far less effective and much slower than the enzymatic activities of present-day proteins. From that moment onward, natural selection came into play by preserving those replicating molecules which were more stable and were more readily reproduced in the given environment. The sequence of their bases thus became a primary genetic code, which afterwards underwent successive changes as a result of individual mutations owing to further selection. The original abiogenic proteins in which NA-replication first began, preexisted in the form of coacervates. These became the means of accumulation of the low molecular weight substances needed for the synthesis. The replicating nucleic acid molecules spread in these primary coacervates, irrespective of whether they accrued in the form of drops or as continuous layers, and produced their own specific protein as deterOr#/ins qf L(fe 6 (I975) 269 271. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordrecht-Holland
270
VLADIM~R
9
.
.'.
N O V A K
.
9
.
"
.
"~149
9.-: . . . ~ 3 9 . ~ . 9 .. ..j .-.. :. . .k; .. . .9
VLASTIMIL
LIEBL
...:. !.-....
~
t . :6 '. ..5~'.. 9" b " . :: . :~o.'. ..:...:: ::
A N D
9 ,
3..-. 9
" .s
.'."/..
: ....Z~.: . ~ : : ! : ."
.
.
9
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9
9
,
9
9
.
9 .: :.\~i.-.
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9
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-.-..p.. ~:..xi~" " - " " . - . . . _b .
."
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.... >.:..- ~ . . . . 9. . 9 . . ~
4..
.... :..-....
'~ :"~'" ~.'~....
5..:.V.:.~f.~.._.... . ".. . . . . . "'" "" ".. ~ :..
.~,=~,. 9 9
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, 9 ,O.~:';i.~ . . . , .
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" 91499
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. ....' .
- 9 9
9 :.~.,~:~...". ":.:.e;.~.3!:... .
.
9
.
9
9 ,~_.~.;~
.
....
Fig. 1. Six gradual stages of the evolution from the self replicating molecule of nucleic acid to the first primitive cell. (1)- a primary coacervate of protein and low molecular weight substances. (2) in connecti0n with protein molecules individual single helix nucleic acid molecules originate. (3) - origin of the double helix DNA molecules. (4) - around each double helix DNA molecule a secondary coacervate originates9 (5) - the secondary hereditary coacervate divides with each DNA molecule replication being composed of the products of its metabolic activity: transcribed RNA, proteins and low molecular weight substances9 (6) - on the surface of the secondary coacervate a surface membrane originates which is later transformed into the cell-membrane of a primitive bacterial cell.
mined by their base sequence 9 T h e new protein remained in contact with the reproducing N A molecules o n the basis of the same physicochemical forces as the original, p r i m a r y coacervate, thereby forming new, secondary coacervates in the p r i m a r y ones, i.e. coacervates in coacervates. As distinct f r o m the p r i m a r y coacervates, the structure and other features of such secondary coacervates were wholly determined by the sequence of bases of the c o r r e s p o n d i n g N A molecule, i.e. b y its genetic code. A l t h o u g h we do not k n o w the structure of the relevant N A molecule, it m a y be presumed that it was originally monohelical. It was p r o b a b l y c o m p o s e d of a m u c h shorter chain o f bases than the present ones k n o w n ; it m a y have resembled recent t r a n s f e r - R N A as suggested by O h n o (1970)9 It was likely to have been o f a D N A type which developed the characteristic double-helix structure only at later stage, however. Its replication was p r o b a b l y catalyzed by the abiogenic protein of the p r i m a r y coacervate which had thus the function o f a presynthetas. T h e protein molecules p r o d u c e d (transcribed) m i g h t n o t have been m u c h different f r o m the pre-existing protein or proteins, but they were not, of course, identical with them. It was these hereditary 'coacervates in coacervate' which underwent further evolution t h r o u g h successive mutations of the genetic code of their D N A molecules. They
ON THE QUESTION OF THE ORIGIN AND EVOLUTIONOF THE GENETIC SYSTEM
271
constitute the necessary intermediate step in evolution from the primary abiogenic coacervates to primary unicellular organisms (with surface membranes) at the level of the phylogenetically most primitive recent bacteria. The structure of various recent viruses and related organisms, from tobacco mosaic virus, via influenza and papatachi virus bodies, bacteriophages, and mycoplasms to L-forms of bacteria and to ricketsiae, shows us the stages through which this evolution probably passed. As evolution progressed, the structure of the genetic code gradually changed and improved. Single strand-DNA molecule was converted to double-strand, messenger RNA was differentiated and in time transfer RNA and ribosomes developed (see Ohno, 1970; Novfik and Liebl, 1971). That was only possible after the formation of a semipermeable membrane on the surface of the secondary coacervate and this was in turn determined genetically by mutations of the DNA molecule. Further differentiation and the more complicated evolution of the future cytoplasm took place within the membrane. Here, various tertiary coacervates, or supercoacervates, originated and formed the starting point for the development of cell organelles, such as ribosomes, chromosomes, the Golgi apparatus, different types of tubules and vesicles, etc. Short as it is, we believe that this brief note shows that all the pieces in the picture of the gradual origin of living organisms on our planet are, bit by bit, falling into place and that it will not be long before science is able to reproduce any of these processes under laboratory conditions. References Crick, P. H. C. : 1968, J. Mol. Biol. 38, 367. Nowik, V. J. A.: 1967, Zurn. Obgd. Biol. 28, 387. Novfik, V. J. A. and LieN, V. : 1971, International Symposium on Origin of Life and Evolutionary Biochemistry, Abstracts, p. 14, Varna, Bulharsko. Ohno, S. : 1970, Evolution by Gene Duplication, Berlin and New York. Orgel, L. E.: 1968, d. Mol. Biol. 38, 381. Woese, C. R. : 1967, The Genetic Code, New York. Woese, C. R. : 1972, Exobiology, N.Holland Publ. Co., A m s t e r d a m - L o n d o n , pp. 301-339.
