E V I D E N C E F O R E X T R A T E R R E S T R I A L A N D
T E R R E S T R I A L A B I O G E N I C P H O T O S Y N T H E S E S
G . M u e l l e r
University of Concepcion, Chile
I . I N T R O D U C T I O N
The formation of complicated organic molecules, from C2N2, C 02, N H3, C O and mixtures of other simple gases, through their irradiation with u.v. and visible light, X-rays, radioactivity or treatment with electrical discharges, has been demonstrated by several workers
( Br i g g s M . H . ; Do e s K . ; Mi l l e r S.L. and Pa r r i s M . ; Pi t t s J.N., Fo o t e J . K . and Wa n J.K.S.; Po n n a m p e r u m a C. and Yo u n g R.S.;
Ri s s i) . The above mentioned gases or the corresponding groups have also been spectroscopically detected in interstellar space, solar, stellar, planetary and cometary atmospheres (for a summary of literature see
Mu e l l e r (1963 A), and are generally known as vulcanic exhalation on the earth. Therefore, it must be anticipated, that pre-biological carbonaceous complexes would synthetize throughout a considerable range of settings of the cosmos, including the earth.
It is believed ( Be r n a l , 1959) that such pre-biological substances may be subsequently utilized by living organisms, provided that the cosmic setting favours the evolution of life.
Since the middle of the nineteenth century the carbonaceous meteorites have been generally believed to contain organic substances.
The presence of organic molecules was proved in Cold Bokkeveldt
( Mu e l l e r , 1953), and since then another six of the twenty-five-odd stones were analysed for these with positive results (see Ma s o n , 1962).
The open question is, whether these carbonaceous complexes are of a pre-biological or post-biological origin. Regarding this, five groups of evidence will be briefly discussed below.
ι. Microfossils
It was claimed ( Na g y and co-workers, 1962) that microfossils were detected in Orgueil and some other carbonaceous meteorites. How- ever, it was found subsequently that all the common forms are of
351
mineral origin, namely, small droplets of glass, rounded or globular olivine crystals and hexagonal troilite plates in various stages of oxidation and terrestrial contaminants ( Mu e l l e r , 1962). It appears, therefore, that the presence of extraterrestrial microfossils in the meteorites has not been proven so far.
2. Biogenic molecules
Ho d g s o n and Ba k e r (1964), using extremely sensitive methods, detected porphyrin-type molecules, probably within the order of overall concentration of ο·οι p.p.m. in the Orgueil meteorite and
Na g y (1964) detected some optical rotation in the same, although
previous attempts at detecting optical rotation with less sensitive methods in Cold Bokkeveldt ( Mu e l l e r , 1953) and Mokoia, ( Br i g g s , et al. y 1962) failed. It appears to the writer that before the above evidence could be interpreted as indicative of the presence of life on the parent bodies of meteorites, the following two factors should be evaluated : (a) the amount of impact and storage and museum handling-con- tamination. This could be determined through simulating falls, etc. on silicate + synthetic carbonaceous substance mixtures, close in con- sistency, porosity and other properties to the various types of meteor- ites, (b) A concrete proof that minute quantities of porphyrins, optically active molecules, etc., do not a-biologically synthetize, given a 'geological timescale'. The second factor is extremely difficult to decide, and for this reason attention is concentrated on the three evidences of * indirect nature', below, which have been hitherto very much neglected.
3. ' Cosmological indicators '
It has been claimed that the presence of water soluble salts and hydrated silicates in Orgueil, indicates an original ' hydrous surround- ing' ( Na g y et al, 1962) which is adapted for the evolution of the living organism. The writer found, however ( Mu e l l e r , 1963b), that the presence of fresh olivine grains within the porous matrix of Orgueil, and the rest of the sixteen carbonaceous chondrites examined so far, precludes the presence of free water on the parent body for more than a few years, or a few decades at the most, in view of our experience on earth, of weathering of olivine. The absence of water for any prolonged period, appears to counterindicate the evolution of life, as we know it here on earth.
ABIOGENIC PHOTOSYNTHESES 353 4. Comparison of terrestrial and meteorite carbonaceous phases
It has been repeatedly indicated since the nineteenth century that the carbonaceous complexes in meteorites closely resemble humic acids of biogenic origin. It appears, however, that the resemblance may prove to be even closer to the tucholites, which are found in pegmatite veins within granitic intrusions, and are generally believed to be of the hitherto best established hydrocarbons of a-biogenic, juvenile origin on earth. The very scanty data regarding both the tucholites and the meteorite hydrocarbons indicate that they both have an anomalously high O/H ratio of 0-5 to ι·ο, when compared to the range of o-o to 0-3 of all the older terrestrial biogenic organic substances. The presence of considerable quantities of aromatic acid molecules are indicated in both the terrestrial and extraterrestrial carbonaceous substances in question.
5. Interrelations between the carbonaceous phase and inorganic phases of meteorites
In the course of work in progress at present, it was found by the author that numerous well denned interrelations exist between the organic and inorganic phases of the meteorites. The most significant of these can be briefly summarized as follows: With increasing C per cent the percentage of H, S, and all the chalchophyl elements increase.
Further, the volatilization curve tends to shift towards lower tempera- ture. The above seems to indicate that the carbonaceous chondrites may originate from different levels of a parent body or rather closely comparable parent bodies; the low carbon stones (with C around 0-5 per cent) originate from relatively deeper and hotter zones, whereas the high carbon group (with C up to 4 7 per cent) originate from the higher, cooler zones. The existence of such a 'fractionation series' of all the volatiles, not the carbonaceous matter alone, is strongly suggestive of a process of non-biological distillation.
It may be summarized that the case for biogenic origin of the carbonaceous complex of any of the meteorites examined in detail so far, has not been proved or strongly indicated ; on the other hand, the evidence tends to support origin during a pre-biological stage of a parent body, which was in all probability too small to furnish con- ditions suitable for the evolution of life, at any stage of its cosmological history. Using the above as a working hypothesis, the conditions under which a given carbonaceous complex formed in a given meteorite,
23
could be partially reconstructed through the following data: (a) deter- mination, as far as experimentally practicable, of the elemental and molecular composition of the carbonaceous complex ; (b) experimental determination of range of conditions (composition of original gas mixture, type of radiation or other energy source) under which the carbonaceous substances condensate, similar in structure to the given meteorite-complex form. It may be mentioned as an illustrative example, that if the carbonaceous complex formed within relatively deep zones of the parent body, then only radioactive radiation and changes in temperature and pressure could have played a part in its condensation, whereas an origin progressively nearer the surface, may have brought into play the effect of cosmic rays, electric discharges, and finally solar radiation. There have already been mentioned in the course of the present meeting certain differences in molecular com- positions of condensates produced through differing high energy agencies, which gives us some confidence that in the future we will be able to distinguish between deep-seated and surface condensates.
According to our theoretical considerations, the condensates in the low-carbon meteorites and the terrestrial tucholites should be of the deep-seated type.
The next question which comes to mind is as to what changes would produce a given organism of terrestrial type biochemistry in a pre-biological carbonaceous condensate ? From the scanty data available it appears that both the terrestrial tucholites and the car- bonaceous complexes of the meteorites seem to be more aromatic and of higher O/H ratio (more acid ?) than the mean for the biogenic terrestrial products. This seems to indicate that the organism tends to hydrogenate the original pre-biological condensates.
These, and numerous further fascinating problems, could be studied in the future through careful comparison of the products of experi- mental non-biogenic photosynthèses, with those found in inferred pre-biological phases from the earth, from the meteorites, and in the near future, also the surface of the moon and other celestial bodies of the solar system.
ABIOGENIC PHOTOSYNTHESES 355 REFERENCES
Be r n a l J.D. (1959) Proc. of the ist Symposium on the Origin of Life on the Earth, Moscow (Pergamon Press), p. 38.
Br i g g s H . M . and Ki t t o G . B . (1962) Nature, Lond. 1 9 3 , 126.
Ho d g s o n G . W . and Ba k e r B . L . (1964) Nature, Lond. 2 0 2 , 125.
Ma s o n B . (1962) Space Sei. Rev. 1 , 621.
Mu e l l e r G . (1953) Geochim et Cosmochim Acta 4, 1 ; (1962) Nature, Lond.
1 9 6 , 929; (1963) Advances in Organic Geochemistry, Pergamon Press, Vol. ι, p. 119.
Na g y B., Cl a u s G . and He n n e s s y D J . (1962) Nature, Lond. 1 9 3 , 129;
(1964) Private communication.
