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CHAPTER 4 BIOLOGICAL MATERIALS IN CARBONACEOUS CHONDRITES HAROLD C. UREY and JAMES R. ARNOLD INTRODUCTION In order to discuss this subject it is necessary to review briefly what our concepts are in regard to the fundamental problem of what life is and the conditions under which it might have evolved. It is possible that in the early history of the Earth, or other planets where life evolved, a situation existed in which chemical reactions proceeded in an ocean in the direction of producing compounds of lower free energy. These compounds of lower free energy may have been returned to the atmosphere where they were converted into compounds of higher energy, and these again reacted in the ocean as before. In a way, this is a metabolic system, but it is not what we call life. Life must be segregated from its surroundings so that we recognize part of the material as living and part of it as non-living. There must be a distinction between the two. On the other hand, without reproduction there cannot be life. This is because any finite body in natural surroundings will in the course of time be destroyed. Thus a reproductive process that permits individuals to die while others survive is a necessary part of the living process. Looking beyond the limits of the Earth and the kind of life that we see here, many questions can be asked. Are the living things based on com- 114

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Biological Materials in Carbonaceous Chondrites 115 pounds of the same chemical elements? We are inclined to answer this question in the affirmative. We do not believe that the chemistry of any other elements except that based on carbon (together with hydrogen, oxy- gen, nitrogen and, to limited extent, other elements), and on aqueous ionic solutions, would duplicate the complicated chemistry that we associate with living organisms. Hence we believe that life would involve roughly the same chemical systems. This chemistry could mean that substances playing the same roles as proteins, fatty acids and carbohydrates will be important con- stituents of living things. Of course these substances may not be the same as those in our terrestrial organisms because the chemistry of carbon has enormous possibilities that are not realized among all the living things of the Earth that exist now or have existed in the past. One might ask whether the reproductive process must involve such substances as ribonu- cleic acid or deoxyribonucleic acid, or whether some other compounds could serve as the carriers of the hereditary information. This again we cannot answer, but we expect such information-carriers to be characteristic of life. The requirements for the evolution of life, in our opinion, include the existence of liquid water and a continuous supply of energy, and it appears to us necessary that the source of energy be the Sun and not radioactivity, because radioactivity has such a destructive effect upon large organic com- pounds. What we specify then is a planet sufficiently large to hold an atmos- phere containing water, illuminated by a star, and maintained at such a temperature that waters will remain liquid on its surface. Considering the situation in the present solar system, this limits us pretty much to the region from Venus to Mars inclusive, and probably excludes Mercury and the asteroids. Though the past history of the solar system may have included times when higher temperatures or lower temperatures prevailed, we can only say the evolution of life in the asteroidal belt would require a con- siderably higher solar temperature. We think we can safely exclude Venus, the Earth and Mars from con- sideration as sources of meteorites showing evidences of life. The Earth and Venus can be excluded because the energy required to remove objects from their surfaces is unreasonably large. Mars is not a possible source, because if life were to evolve on this planet, water must have been present at some time in the past; in the course of time it must have escaped into space; and in this case erosion of the surface of the planet should have occurred, sedimentary rocks should have been produced, and if the car- bonaceous chondrites could have been removed from Mars, sedimentary rocks should also have been removed, and they should appear among our meteorites. No such meteorites have been observed. Also, the energy of removal of objects from Mars, though not so large as that from Earth,

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116 THE COSMIC SETTING is still substantial. The carbonaceous chondrites must come from smaller bodies, that is, the asteroids or the Moon. THE CARBONACEOUS CHONDRITES In discussing the meteorites from the standpoint of the possibility of extra- terrestrial life, we can exclude the great majority of the metals, the stony irons and most of the stone meteorites. These objects are strictly inorganic in characteristics, consisting of minerals that have been heated to high tem- perature and apparently have been agglomerated in the almost complete absence of water and so remained from the time that they were formed until the present. Only the carbonaceous chondrites contain much water and give us the suggestion that at least some of them were probably immersed in water at some time in the past. The carbonaceous chondrites consist of several rather distinct types as pointed out by Wiik [1956] who classified them as Types 1, 2, and 3. Mason [1962] has accepted the first two types, but the Type 3 he prefers to classify as a non-carbonaceous group. All contain some carbon, but the first types contain substantial amounts of water and little metal. It is par- ticularly the first two types, and especially Type 1, that we are concerned with in this discussion. Table 1 gives the composition in atomic percentages of samples of Type 1 and Type 2 carbonaceous chondrites as analyzed by Wnk, as well as a high iron group ordinary chondrite, and Table 2 gives the percentages by weight of carbon, water and sulfur in these types. It will be seen that the carbonaceous chondrites have a composition with respect to the elements other than water, sulfur and carbon that is very similar to a high iron group chondrite. It is evident from this that most of the elements have not been sorted, as is the case on the surface of the Earth. There is no appreciable sorting by water or by melting processes. These objects are not similar to sedimentary rocks and they are not similar to igneous and metamorphic rocks such as basalts, granites, gneisses, etc. In fact, the carbonaceous chondrites have abundances of some of the rarer elements that resemble more what we expect from our studies of the fundamental solar abundances than is true for the ordinary chondrites and the achon- drites. In this respect they would appear to be a more primitive material than the other stone meteorites. There are discrepancies with respect to solar abundances, most noticably in the case of iron, for the carbonaceous chondrites contain about four or five times as much iron with respect to silicon as does the Sun. It is important to recognize these facts in connection with biological material in meteorites because the internal evidence shows that if living

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Biological Materials in Carbonaceous Chondrites 117 TABLE 1. Composition of Carbonaceous and High Iron Chondrites Atomic Percentages, S, C, H2O Eliminated Typel Type II Average H.Chondrites Orgueil Meghei Fe (Metal) 0.00 0.00 16.18 Fe (Oxide- Sulfide) 27.34 26.18 10.05 Ni 1.37 1.41 1.57 CO 0.07 0.06 0.09 Si 31.12 31.85 33.12 Ti 0.09 0.09 0.09 Al 2.68 2.90 3.60 Mn 0.22 0.19 0.25 Mg 32.48 33.19 31.68 Ca 1.81 2.04 1.60 Na 1.97 1.40 1.56 K 0.12 0.07 0.19 P 0.33 0.29 0.19 Cr 0.40 0.33 0.33 Total 100.00 100.00 100.00 TABLE 2. Composition of Carbonaceous and High Iron Chondrites Weight Percentages Typel Orgueil Type II Average H.Chondrites Meghei S H2O 5.50 19.89 6.96 3.66 12.86 2.48 1.57 0.37 Carbonaceous organisms are present in these objects, they must have been introduced into the objects without any processes of sedimentation or sorting by running water. Included in the Type 1 carbonaceous chondrites are some minerals that seem to have crystallized from water. The silicate minerals are the hydrated clay types. The meteorites contain a substantial amount of magnesium sulfate, which is soluble in water, and carbonate minerals of calcium, mag- nesium and ferrous iron have been observed in Orgueil. Magnetite also is present, and this is the first oxidation product produced by water acting on iron. Ammonium salts were reported by early observers of these objects. This whole pattern indicates that these objects have been subjected to

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118 THE COSMIC SETTING liquid water. This was particularly shown by the work of DuFresne and Anders [1962]. What appears to be true is that some material of approxi- mately primitive composition was acted upon by liquid water and a mild oxidizing agent to produce a mixture of reduced and oxidized materials. DuFresne and Anders show that this mixture of minerals might well be an equilibrium one in spite of its odd composition. It seemed probable that these materials were produced on the surface of a planetary object, sub- jected to ultraviolet light of the Sun, which would have supplied the energy to produce hydrogen peroxide, and this in turn may have oxidized sulfides and carbon compounds to sulfates and carbonates. In the presence of a reducing atmosphere, we might expect ferrous iron to be present in the water. Where might such a situation have occurred? Vrey [1962] suggests that it was on the surface of a planetary object and that these minerals are the residue of a primitive ocean that existed only for a short time and under conditions which did not produce sedimentary rocks or sorting by running water. Anders [1963] suggests that this mixture was produced on the interior of an asteroidal object due to water rising from a heated interior and acting upon mineral material nearer the surface of the asteroid. The suggestion by Urey may not be inconsistent with the possibility of the development of life. Anders' suggestion seems to be clearly inconsistent with the possible development of life in such objects. The analysis of these carbonaceous chondrites goes back over a century. In 1834, Berzelius investigated the Alais (Type 1) meteorite and he recorded that it contained "humus and traces of other organic compounds." Wohler and Hornes [1859] studied Kaba (Type 3) and found humic and bituminous matter in an alcohol extract. Cloez [1864] analyzed the organic matter in Orgueil and reported that some 6.41 per cent was similar to peat and lignite. Berthelot [1868] recorded hydrocarbons in Orgueil. Mueller [1953] examined Cold Bokkeveld, which is a Type 2, and secured carbonaceous matter having the following analysis: C, 19.84 per cent; N, 3.18 per cent; S, 7.18 per cent; H, 6.64 per cent; Cl, 4.81 per cent; ash, 18.33 per cent; and O, by difference, 40.02 per cent. He expressed the view that this consisted of salts of carboxylic acids. It is a very old suggestion that this material was produced by living organisms. None of the older researches proved this. It should be remembered that during the early 19th century it was accepted that complex carbon compounds could not be produced except by the aid of living organisms. During the last century we have found that many kinds of very complex compounds are made in our chemical laboratories and that similar processes can occur in nature. The more recent work started in 1961 with a paper by Nagy, Meinschein and Hennessy [1961]. They had been working on organic geochemistry,

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Biological Materials in Carbonaceous Chondrites 119 especially that pertaining to the origin of petroleum. The analytical methods which they used were those standard for this sort of work. Molecules and their fragments were volatilized and subjected to a high energy electron beam. The ionized products were analyzed in a mass spectrometer. Many peaks are secured in procedures of this kind. The data secured is difficult to interpret but in the opinion of these authors the pattern of peaks was very similar to those which they had seen in connection with their petroleum investigations. They concluded that the material was of biogenic origin, just as they believe and others believe is true for all deposits of petroleum and most of the carbonaceous material on the surface of the Earth. For the reasons that have been outlined above, students of meteorites did not believe this could be true. This observation was followed by work by Claus and Nagy [1961] and by Nagy, Claus and Hennessy [1962] in which they described microscopic particles from Orgueil and Ivuna, both Type 1 carbonaceous chondrites. They suggested that these were microfossils. This was followed by papers by other authors. Staplin [1962] and Timofejew [1963] also maintained that they had found biological materials. There did not seem to be much overlapping in the type of materials described by the different authors. Some of the objects described by Nagy and his colleagues are certainly contaminants as was shown by Anders and Fitch [1962], and indeed the more complicated ones appear certainly to be contaminations by present- day biological material, ragweed pollen particularly. There remains a con- siderable number of the less complicated types of objects that appear to be indigenous to these meteorites. These latter objects are mineralized with limonite but they also contain residues insoluble in hydrochloric acid and hydrofluoric acid [Nagy et al., 1963]. The bodies which remain after such acid treatment have the shape of the mineralized body. They do not contain elements of high atomic weight as indicated by the electron microprobe analysis. That is, there appears to be organic matter left after demineraliza- tion. These objects also give absorption spectra in the ultraviolet that are similar to absorption spectra of biological material. The spectra are of an indefinite type, as is characteristic of such spectra, and it is difficult to be sure whether the material is of biological origin or not. What seems very probable is that it is, nevertheless, carbonaceous matter of some complex kind similar in a general way to biological material. Some main- tain that these objects are artifacts and they may be so. But, of course, it would be far more interesting if they were the residue of biological material, as the authors of these papers indicate. In further work, Nagy et al. [1964] have reported the presence of levo- rotatory materials in extracts from the Orgueil stone. The chemical com-

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120 THE COSMIC SETTING position of this material as determined by thin-layer chromatography appears to be quite different from terrestrial biological material with which it was compared. Some probable contaminants such as ragweed pollen, dust from the museum in which the object was stored, washings from human hands, were all dextrorotatory, whereas the material from the Orgueil stone was found to be levorotatory. Of course it is possible that some biological contaminants consumed the dextrorotatory compounds leaving the levorotatory compounds behind. If so, they did not leave behind any residues that have been identified in other respects (see the discussion of porphyrins and amino acids below). Recently, Hayatsu [1964a] studied extracts of Orgueil using material from the very center of a fairly large stone. He has been unable to find any evidence for optical activity. Hayatsu, however, did not use the exact chemical procedures used by the previous authors. Nagy has repeated part of Hayatsu's work exactly and has obtained quite different chemical results with respect to the separation of sulfur and has found materials that are levorotatory, as reported previously. If optical activity is indigenous to the object, it would be strong evidence for biological activity, but at the present time this cannot be regarded as certain because of Hayatsu's work and because of the possibility (as mentioned above) that biological organisms growing in the meteorite after it landed on Earth destroyed preferentially the one optical isomer and left the other. At present this problem is not conclusively settled. Nagy and Bitz [1963] showed by the use of infrared absorption spectra and gas chromatography that fatty acids were present in extracts from the Orgueil meteorite, and Hayatsu reports that he has confirmed this observa- tion. Recently, Or6 [1965] has extracted hydrocarbons from the Orgueil meterorite, studied them with a mass spectrometer, and finds that w-alkane hydrocarbons running from Ci5 to C^ are present. In the material from this meteorite he finds that the odd carbon-chain hydrocarbons are somewhat more abundant than the even carbon-chain hydrocarbons. Hodgson and Baker [1964], on the basis of spectroscopic analysis, chromatographic separation and the chemical properties, found pigments in the Orgueil meteorite that are indistinguishable from vanadyl porphyrin. They also found that chlorins, which are prominent in all recent biological deposits, were not present in their samples. Incidentally, two of these samples were identical with those in which Nagy and his colleagues found optical activity. It is their conclusion that the porphyrin is indigenous and that it is not of recent origin. The Soret band in the ultraviolet is similar to such bands found in ancient terrestrial sediments. The wavelength of the Soret band changed with the solvent in which it was dissolved in the same way as the vanadyl porphyrin band does. The material from the meteorite

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Biological Materials in Carbonaceous Chondrites 121 was extracted in the same chemical fraction as is the porphyrin from terres- trial materials. Moreover, one sample that had been used by Nagy for other purposes did not show the presence of such a vanadyl porphyrin band. Subsequent to this, Hodgson and Baker subjected a non-terrestrial sample containing vanadyl porphyrin to the same chemical procedures as used by Nagy and they found that the vanadyl porphyrin band disappeared. It should be noted that terrestrial material containing porphyrin in any form is always regarded as being of biologic origin. Calvin [1961] reported the presence of cytosine-like substances in Murray, but Oro [1963] argued that this was a laboratory contaminant. Calvin maintains that this is incorrect and the matter has not been settled. Hayatsu [19646] reported the finding of adenine and guanine in Orgueil and thought he found a uracil type of compound. These are the four bases of RNA and the Watson-Crick model requires that the base pairs are adenine-uracil and guanine-cytosine. The presence of these bases in terres- trial material would lead to a strong presumption that it had been pro- duced by biological systems. Other nitrogen compounds were reported by Hayatsu and he argues that this is evidence for a non-biological origin, but it should be noted that in terrestrial material of biological origin many com- pounds are present that, so far as is known, have no biological significance. This is therefore not a proof of the material being of abiologic origin. If the Orgueil meteorite had become contaminated, by bacteria or other biological organisms, during the time that it has been on the Earth, one would expect to find the remnants of amino acids. Calvin [1961] and Briggs [1961] failed to detect amino acids, and recently Hamilton [1964], using very careful techniques, has also been unable to find these. Kaplan et al., [1963] and Vallentine [1965] reported the presence of amino acids in Orgueil, but Kaplan et al. concluded that they were due to contamination of their sample and reported that the material from which these were extracted developed more amino acids in time, indicating definite con- tamination. Amino acids might be preserved for long periods of time if suitably protected (Abelson [1956] found amino acids inside cretaceous fossil shells), but might very well not be preserved for the long time that would be indicated if they were indigenous to the meteorite. Their absence indicates that the samples of Calvin, Briggs, and Hamilton were not con- taminated with present-day organisms, and hence that some samples, at least, are not so contaminated. These various lines of evidence are not entirely satisfactory and are certainly not conclusive. Surely if such material were of terrestrial origin very little doubt would be expressed about its biological origin. But we believe that life evolved from inanimate matter on the Earth. The initial stages of the evolution of life may have occurred in other places. Either the

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122 THE COSMIC SETTING materials reported to be in the carbonaceous chondrites are of biotic origin, or they are materials of abiotic origin that are very suggestive of compounds of biotic origin. It would appear that the compounds that have been reported would indicate that at least considerable progress toward the evolution of life had occurred. The simpler fossil-like objects that are present in these materials certainly look much like micro-fossils and some of them are securely embedded in the matrix of the meteorite. The diffi- culties in deciding whether material of this kind is of biotic or abiotic origin suggest strongly that similar difficulties may be expected to occur if material is returned to us from the Moon or from the asteroidal belt or from Mars. Evidence of optical activity would probably be most definitive, but it is again difficult to exclude completely the possibility of contaminants. As indicated in the introduction to this discussion, it seems very difficult to believe that biological material could have evolved in the asteroidal belt and that we could have received samples of this material without at the same time receiving some meteorites that showed some sorting of minerals due to running water. In particular, it is difficult to understand how life could have evolved and been deposited in material that is so primitive as is suggested by the inorganic composition of the Type 1 and Type 2 carbon- aceous chondrites. It is the contention of Urey that the whole situation is consistent only with the hypothesis that life evolved on one object and con- taminated for a brief period of time a more primitive object, and he sug- gested that the Earth was the object on which life evolved, and that the Moon may have been the primitive object contaminated by material of this kind. Today, some three years later, he maintains the same position. If it can be shown that the meteorites do not come from the Moon, it is exceed- ingly difficult to understand all the circumstances that would be consistent with life appearing in the carbonaceous chondrites. It should be noted that we do not know what the composition of the surface of the Moon is. Evidence secured so far is not conclusive in regard to the questions discussed here. Certain features of the Ranger VII photo- graphs are not inconsistent with fragmented material filling the mare basin where the spacecraft landed. These pictures are not conclusive in a positive sense either. It seems likely that the chemical analyses planned for the Surveyor program may give indications of a positive character in regard to this question, but probably the Apollo project must be successfully carried out in order to get definite answers. That the Moon may have been contaminated from the Earth is a pos- sibility. Two proposals for the origin of the Moon are considered seriously today. The first proposal is that the Moon escaped from the Earth some time during its early history when metallic iron, originally assumed to be distributed throughout the Earth, sank catastrophically to form the core,

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Biological Materials in Carbonaceous Chondrites 123 thus increasing the angular velocity of rotation. Most students of the subject do not subscribe to this view as a reasonable possibility. But if it did occur, the Moon would surely be contaminated with primitive oceans of the Earth and with any biological material that was present at that time. On the other hand, if the Moon was captured by the Earth, then it is a reasonable proposal that many moons were about and possibly a three- body perturbation enabled the Earth to capture the Moon. But if objects of this kind were about, they would be captured by the Earth with very violent processes in which material from the Earth may have even been thrown to great distances and possibly completely away from the Earth- Moon system. In such processes some capture of material from the Moon might occur, and if only small amounts of biological material escaped high temperature and other destructive processes and were captured by the Moon, they would rapidly multiply in any temporary seas and again con- taminate the Moon. It is possible, and many think certain, that the maria of the Moon have not been subjected to water in this way. If this can be proved, then of course the carbonaceous chondrites cannot come from the Moon. Questions of this kind can be answered only by the much more ex- tensive studies of the Moon that are being planned by the National Aero- nautics and Space Administration. REFERENCES Abelson, P. H. (1956) Sci. Amer. 195, 83. Anders, E. (1963) Origin of the Solar System, Academic Press, New York. Anders, E., and Fitch, F. W. (1962) Science 138, 1392. Berthelot, M. (1868) Compt. rend. 67, 849. Berzelius, J. J. (1834) Ann. Phys. Chem. 33, 113. Briggs, M. H. (1961) Nature 191, 1137. Calvin, M. (1961) Chem. Eng. News 39, 21, 96. Cloez, S. (1864) Compt. rend. 59, 37. DuFresne, E., and Anders, E. (1962) Geochim. Cosmochim. Acta 26, 1085. Hamilton, P. B. (1964) Unpublished data. Hayatsu, R. (1964a) Preprint. Hayatsu, R. (19646) Science 146, 1291. Hodgson, G. W., and Baker, B. L. (1964) Nature 202, 125. Kaplan, I. R., Degens, E. T., and Renter, J. H. (1963) Geochim. Cosmochim. Acta 27, 805. Mason, B. (1962) Meteorites, John Wiley & Sons. New York. Mueller, G. (1953) Geochim. Cosmochim. Acta4, 1. Nagy, B., Claus, G., and Hennessy, D. J. (1962) Nature 193, 1129. Nagy, B., and Bitz, M. C. (1963) Arch. Biochem. Biophys. 101, 240. Nagy, B., Fredriksson, K., Urey, H. C., Claus, G., Andersen, C. A., and Percy, J. (1963) Nature 198, 121.

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124 THE COSMIC SETTING Nagy, B., Meinschein, W. G., and Hennessy, D. J. (1961) Ann. N. Y. Acad. Sci. 93, 25. Nagy, B., Murphy, M. T. J., Modzeleski, V. E., Rouser, G., Claus, G., Hennessy, D. J., Colombo, U., and Gazzarrini, F. (1964) Nature 202, 228. Or6, J. (1963) Nature 197, 756. Or6, J. (1965) Private communication. Staplin, F. L. (1962) Micropaleontol. 8, 343. Timofejew, B. W. (1963) Grana. Polynol. 4, 92. Urey, H. C. (1962) Nature 193, 1119. Vallentine, J. R. (1965) In press. Wfik, H. B. (1956) Geochim. Cosmochim. Acta 9, 279. Wohler, M. F., and Homes, M. (1859) Sitzber. Akad. Wiss. Wien, Math- Nature. Kl. 34, 7.

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PART IV RECOGNITION OF LIFE AND SOME TERRESTRIAL PRECEDENTS

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