Ir'ARUS 19, 341-346 (1973) Directed Panspermia I?. H. C. CRICK Medical Research Council, Laboratwy of Molecular Biology, Hills Road, Cambridge, England AND L. E. ORGEL The Salk Institute for Biological Studies, P.O. Box 1809, San Diego, California 92112 Received June 22, 1972; revised December 20, 1972 It now seems unlikely that extraterrestrial living organisms could have reached the earth either as spores driven by the radiation pressure from another star or as living organisms imbedded in a meteorite. As an alternative to these nineteenth-century mechanisms, we have considered Directed Panspermia, the theory that organisms were deliberately transmitted to the earth by intelligent beings on another planet. We conclude that it is possible that life reached the earth in this way, but that the scientific evidence is inadequate at the present time to say anything about the probability. We draw attention to the kinds of evidence that might throw additional light on the topic. INTRODUCTION It was not until the middle of the nine- ~~enth century that Pasteur and Tyndall completed the demonstration that spon- t ;\ueous generation is not occurring on the IS&h nowadays. Darwin and a number of o(,her biologists concluded that life must ll;l.ve evolved here long ago when condi- tions were more favourable. A number of scientists, however, drew a quite different c,onclusion. They supposed that if life does rtot evolve from terrestrial nonliving matter nowadays, it may never have done so. Hence, they argued, life reached the (*;Lrth as an "infection" from another I'lanet (Oparin, 1957). Arrhenius (1908) proposed that spores It;Ld been driven here by the pressure of i.he light from the central star of another IJanetary system. His theory is known as I'anspermia. Kelvin suggested that the first organisms reached the Earth in a tneteorite. Neither of these theories is :~bsurd, but both can be subjected to severe c:dticism. Sagan (Shklovski and Sagan, 1966; Sagan and Whitehall, 1973) has shown that any known type of radiation- resistant spore would receive so large a dose of radiation during its journey to the Earth from another Solar System that it would be extremely unlikely to remain viable. The probability that suf- ficiently massive objects escape from a Solar System and arrive on the planet of another one is considered to be so small that it is unlikely that a single meteorite of extrasolar origin has ever reached the surface of the Earth (Sagan, private communication). These arguments may not be conclusive, but they argue against the "infective" theories of the origins of life that were proposed in the nineteenth century. It has also been argued that "infective" theories of the origins of terrestrial life should be rejected because they do no more than transfer the problem of origins to another planet. This view is mistaken; the historical facts are important in their own right. For all we know there may be other types of planet on which the origin of life a6 initio is greatly more probable than on our own. For example, such a planet may possess a mineral, or compound, of crucial catalytic importance, which is rare on (`opyright 0 1973 by Academic Press, Inc. AII rights of reproduction in any form reserved. Printed in Great Britain. 341 342 CRICKAND ORQEL Earth. It is thus important to know whether primitive organisms evolved here or whether they arrived here from some- where else. Here we reexamine this problem in the light of more recent biological and astronomical information. OUR PRESENT KNOWLEDGE OF THE GALAXY The local galactic system is estimated to be about 13 x 109yr old (See Metz, 1972). The first generation of stars, because they were formed from light elements, are unlikely to have been accompanied by planets. However, some second generation stars not unlike the Sun must have formed within 2 x 10gyr of the origin of the galaxy (Blaauw and Schmidt, 1965). Thus it is quite probable that planets not unlike the Earth existed as much as 6.5 x 10gyr before the formation of our own Solar System. We know that not much more than 4 x 10qyr elapsed between the appearance of life on the Earth (wherever it came from) and the development of our own tech- nological society. The time available makes it possible, therefore, that tech- nological societies existed elsewhere in the galaxy even before the formation of the Earth. We should, therefore, consider a new "infective" theory, namely that a q---------The Present Formation of Solar System Time Available for Primary Origin of Life, Development of Technology and Passage between Planets. Formation of First Earthlike Planets FIG. 1. An approximate time-scale for the events discussed in the paper. To simplify illustration the age of the galaxy has been some- what arbitrarily taken as 13 x 109yr. primitive form of life was deliberal(*i~ planted on the Earth by a technologiczr II! advanced society on another planet. Are there many planets which coul~l IM* infected with some chance of success? I I iii believed, though the evidence is weak ii1111 indirect, that in the galaxy many stars. 01 a size not dissimilar to our Sun, II:IVV planets, on a fair fraction of wl\ic% temperatures are suitable for a form of' Ii lb based on carbon chemistry and licirli4 water, as ours is. Experimental studies of the production of organic chemicals UIIC Iftr prebiotic conditions make it seem likc~l> that a rich prebiotic soup accumulates ot I rl high proportion of such Earthlike plaml H. Unfortunately, we know next to nothi~~g about the probability that life evol\r(*ti within a few billion years in such a so I I 11, either on our own special Earth, or still 1~ on other Earthlike planets. If the probability that life evolves in :L suitable environment is low we may 111: able to prove that we are likely to be aloncl in the galaxy (Universe). If it is high ttw galaxy may be pullulating with life III' many different forms. At the moment M'V have no means at all of knowing which 01' these alternatives is correct. We are thus free to postulate that there have been (an I I still are) many places in the galaxy where! life could exist but that, in at least ;L fraction of them, after several billion years the chemical systems had not evolved t,o the point of self-replication and naturul selection. Such planets, if they do exist, would form an excellent breeding grountl for external microorganisms. Note tha,b because many if not all such planets woulcl have a reducing atmosphere they woulcl not be very hospitable to the higher forms of life as we know them on Earth. OURPROPOSAL The possibility that terrestrial life de- rives from the deliberate activity of an extraterrestrial society has often been considered in science fiction and more or less light-heartedly in a number of soien- tifio papers. For example, Gold (1960) has suggested that we might have evolved from the microorganisms inadvertently DIRECTED PANSPERMIA 343 left behind by some previous visitors from another planet (for example, in their garbage). Here we wish to examine a very specific form of Directed Panspermia. ( `ould life have started on Earth as a result of infection by microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship? To show that this is not totally implausible we shall use the theorem of detailed cosmic reversibility ; if we are capable of infecting uti as yet lifeless extrasolar planet, then, given that the time was available, another lrchnological society might well have ihfected our planet when it was still lifeless. THE PROPOSED SPACESHIP The spaceship would carry large samples of a number of microorganisms, each h;bving different but simple nutritional rckquirements, for example blue-green ;\lgae, which could grow on CO, and water itI "sunlight." A payload of IOOOkg might IN: made up of 10 samples each containing l~`~microorganisms, or 100 samples each ot' lOI microorganisms. It would not be necessary to accelerate the spaceship to extremely high velocities, since its time of arrival would not be important. The radius of our galaxy is &out lo5 light years, so we could infect most planets in the galaxy within lOa yr by cleans of a spaceship traveling at only olle-thousandth of the velocity of light. Several thousand stars are within a hundred light years of the Earth and could IN: reached within as little as a million pars by a spaceship travelling at only rl(),OOOmph, or within 10,OOOyr if a speed of one-hundredth of that of light were possible. The technology required to carry out sllch an act of interstellar pollution is not available at the present time. However, it stems likely that the improvements in astronomical techniques will permit the I~~cation of extrasolar planets within the Istxt few decades. Similarly, the problem of sending spaceships to other stars, at velocities low compared with that of light, should not prove insoluble once workable nuclear engines are available. This again is likely to be within a few decades. The most difficult problem would be presented by the long flight times ; it is not clear how long it will be before we can build components that would survive in space for periods of thousands or millions of years. Although there are some technological problems associated with the distribution of the microorganisms in viable form after a long journey through space, `none of them seems insuperable. Some radiation protection could be provided during the journey. Suitable packaging should guarantee that small samples, including some viable organisms, would be widely distributed. The question of how long microorganisms, and in particular bacterial spores, could survive in a spaceship has been considered in a preliminary way by Sneath (1962). He concludes "that life could probably be preserved for periods of more than a million years if suitably protected and maintained at temperatures close to absolute zero." Sagan (1960) has given a comparable estimate of the effects of radiation damage. We conclude that within the foreseeable future we could, if we wished, infect another planet, and hence that it is not out of the question that our planet was infected. We can in fact go further than this. It may be possible in the future to send either mice or men or elaborate instruments to the planets of other Solar Systems (as so often described in science fiction) but a rocket carrying microorganisms will always have a much greater effective range and so be advantageous if the sole aim is to spread life. This is true for several reasons. The conditions on many planets are likely to favour microorganisms rather than higher organisms. Because of their ex- tremely small size vast numbers of micro- organisms can be carried, so much more wastage can be accepted. The ability of microorganisms to survive, without special equipment, both storage for very long periods at low temperatures and also an abrupt change back to room temperatures is also a great advantage. Whatever the potential range for infection by other organisms, microorganisms can almost 344 CRICKANDORUEL certainly be sent further and probably much further. It should be noted that most of the earliest "fossils" so far recognized are somewhat similar to our present bacteria or blue-green algae. They occur in cherts of various kinds and are estimated to be up to 3 x 109yr old. This makes it improbable that the Earth was ever infected merely by higher organisms. MOTIVATION Next we must ask what motive we might have for polluting other planets. Since-we would not derive any direct advantage from such a programme, presumably it would be carried through either as a demonstration of technological capability or, more probably, through some form of missionary zeal. It seems unlikely that we would deliber- ately send terrestrial organisms to planets that we believed might already be in- habited. However, in view of the precarious situation on Earth, we might well be tempted to infect other planets if we be- came convinced that we were alone in the galaxy (Universe). * As we have already explained we cannot at the moment estimate the probability of this. The hypothetical senders on another planet may have been able to prove that they were likely to be alone, and to remain so, or they may have reached this conclusion mis- takenly. In either case, if they resembled us psychologically, their motivation for pol- luting the galaxy would be strong, if they believed that all or even the great majority of inhabitable planets could be given life by Directed Panspermia. The psychology of extraterrestrial so- cieties is no better understood than terrestrial psychology. It is entirely poss- ible that extraterrestrial societies might infect other planets for quite different reasons than those we have suggested. Alternatively, they might be less tempted than we would be, even if they thought 1 In a somewhat different context the seeding of Venus and other solar planets has been suggested by C. Sagan (1961), and T. Gold, private communication. that they were alone. The arguments givcrl above, together with the principle 01 cosmic reversibility, demonstrate the ponx- ibility that we have been infected, but (lo not enable us to estimate the probability. POSSIBLEBIOLOGICALEVIDENCE Infective theories of the origins 01 terrestrial life could be taken rnurcl seriously if they explained aspects of bicj- chemistry or biology that are otherwinct difficult to understand. We do not have* any strong arguments of this kind, 1~1, there are two weak facts that could 1118 relevant. The chemical composition of livitl:: organisms must reflect to some extent i,tI(' composition of the environment in whid) they evolved. Thus the presence in livi~q organisms of elements that are extremcl! rare on the Earth might indicate that lifi: in extraterrestrial in origin. Molybdenum in an essential trace element that plays iI II important role in many enzymatic I'(' actions, while chromium and nickel :IIY relatively unimportant in biochemists>-. The abundance of chromium, nickel, at111 molybdenum on the Earth are 0.20, 3. I (i, and 0.02%, respectively. We cannot COII. elude anything from this single example, since molybdenum may be irreplaceable i I I some essential reaction-nitrogen fixatiotl, for example. However, if it could be show I I that the elements represented in terrestriill living organisms corelate closely with thos~s that are abundant in some class of star molybdenum stars, for example-we mig I I I look more sympathetically at "infective*" theories. Our second example is the genetic codv. Several orthodox explanations of ~JIV universality of the genetic code can 1~ suggested, but none is generally acceptcbtl to be completely convincing. It is a lit& surprising that organisms with sornewl~;~l different codes do not coexist. The 1111 i. versality of the code follows natural 1,~ from an "infective" theory of the origins 01 life. Life on Earth would represent a clortr~ derived from a single extraterrestri:ll organism. Even if many codes WCI'O represented at the primary site where lil'c: DIRECTED PANSPERMIA 345 began, only a single one might have oper- rued in the organisms used to infect the l&uth. CONCLUSION ln summary, there is adequate time for tcbchnological society to have evolved twice in succession. The places in the galaxy wltere life could start, if seeded, are lrrobably very numerous. We can foresee I It& we ourselves will be able to construct ro(nkets with sufficient range, delivery ftltility, and surviving payload if micro- ctrzanisms are used. Thus the idea of Di- rc*c:ted Panspermia cannot at the moment IN, rejected by any simple argument. It is ra(lically different from the idea that life ~larted here ab in& without infection I`l.orn elsewhere. We have thus two sharply clil`ferent theories of the origin of life on Lath. Can we choose between them? At the moment it seems that the experi- rtlcutal evidence is too feeble to make this ~liscrimination. It is difficult to avoid a lN>rsonal prejudice, one way or the other, Illrt such prejudices find no scientific Nl~l'port of any weight. It is thus important 11~;lt. both theories should be followed up. \\`ork on the supposed terrestrial origin of lifib is in progress in many laboratories. hs far as Directed Panspermia is concerned !v(' can suggest several rather diverse lines III` research. The arguments we have employed here II~I~,. of necessity, somewhat sketchy. Thus 1111~ detailed design of a long-range space- whip would be worth a careful feasibility HI 11tly. The spaceship must clearly be able I.II home on a star, for an object with any rrppreciable velocity, if dispatched in a random direction, would in almost all VIIXS pass right through the galaxy and otrf the other side. It must probably have 10 clecelerate as it approached the star, in I WC I er to allow the safe delivery of the pay- IlNk(l. The packets of microorganisms must IN, made and dispersed in such a way that ~-lrc*.v can survive the entry at high velocity illi o the atmosphere of the planet, and yet IN* ;Lble to dissolve in the oceans. Many 1114ul feasibility studies could be carried OIII on the engineering points involved. On the biological side we lack precise information concerning the life-time of microorganisms held at very low tempera- tures while traveling through space at relatively high velocities. The rocket would presumably be coasting most of the time so the convenient temperature might approximate to that of space. How serious is radiation damage, given a certain degree of shielding'? How many distinct types of organism should be sent and which should they be! Should they collectively be capable of nitrogen fixation, oxidative phosphorylation and photosynthesis? Al- though many "soups" have been produced artifically in the laboratory, following the pioneer experiments of Miller, as far as we know no careful study has been made to determine which present-day organisms would grow well in them under primitive Earth conditions. At the same time present-day organisms should be carefully scrutinized to see if they still bear any vestigial traces of extra- terrestrial origin. We have already men- tioned the uniformity of the genetic code and the anomalous abundance of molyb- denum. These facts amount to very little by themselves but as already stated there may be other as yet unsuspected features which, taken together, might point to a special type of planet as the home of our ancestors. These enquiries are not trivial, for if successful they could lead to others which would touch us more closely. Are the senders or their descendants still alive? Or have the hazards of 4 billion years been too much for them? Has their star inexorably warmed up and frizzled them, or were they able to color&e a different Solar System with a short-range spaceship? Have they perhaps destroyed themselves, either by too much aggression or too little? The difficulties of placing any form of life on another planetary system are so great that we are unlikely to be their sole descendants. Presumably they would have made many attempts to infect the galaxy. If the range of their rockets were small this might suggest that we have cousins on planets which are not too distant. Perhaps the galaxy is lifeless except for a local village, of which we are one member. 346 CRICKANDORBEL One further point deserves emphasis. We feel strongly that under no circumstances should we risk infecting other planets at the present time. It would be wise to wait until we know far more about the prob- ability of the development of life on extra- solar planets before causing terrestrial organisms to escape from the solar system. ACKNOWLEDGMENTS We are indebted to the organisers of a meeting on Communication with Extraterrestrial Intel- ligence, held at Byurakan Observatory in Soviet Armenia in September 1971, which crystallized our ideas about Panspermia. We thank Drs. Freeman Dyson, Tommy Gold, and Carl Sagan for discussion and important com- ments on our argument. REFERENCES ARRHENIUS, S. (1908). "Worlds in the Making." Harper and Row, New York. BLAA~~, A., AND SCHMIDT, M. (1965). "G&c& Structure." University of Chicago PTQHN, Chicago. GOLD, T. (May 1960). "Cosmic Garbage" "i\ir Force and Space Digest", p. 65. METZ, W. D. (1972). Reporting a speech by Allan Sandage. Science 178, 600. OPARJ.N, A. I. (1957). "The Origins of Life (III Earth." Academic Press, New York, N4.w York (gives a general discussion on 1'11n. spermia). SAGAN, C. (1960). Biological contamination ef the Moon. Proc. Nat. Acad. Sci. 46, 396. SAGAN, C. (1961). The planet Venus. Science 133. 849. SAGAN, c., AND WHITEHALL, L. (1973). To ~I,I submitted to Icam. SHICLOVSKII, I. S., AND SAGAN, C. (1966 1111tl 1967). "Intelligent Life in the Universlt." p. 207. Holden-Day, San Francisco and 111~11 Publishing Co., New York. SNEATH, P. H. A. (1962). Longevity of rnicnl. organisms. Na;ture (London) 195, 643.