Note: Because some of the information in this article may be outdated, it has been archived.
1. Introductory remarks
A likely paradigm shift from a warm little terrestrial pond to an extraterrestrial source of microbial life was signalled in August 1996 by D.S. McKay et al.[ who announced the “discovery” of microbial fossils within a meteorite (ALH84001) originating from the planet Mars. Although that claim itself has since been challenged, the impact of the initial announcement has not diminished in the intervening years. Astrobiology has suddenly emerged as a new scientific discipline and several international organisations including NASA have expressed their commitment to research in this area. Tying an impending paradigm shift in a roundabout way to Mars can be seen to have been politically astute, because by tradition the public is tuned to the concept of life on Mars, tuned at any rate from the time of H.G. Wells. The point being that the world was ready for the shift and this seemed the smoothest way of bringing it about.
Ideas concerning the existence of life outside Earth have spanned many centuries and many different cultures. In most ancient philosophies of the orient, for instance in Vedic and Buddhist writings, the cosmic character of life is taken for granted. It is regarded as an inherent property of an infinite, timeless, and eternal Universe. A similar concept made only a brief appearance in Western Philosophy. The idea of living seeds or “spermata” being ever present in the cosmos was posited by the pre-Socratic philosopher Anaxoragas as early as the 5th century BC, but soon came to be replaced by an Aristotelean Earth-centred world view.
In modern times from the late 1970’s onwards, we2 have assembled and analysed evidence from astronomy, geology and biology that have a bearing on the cosmic nature of life. In the present article we shall consider these ideas in an historical context, and proceed to argue that a major paradigm shift is well on the way to being accomplished.
2. Early history of panspermia
Until the late 19th century panspermia meant the passage of organisms through Earth’s own atmosphere, not an incidence from outside Earth. In this form it seems to have been used first by the Abbee Lazzaro Spallanzoni (1729-99). But almost a century before that, Francesco Redi had carried out what can be seen as a classic experiment in the subject. He had shown that maggots appear in decaying meat only when the meat is exposed to air, inferring that whatever it was that gave rise to the maggots must have travelled to the meat through the air.
A very long wait until the 1860’s then ensued, until Louis Pasteur’s3 experiments on the souring of milk and the fermentation of wine showed that similar results occurred when the air-borne agents were bacteria, replicating as bacteria but not producing a visible organism like maggots. The world then permitted Pasteur to get away with a huge generalisation, and honoured him greatly both at the time and in history for it. Because by then the world was anxious to be done with the old Aristotelian concept of life emerging from the mixing of warm earth and morning dew. The same old concept was to arise again in the mid-twenties of the past century, however, but with a different name. Instead of Aristotle’s warm earth and morning dew it became “a warm organic soup.”
Pasteur’s far-ranging generalisation implied that each generation of every plant or animal is preceded by a generation of the same plant or animal. This view was taken up enthusiastically by others, particularly by physicists, among them John Tyndall, who lectured frequently on the London scene. The editorial columns of the newly established Nature (e.g., issue of January 27, 1870) objected with some passion to Tyndall’s Friday evening discourse at the Royal Institution on January 21, 1870. Behind the objection was the realisation that were Pasteur’s paradigm taken to be strictly true, the origin of life would need to be external to Earth. For if life had no spontaneous origin, it would be possible to follow any animal generation-by-generation back to a time before Earth existed, the origin being therefore required outside Earth.
This was put in remarkably clear terms in 1874 by the German physicist Hermann von Helmholtz4:
“It appears to me to be a fully correct scientific procedure, if all our attempts fail to cause the production of organisms from non-living matter, to raise the question whether life has ever arisen, whether it is not just as old as matter itself, and whether seeds have not been carried from one planet to another and have developed everywhere where they have fallen on fertile soil….”
Sir William Thomson (Lord Kelvin)5 said of Pasteur’s paradigm: “Dead matter cannot become living without coming under the influence of matter previously alive. This seems to me as sure a teaching of science as the law of gravitation…”
So if life had preceded Earth, how had it arrived here and where had it come from? Earlier in the 19th century the German physician R.E. Richter6 had suggested that living cells might travel from planet to planet inside meteorites. Inadequacies in Richter’s dynamics permitted J. Zollner in the 1870’s to raise objections, eagerly seized on by orthodox opinion. But Kelvin’s superior knowledge of dynamics allowed him to see that there was nothing to Zollner’s objections, in particular that evaporation from the outside of a large meteorite keeps its interior cool, thereby reasserting the possibility of organisms being carried from planet to planet inside meteorites. In his presidential address to the 1881 meeting of the British Association, Kelvin drew the following remarkable picture:
“When two great masses come into collision in space, it is certain that a large part of each is melted, but it seems also quite certain that in many cases a large quantity of debris must be shot forth in all directions, much of which may have experienced no greater violence than individual pieces of rock experience in a landslip or in blasting by gunpowder. Should the time when this earth comes into collision with another body, comparable in dimensions to itself, be when it is still clothed as at present with vegetation, many great and small fragments carrying seeds of living plants and animals would undoubtedly be scattered through space. Hence, and because we all confidently believe that there are at present, and have been from time immemorial, many worlds of life besides our own, we must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instant no life existed upon Earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation.”
Thus almost 120 years ago the ideas that have recently come to the forefront of scientific discussion were already well known. Demonstrating yet again the oft-repeated lesson that before advancing new theories it is as well to learn what has gone before.
It is a weakness of science that unless an idea has a means of advancing itself through observation or experiment it stultifies, almost regardless of how good the idea may be in itself. Unfortunately there was no way at that date, 1881, whereby observation or experiment could be brought seriously to bear on Kelvin’s formulation of panspermia.
3. Enter Svante Arrhenius
The next facet in the story is associated with the Swedish Chemist and Nobel laureate Svante Arrhenius, whose book Worlds in the Making8 appeared in English in 1908. Arrhenius’ contribution rested on two main points, one good, and one not so good. The good point was that microorganisms possess unearthly properties, properties that cannot be explained by natural selection against a terrestrial environment. Arrhenius demonstrated this by taking seeds down to temperatures close to zero degrees Kelvin, and then showing their viability when reheated with sufficient care. Many other ‘unworldly’ properties have come to light over the years to which we shall have occasion to refer below.
The not-so-good point was that Arrhenius conceived of microorganisms travelling individually and unprotected through the galaxy from star system to star system. He noticed that organisms with critical dimensions of 1 micron or less are related in their sizes to the typical radiation wavelengths from dwarf stars in such a way that radiation (light) pressure can have the effect of dispersing these particles throughout the galaxy. But space-travelling individual bacteria would be susceptible to deactivation and damage from the ultraviolet light of stars, a fact already known in the first decades of the century. P. Becquerel9 mounted an attack on Arrhenius’ views in 1924, on the basis of possible ultraviolet damage and this attack was widely accepted and repeated many times since. But several other facts of relevance to this problem were not known at the time.
4. Extreme hardihood of bacteria
On the whole, microbiological research of the past 15 years has shown that bacteria and other microorganisms are remarkably space-hardy, far more than Arrhenius may have ever imagined.10 Microorganisms known as thermophiles and hyperthermophiles are present at temperatures above boiling point in oceanic thermal vents. Entire ecologies of microorganisms are present in the frozen wastes of the Antarctic ices. A formidable total mass of microbes exists in the depths of Earth’s crust, some 8 kilometres below the surface, greater than the biomass at the surface11. A species of phototropic sulfur bacterium has been recently recovered from the Black Sea that can perform photosynthesis at exceedingly low light levels, approaching near total darkness12. There are bacteria (e.g., Deinococcus radiodurans) that thrive in the cores of nuclear reactors. Such bacteria perform the amazing feat of using an enzyme system to repair DNA damage, in cases where it is estimated that the DNA experienced as many as a million breaks in its helical structure.
There is scarcely any set of conditions prevailing on Earth, no matter how extreme that is incapable of harbouring some type of microbial life. Under space conditions, microorganisms are very easily protected against ultraviolet damage. A carbonaceous coating of only a few microns thick provides essentially a total shielding against ultraviolet light, and there are several modern experiments that have demonstrated precisely that. Next, let’s note that many types of microorganisms are not really killed by ultraviolet light, they are only deactivated. And this happens through a shifting of certain chemical bonds contained in the genetic structures of the organisms, without destroying genetic arrangements. This also permits the original properties to be recovered once the ultraviolet radiation has been shut off. Furthermore, we know that microorganisms that are normally sensitive to ultraviolet light can, through repeated exposures, be made just as insensitive as the more resistant kinds — yet another unearthly property.
Many other tests of the space hardihood of bacteria and viruses have recently been made. In one such test the bacterium Bacillus subtlis was exposed for nearly six years in space aboard NASA’s Long Duration Exposure Facility and was found to retain viability. In other experiments currently in progress13 bacteria impacting sand with speeds of 0.3-0.8km/s have been shown to survive. These speeds are in comfortable excess of the terminal velocities of micrometre sized particles following atmospheric braking. Experiments such as these show unequivocally that the transfer of microbes from a comet to Earth can take place without a significant loss of viability.
5. Coalification of bacteria and interstellar organic molecules
Notwithstanding the remarks of the previous section bacteria which have no protective coatings and which are continually exposed to cosmic rays and to the background of starlight in open regions of interstellar space, in the so-called “diffuse” clouds, must be subject to degradation and eventual destruction. The process would be analogous to coalification and graphitization of living material. First, microorganisms expelled from any galactic source into unshielded regions of interstellar space will become deactivated. Then, the deactivated particles will be subject to steadily increasing degradation, ending in a release of free organic molecules and polymers, similar to what astronomers have been discovering since the late 1960’s. The ultimate end product will be a transformation of a viable bacterium to a submicron-sized particle of coal.
Today an impressive array of interstellar molecules has been detected and among the list are a host of hydrocarbons, polyaromatic hydrocarbons, the amino acid glycine, vinegar and the sugar glycoaldehyde14. Such organic molecules that pervade interstellar clouds make up a considerable fraction of the available galactic carbon. Theories of how interstellar organic molecules might form via non-biological processes are still in their infancy and, in terms of explaining the available facts, they leave much to be desired.
The overwhelming bulk of organic matter on Earth is indisputably derived from biology, much of it being degradation products of biology. Might not the same processes operate in the case of interstellar organic molecules? The polyaromatic hydrocarbons that are so abundant in the cosmos could have a similar origin to the organic pollutants that choke us in our major cities - products of degradation of biology, biologically generated fossil fuels in the urban case, cosmic microbiology in the interstellar clouds. The theory of cosmic panspermia that we have proposed leads us to argue that interstellar space could be a graveyard of cosmic life as well as its cradle. Only the minutest fraction (less than one part in a trillion) of the interstellar bacteria needs to retain viability, in dense shielded cloudlets of space, for panspermia to hold sway. Common sense dictates that this survival rate is unavoidable.
6. Interstellar dust: from inorganic models to organic dust
Our own personal encounter with theories of cosmic life began with attempts to understand the nature of cosmic dust, dust that is clearly evident in the form of conspicuous dark clouds in the Milky Way such as in Figure 1.
Figure 1: Interstellar dust clouds
We embarked on this work in the 1960’s, and from then on the scope of a project that started as a simple astronomical investigation expanded to proportions that could not have been anticipated. Cosmic dust grains populate the vast open spaces between stars of the Milky Way, showing up as a cosmic fog, dense enough in many directions to blot out the light of distant stars. Remarkably, the dust grains in these clouds appear to be much the same in their physical and chemical characteristics in whichever direction one looks. They are of a size that would be typical for a bacterium, a micrometre or less.
A fact that impressed us from the outset was that the total mass of interstellar dust in the galaxy is as large as it possibly can be if all (or nearly all) the available carbon, nitrogen and oxygen in interstellar space is condensed in the grains. The amount is about three times too large for the grains to be mainly made up of the next commonest elements, magnesium and silicon, although magnesium and silicon could of course be a component of the particles, as would hydrogen, and also many less common elements in comparatively trace quantities15.
If one now asks the question “Of what precisely are the cosmic dust grains made?” a number of inorganic molecules composed of C,N,O in combination with hydrogen present themselves as possible candidates. These include water ice, carbon dioxide, methane, ammonia, all these materials being easily condensable into solids at temperatures typically of about 20-50 degrees Kelvin, which is the average temperature of the interstellar dust grains. During the decade starting from the early 1960’s we studied the properties of a wide range of inorganic grain models, comparing their electromagnetic properties against the formidable number of observations that were beginning to emerge. Such models stubbornly refused to fit the available data to anything like the precision that was required. The correspondences between predictions for assemblies of inorganic particles and the observations could be lifted to a certain moderately acceptable level of precision but never beyond that, no matter how hard one tried16.
It was a milestone in our progress towards interstellar panspermia when one of us (NCW) realized that there is another very different class of materials that can be made from the same four commonest elements — C,N,O,H, namely organic materials, possibly of a polymeric type17. Of course there are a vast number of organic compositions that are possible, requiring a great number of further investigations. With our experience of the prevalence of biogenic terrestrial organics, it is fair to say that we had our eyes on a possible biological origin from the outset. By the mid-1970’s, astronomical observations were spanning a large range in wavelength, from 30 microns in the infrared, through the near infrared, into the visible spectrum, and further into the ultraviolet. So a satisfactory theory of the nature of grains had by now satisfied a large number of observational constraints.
In 1979 we stumbled on a result that led to many further discoveries, all pointing clearly in the direction of panspermia. When we examined the light scattering properties of freeze-dried bacterial particles (hollow organic grains) of the type one might expect to occur in space, a remarkable correspondence with astronomical data emerged. Such a precise correspondence was not found possible for any inorganic, non-biological grain model. Another piece of evidence that we had uncovered at about the same time was that a broad absorption feature in interstellar dust centred on the wavelength 2175A (which we originally attributed to graphite) matched a large class of aromatic molecules, quinoline and quinozoline being the first examples we investigated.
Flux from the galactic centre source GC-IRS7 obtained by D.A. Allen and D.T. Wickramasinghe (1981) compared with predictions for a bacterial grain model 25.
Perhaps the most surprising confirmation of the bacterial model followed the pioneering observations by David A. Allen and Dayal T. Wickramasinghe of a source of infrared radiation, GC-IRS7, located near the centre of our galaxy18. The spectrum of this source revealed a highly detailed absorption profile extending over the 2.9-3.8 micrometre wavelength region, indicative of combined CH, OH and NH stretching modes. A laboratory spectrum of the desiccated bacterium E. Coli, obtained by us some months earlier, together with a simple modeling procedure, provided an exceedingly close point-by-point match to astronomical data over the entire 2-4 micron waveband (See references in Hoyle and Wickramasinghe19). This agreement is shown in Figure 2. At this stage we realized that a large fraction of the interstellar dust was not merely hollow and organic, but it must spectroscopically be indistinguishable from freeze-dried bacterial material in combination with their degradation products. In our galaxy alone the total mass of this bacterial type material had to be truly enormous, weighing a formidable 1033 tonnes. Recent studies have shown that while the cosmic dust is largely dominated by the infrared spectral properties of freeze-dried bacteria, a sequence of degradation steps, from semi-anthracite to coals of varying grades clearly shows up under different astrophysical conditions. To attempt explanations of this sequence on the hypothesis of “coalifying” graphitic material (as is currently being done) involves a violation of basic thermodynamics and is unlikely to prove correct.
7. Replication properties of bacteria
By far the simplest way to produce such a vast quantity of small organic particles (with properties ranging from prestine bacteria to coals) and with sizes appropriate to bacteria is from a bacterial template. The power of bacterial replication is immense. Given appropriate conditions for replication, a typical doubling time for bacteria would be two to three hours. With a continuing supply of nutrients, a single initial bacterium would generate some 240 offspring in four days, yielding a culture with the size of a cube of sugar. Continuing for a further four days and the culture, now containing 280 bacteria would have the size of a village pond. Another four days and the resulting 2120 would have the scale of a large comet. Yet another four days and the resulting 2160 bacteria would be comparable in mass to a molecular cloud like the Orion Nebula. Add four more days, for a total of 20 days since the beginning of the process, and the bacterial mass would be that of a million galaxies. No abiotic process remotely matches this replication power of a biological template. Once the immense quantity of organic material in the interstellar material is appreciated, a biological origin for it becomes an almost inevitable conclusion.
8. Cometary panspermia
According to the theory we have developed, the sources of biological particles in interstellar clouds are comets. An individual comet is a rather insubstantial object. But our solar system possesses so many of them, perhaps more than a hundred billion of them, that in total mass they equal the combined masses of the outer planets Uranus and Neptune, about 1029 grams. If all the dwarf stars in our galaxy are similarly endowed with comets, then the total mass of all the comets in our galaxy, with its 1011 dwarf stars, turns out to be some 1040 grams, which is just the amount of all the interstellar organic particles that are present in dust clouds within the galaxy.
How would microorganisms be generated within comets, and then how could they get out of comets? We know as a matter of fact that comets do eject organic particles, typically at a rate of a million or more tons a day. This was what Comet Halley was observed to do on March 30-31, 1986. And Comet Halley went on doing just that, expelling organic particles in great bursts, for almost as long as it remained within observational range. The particles that were ejected in March 1986 were well placed to be observed in some detail. No direct tests for a biological connection had been planned, but infrared observations pointed unexpectedly in this direction. The infrared emission spectrum of dust from Comet Halley obtained by Dayal Wickramasinghe and David Allen20 in March 1986 matched precisely the laboratory spectrum of bacterial grains as shown in Figure 3. An independent analysis of dust impacting on mass spectrometers aboard the spacecraft Giotto also led to a complex organic composition, a composition that was fully consistent with the biological hypothesis. Broadly similar conclusions have been shown to be valid for other comets as well, in particular Comet Hyakutake and Comet Hale-Bopp. Thus one could conclude from the astronomical data that cometary particles, just like the interstellar particles, are spectroscopically identical to bacteria, existing in various combinations with their degradation products.
Emission by dust coma of Comet Halley observed by D.T. Wickramasinghe and D.A. Allen on March 31, 1986 (points) compared with bacterial models.
Cosmic amplification cycle of biology
The logical scheme for the operation of cometary panspermia is summarized in Figure 4. The dust in interstellar clouds must always contain the minutest fraction of bacteria (less than a trillionth) that retain viability despite the harsh radiation environment of space. This exceedingly modest requirement of survival is unlikely to be invalidated, so panspermia becomes inevitable. When a new star system (e.g. a solar system) forms from interstellar matter, comets condense in the cooler outer periphery as a prelude to planet formation. Each such comet incorporates, at the very least, a few billion viable bacterial cells, and these are quickly reactivated and begin to replicate in the warm interior regions of the comets, thus producing vast numbers of progeny. As a stellar or planetary system develops, comets that plunge from time to time into the inner regions of the system would release vast quantities of bacteria in the manner discussed earlier for our own solar system. Some of the evaporated bacterial material is returned into the interstellar medium. New stars and star systems form and the whole cycle continues, producing biologically processed material.
9. Microfossils in meteorites
In the mid-1960’s Claus and Nagy21 in collaboration with G. Claus, B. Nagy and D.L. Europa22 examined the Orgueil carbonaceous meteorite which fell in France in 1864, microscopically as well as spectroscopically. They claimed to find evidence of organic structures that were similar to fossilized microorganisms, algae in particular. The evidence included electron micrograph studies, which showed substructure within these so-called “cells.” Some of the structures resembled cell walls, cell nuclei, flagella-like structures, as well as constrictions in elongated objects to suggest cell division. If these “organized elements” were indeed microbial fossils, the question arises as to how such structures were included within carbonaceous meteorites. This question could not be satisfactorily answered in 1960, although with the wisdom of hindsight we could now say the answer was evident: carbonaceous chondrites, typified by Orgueil, represent the sedimented residue of comets that once contained microbial life thriving within subsurface pools. Carbonaceous chondrites can thus be thought of as fragments of biological comets that have been progressively stripped of volatiles, and within which sedimentation and compaction of microorganisms may have occurred over hundreds of orbits around the Sun.
Unfortunately many scientists were determined to stop a seemingly inevitable trend towards panspermia and they used these early claims of meteoritic microfossils23,24 to support their arguments. The tactic employed was to point to a very small number of alleged microfossils that were most likely to be terrestrial contaminants. This still left an overwhelming number of organic structures for which no satisfactory explanation could be offered.
In the early 1980’s the German paleontologist Hans D. Pflug25 reopened the issue of microbial fossils in carbonaceous meteorites. Pflug used techniques that were distinctly superior to those of Claus and his colleagues and found a profusion of organized elements comprised of organic matter in thin sections prepared from a sample of the Murchison meteorite. The method adopted by Pflug was to dissolve-out the bulk of the minerals present in a thin section of the meteorite using hydrofluoric acid, doing so in a way that permits the insoluble carbonaceous residue to settle with its original structures in tact. It was then possible to examine the residue in an electron microscope without disturbing the system from outside. The patterns that emerged were stunningly similar to certain types of terrestrial microorganisms. Scores of different morphologies turned up within the residues, many resembling known microbial species. It would seem that contamination could now be excluded by virtue of the techniques used. No convincing non-biological alternative to explain all the features were offered by the critics, although a statement that they were all “mineralogical artifacts” that somehow trapped organics from a surrounding medium came to be widely publicized. Despite all these criticisms, a renewed attempt to explore the question of microfossils in carbonaceous meteorites was undertaken in 1997 by R.B. Hoover of the NASA Marshal Space Flight Centre. The new work appears to corroborate Pflug’s earlier findings of microfossils in the deep interiors of carbonaceous chondrites.
10. The Mars meteorite ALH 84001
As mentioned at the beginning of this article the latest chapter in the exploration of panspermia was opened in August 1996 with studies of a 1.9kg meteorite, which is believed to have originated from Mars. The meteoriote ALH84001 is just one of a group of meteorites discovered in 1984 in Allan Hills, Antarctica, which is thought to have been blasted off the Martian surface by an asteroid or comet impact some 15 million years ago. The resulting ejecta orbited the sun until 13,000 years ago when it plunged into the Antarctic and remained buried there in ice until its discovery. The presumed Martian origin of these meteorites (also known as SNC meteorites) seems to have been confirmed by several independent studies.
A team of investigators from NASA and Stanford, led by NASA’s David S. McKay1, have found that within the meteorite ALH 84001 there are sub-micron sized carbonate globules around which complex organic molecules are deposited. As we have already noted these molecules, including polyaromatic hydrocarbons, are characteristic products of the degradation of bacteria. The most striking evidence shows up as strings of elongated structures that are similar to “nanobacteria” or “nanobes,” a class of terrestrial bacteria that has only recently been recognized and identified, one that represents perhaps the smallest of living forms.
McKay and his colleagues admit that their proposed identification involves a process of multi-factorial assessment. The totality of the available evidence, in their view, points to a microbial origin, although each single piece of evidence may be capable of more conservative interpretation. Many such interpretations have since been offered and consensus opinion seems to be veering cautiously towards rejecting rather than accepting the original McKay claims. The jury is still out and arguments rage concerning many issues, for instance the temperature at which the carbonate globules condensed, and whether the putative biological structures could survive these temperatures27. McKay and his colleagues28 still vigorously defend their original contention and are advancing even stronger arguments in its support. The debate seems destined to continue, however, perhaps until the day when Martian samples are returned to Earth.
If the explanation of McKay et al is eventually upheld, the deposition of the microfossils coincident with the condensation of carbonate globules can be dated at 3.6Ga BP. So one might conclude that microbial life existed on Mars some 3600 million years ago, probably concurrently with evidence of microbial fossils on Earth. In accordance with the theory of cometary panspermia it would appear likely that both Earth and Mars came to be seeded with bacterial life almost at the same time.
11. Planetary panspermia
An alternative version of panspermia that is becoming increasingly popular follows closely along the lines of Kelvin quotation in Section 2. The trend is based on a growing body of evidence that planetary material could be exchanged between the inner planets of the solar system. There are meteorites recovered on Earth that originated on the Moon (lunar meteorites) and others that originated on the planet Mars, such as the SNC meteorites previously described. We also know from studies of lunar craters that both the Earth and the Moon were subject to intense cometary and asteroid bombardment prior to 4 Gy ago. And the same process continued at a much-reduced intensity in later epochs.
In a typical impact of a 10 km-sized comet with a planet such as Mars (which occurs on the average every few tens of Ma at the present time) most of the material of the impactor and crater will be vaporized. However, material at the periphery of an impact crater will be ejected in the form of rocks and boulders that would be subject only to mild shocking. Such rocks could harbour viable microbes and microbial spores in their interiors and be ejected in many directions in a wide range of velocities. A fraction of boulders that have velocities in excess of the planetary escape speed (5 km/s for Mars, 11.2 km/s for Earth) would be spread over a large volume of interplanetary space, and thus could impact other planetary bodies. Microbes within boulders that survived the trauma of the initial comet impact and subsequent travel outwards through the atmosphere of the parent planet face a further hazard on re-entering the atmosphere of a receiving planet. But this hazard will be overcome for boulders of the size of a metre or more: only the outer layers become ablated, the interior remaining cool. Since there is now no doubt that ALH84001 was a fragment of rock blasted off the Martian surface, and since fragile chemical structures were found to survive the transit Earth, the survival of microbes or spores surviving in the interiors of similar interplanetary impactors is no longer in doubt.
These considerations have led to speculations that life might have started first on Mars and then been transferred to Earth via an ALH84001 type missile some 3600 million years ago. This begs the question of how life got started on Mars. Although the transference of life between planets is possible, cometary panspermia would seem to be the stronger process in transferring life within the solar system.
Geological studies over the past decade have now pushed back the earliest evidence for terrestrial life beyond 3.83 billion years BP, well into an epoch when we know with certainty that Earth was severely pummeled by comet and meteorite impacts. While the early epoch of heavy bombardment would not have been conducive to prebiotic chemistry, it would nevertheless have offered ample scope and many occasions for the transfer of cometary life to Earth.
12. Improbability of life’s origins: cosmic evolution
Our hypothesis is that viable bacteria are of cosmic origin. They were present already in the material from which the solar system condensed and their number was then topped up substantially by replication in cometary material. Thus the impacts of cometary material would have brought them to Earth. The interiors of large enough impactors are known to remain cool and relatively undisturbed in such impacts. The wiping out of resident cultures was then of no overall consequence because the destroyed cultures were replaced by new arrivals.
The hypothesis questions the viability of chemical processes in a warm little pond. Would these processes yield the molecular arrangements of such observed biological structures as DNA and RNA, or at the enzymes for which such structures code? A typical enzyme is a chain with about 300 links; each link being an amino acid of which there are 20 different types used in biology. Detailed work on a number of particular enzymes has shown that about a third of the links must have an explicit amino acid from the 20 possibilities, while the remaining 200 links can have any amino acid taken from a subset of about four possibilities from the bag of 20. This means that with a supply of all the amino acids supposedly given, the probability of a random linking of 300 of them yielding a particular enzyme is as little as
The bacteria present on Earth in its early days required about 2000 such enzymes, and the chance that a random shuffling of already-available amino acids happens to combine so as to yield all the required 2000 enzymes is
which works out at odds of one part in about 10500,000 , with the factorial hardly making any difference, large as it might seem.
A probability as small as this cannot be contemplated. So to a believer in the paradigm of the warm little pond there has to be a mistake in the argument. So although it is known that the bacteria present on Earth, almost from the beginning, were ordinary bacteria, everyday bacteria as one might say, it is argued that the first organisms managed to be viable with considerably fewer than 2000 enzymes31.
The number has been reduced from 2000 to 256 (an amazing but illusory degree of accuracy). Additionally one can reduce the lengths of required chains of amino acids. Suppose, for example, one reduces the length as much as tenfold, to only 30 links. Then the chance of obtaining such a severely sawndown enzyme is
Neglecting the effect of the factorial, this amounts only to one part in 106900, still not a bet one would advise a friend to take. For comparison, there are about 1079 atoms in the whole visible universe, in all the galaxies visible in the largest telescopes. This comparison shows in our opinion that life must be a cosmological phenomenon, not at all something which originated in a warm little terrestrial pond.
In a spatially infinite universe, a universe that ranges far beyond the largest telescopes, there is the very small chance that a replicative primitive cell will bear fruit somewhere and, when it does, replication will cause an enormous number of the first cells to be produced, as we have shown in the example of cometary interiors in section 7. It is here that the immense replicative power of biology shows to great advantage, particularly since we can distribute the products of such replication over millions of galaxies. Each minute innovative step in the development of life — every gene — can generate and disperse enough copies of itself the over a cosmic scale for a second highly improbable event to occur somewhere in one of the profusion of offspring. And so, by an extension of the argument to the third, fourth, fifth improbable events. Indeed to a whole chain of improbable occurrences, which result at last in the magnificent range and variety of genes we have today, the genes that were already present at the formation of Earth.
With the genetic components of life distributed widely throughout the universe, it is a matter for each local environment to pick out arrangements that best fit the particular circumstances. In a case like Earth, a complicated fitting together of the components occurred over the last several hundred million years, by a process which biologists refer to as evolution32.
On this view of the origin of life there would be little variation in the forms to which the process gives rise, at least so far as basic genes are concerned, over the whole of our galaxy. Or indeed, over all nearby galaxies. The rest of the story concerns the many ways in which the same basic genes can combine to produce rich varieties of living forms from one environment to another, always remembering that because of the large numbers involved — large numbers of stars, large numbers of planets and large numbers of galaxies, the system can afford many failures.
Actionbioscience Editor’s Note: To read a commentary on the above paper by the peer reviewer, click here. The peer reviewer’s comments are not intended to discredit the authors, but to present alternative views to some of the points in the authors’ model.