Introduction

Meteorites and meteoric dust are continually falling on the Earth and these materials obviously present a variety of scientific problems for the astronomer, the mineralogist, the meteorologist, and the chemist. It is not widely realised, however, that meteorites pose several problems of biological importance that raise undecided issues fundamental to several fields. For example, if it could be shown that meteorites contained the remains of living organisms, then the problem of the origin of terrestrial life would need complete rethinking.

It is the purpose of this article to pose just this kind of question for there have been several recent reports of biogenic materials within meteorites. Before considering these recent data a short review is given of previous biological studies of the meteorites.

Towards the end of the nineteenth century, it was claimed (Hahn, 1880; 1882) that certain stony meteorites contained indigenous fossil organisms. The only criterion put forward in support of this claim was the morphology of various structures observed within the meteorites. This criterion was considered insufficient and it was generally assumed that the structures were inorganic artefacts. It was shown, for example, that some of the structures were merely chondrules.

Interest in meteorites also arose with the apparently conclusive demonstration by Louis Pasteur that spontaneous generation does not occur on the Earth. Many nineteenth-century scientists jumped to the conclusion that spontaneous generation could therefore never have occurred. This conclusion is obviously not necessarily true from the demonstration that micro-organisms do not now spontaneously appear under laboratory conditions. Yet many scientists were led to postulate that the first terrestrial organism arose elsewhere in space and was then carried to the earth within a meteorite. Such speculations naturally led to the examination of the interiors of meteorites for viable micro-organisms. One of the first investigations into the possible occurrence of bacterial spores in stony meteorites was apparently conducted by Pasteur (see Becquerel, 1924). Negative results came from both this and later studies (Gallipe and Souffland, 1921). C. B. Lipman (1932: 36), however, claimed success and reported the detection of a number of bacterial species from the centres of surface-sterilised meteorites. Some species were unidentifiable, and one had the ability to decompose paraffin oils. Assuming the technique to be above criticism, this result could be explained either by contamination of the meteorites after fall, or that the bacteria were extraterrestrial organisms.

The findings naturally resulted in some controversy and Lipman's work was repeated by Roy (1935) who reported the isolation of bacteria from meteorite samples in three out of twelve experiments. However, Roy was able to identify his organisms as common species (Bacillus subtilis and Staphylococcus albus).

More recent attempts to culture living micro-organisms from meteorites include those of Briggs and Kitto (1962) who were attempting to check on the sterility of the interior of a carbonaceous meteorite (Mokoia). No viable organisms were detected in this study. However, the presence of an as yet unidentified aerobic bacterial species has been reported by Sisler (1962), who isolated the organism from inside surface-sterilised samples of another carbonaceous meteorite (Murray). Sisler worked in the germ-free laboratory of the U.S. National Institute of Health and contamination of the material during preparation can be ruled out.

Recently, there have been reports in the Soviet press (Rubchikova, 1962) of the isolation by Russian scientists of viable unidentified micro-organisms from the interior of a further carbonaceous meteorite (Mighei) first sterilised by heating at 150°C.

It is difficult to draw conclusions from these experiments. There is no prima facie reason why carbonaceous meteorites should not contain extraterrestrial organisms. As will be shown later, this group of meteorites is unusual in having been formed under conditions of low temperature and have never been submitted to heat sufficient to destroy indigenous organisms. During entry through Earth's atmosphere, the surface of the meteorites has fused, page 3 but the presence of large quantities of thermolabile materials within samples of the stones proves that the short time of flight and their low thermal conductivity has protected the interiors. Consequently, if these meteorites originated somewhere where a life-form occurred, then they could well contain surviving organisms. On the other hand, if the organisms detected within the meteorites prove to be terrestrial species, then the meteorites have been contaminated after arrival on Earth. To conclude that viable organisms isolated from within meteorites had an extraterrestrial origin would be possible only if the organisms proved unidentifiable and possessed some properties that clearly showed them as unearthly. It is apparent that the evidence outlined above does not meet these criteria and terrestrial contamination is the obvious explanation of most of the positive results. On the other hand, most of the experiments gave negative results and demonstrate the lack of widespread microbiological contamination of stony meteorite interiors during storage under museum conditions. This is an important point that will be returned to later.

The Carbonaceous Chondrites

Some of the experiments conducted above were on specimens of a fairly rare class of meteorites: the carbonaceous chondrites. As it is on this group that the current biological investigations are being conducted, a brief discussion on the properties of these meteorites will be given.

The nature of the organic material present within the carbonaceous chondrites presents several problems. Assuming it to be extraterrestrial it could be of either biological or nonbiological origin. To establish a biogenic origin it would have to be demonstrated that the composition of the material is identical with that which would be produced by the disruption of a terrestrial life-form. If the composition is different. it follows that the material is either of abiogenic origin, or is derived from an extraterrestrial life-form of different chemical composition to known terrestrial forms. If the latter hypothesis be adopted, it then becomes impossible to use the mere presence of organic matter within meteorites as a criterion of extraterrestrial biogenic processes, for, obviously, any chemical substance could be said to be a part of a completely unknown and hypothetical alien life-form.

This latter point seems of importance, for there is little doubt that in several features the meteorite organic compounds do not resemble terrestrial biological debris. On the other hand, there are demonstrated simple abiogenic syntheses for the majority of the meteorite compounds.

Quite a variety of organic compounds have been identified in meteorite extracts (Berzelius (1843), Berthelot (1868), Smith (1876), Meuller (1953). Calvin and Vaughan, Calvin (1961), Briggs (1961)). As the nature and problems of identification of these compounds are discussed elsewhere (Briggs, 1962c) only a brief list of the general classes of compounds is given here: aromatic and heterocyclic derivatives, straight-chain and linear paraffins, urea and acetamide, low molecular weight aliphatic acids and esters. All are optically inert. Most of these compounds can be formed by radiation — or discharge — induced reaction from simple materials.

The paraffin hydrocarbons present a more complex problem for several reasons. First, the available evidence indicates that the meteorite compounds are largely unbranched. Now abiogenic syntheses for paraffins yield a mixture of branched and unbranched compounds, the former predominating. Consequently, if the meteorite paraffins are abiogenic, an explanation for the absence of branched hydrocarbons in the meteorite extract is required. Secondly, it is known that the paraffins of most terrestrial organisms are unbranched and show a definite pattern of compounds with different carbon atoms per molecule. It has been claimed by Nagy, et al. (1961) page 8 and Meinschein et al. (1962) that the distribution of the various molecular species of hydrocarbons from both the Orgueil and Murray meteorite resemble the hydrocarbon distribution of organisms, and also of recent sediments, which contain hydrocarbons of presumed biogenic origin. (See Table 6.) The agreement between the distribution patterns of the two hydrocarbon mixtures, however, is not very close and has been criticised on several grounds (Anders, 1961).

The problem therefore arises of whether there is some mechanism whereby unbranched paraffins, similar to those present in the carbonaceous meteorites, could be formed abiogenically. The only experimental demonstration of the synthesis of high molecular weight hydrocrabon-like material is by Wilson (1960). It was shown by this worker that electric discharges acting on a mixture of methane, ammonia and hydrogen above a conducting salt solution, such that the discharge continually strikes the liquid-gas interface, will produce very high molecular-weight substances composed largely of carbon and hydrogen. The mechanism of the synthesis appears to involve the initial formation of simple low molecular weight substances which collect at the interface and become cross-linked into a two-dimensional molecular reticulum by the discharge.

It is a fairly well-known fact^* that thin layers of waxy material tend to accumulate on surfaces within electron microscopes in the electron-beam pathway. This material is probably of a similar nature to the discharge polymers: the reaction being induced in this case by the high energy electrons. It is probably significant that waxy coatings have recently been identified on various cosmic nickel fragments.^†. Again, this material is probably a form of hydrocarbon polymer synthesised abiogenically by radiation in space. Moreover, the presence of this material on objects of extraterrestrial origin immediately raises the question of whether similar substances occur in meteorites. Experiments on three samples of carbonaceous chondrites, from which all low-molecular-weight organic compounds have been removed by solvent extraction, have demonstrated the presence of carbon in a form amorphous to X-rays. Moreover, microscopic examination of the meteorite samples reveals the presence of flat. translucent fragments a few microns in size that char on heating. This is evidence for the presence within the carbonaceous meteorites of high molecular weight organic material probably similar to the discharge polymers and to the organic coatings on cosmic nickel fragments mentioned above.

While the above scheme is partly hypothetical, it would seem a possible abiogenic explanation for the meteorite hydrocarbon analyses.

Organic Microstructures

From the previous discussion it will be seen that the conclusive identification of biological materials within meteorites cannot rest on the mere presence, or even the nature, of the organic compounds present.

However, a completely new class of evidence has now been introduced. It has been claimed (Claus and Nagy, 1961) that certain microscopic structures within the carbonaceous chondrites are the fossilised remains of an extraterrestrial life form. The meteorites studied were samples of Orgueil, Ivuna, Murray and Mighei. It was reported that both Orgueil and Ivuna contained very large numbers (over 1,000 per milligram) of microstructures in the 5 to 20μ size range that exhibited a complex morphology, fluoresced in ultra violet light, took up a range of biological stains, and yet could not be identified as any known terrestrial forms. (See Table 7.)

The techniques used to study these so-called ‘organised elements’ were relatively simple. In some experiments small samples of the meteorites were crumbled in water or in glycerol on to microscope slides and then examined. Alternately, some of the organised elements were separated from the meteorites by extraction with organic solvents.

The publication of these results led to the calling of several symposia to discuss the implications. One of these symposia was a page 11 collection of papers in Nature (Symposium, 1962) while another was called by the New York Academy of Sciences (see Urey, 1962). In the Nature papers, Briggs and Kitto (1962) reported their inability to find anything resembling micro fossils in Mokoia, though the meteorite did contain numerous scraps of irregularly shaped material that took up biological stains and charred on heating. A group at the University of Chicago (Fitch, et al., 1962) reported results of microscopic examinations of Orgueil and Ivuna and failed to confirm the original findings of Claus and Nagy. The Chicago group suggested that the objects stated to be ‘organised elements’ were in fact several different classes of micro structures; including rounded silicate grains, particles of magnetite and troilite, and droplets of elemental sulphur associated with fluorescent hydrocarbons.

In this symposium, Professor Bernal (1962) reported that examination of Claus's preparations by himself and staff members of the British Museum had convinced them of the biological nature of the organised elements. Similar materials had also been found in preparations from a sample of the Orgueil meteorite held in the museum's collection.

A second paper from the New York group (Nagy, et al., 1962) reported further details of the organised elements. It was stated that concentration of the microstructures could be achieved by various density floatations, and, moreover the objects had now been observed embedded within the minerals of the meteorite on examination of thin sections. These findings were presented in more detail at the New York Symposium (Urey, 1962) when the data from the British Museum group was also presented by Ross.

Since that time an examination of the Orgueil meteorite has been made and published by Staplin (1962) who is an authority on palynology. He has reported the separation from the meteorite of a variety of microfossils ‘of unknown affinities or age’. Details are given in Table 8.

However, the results have not passed unchallenged. Pearson (1962) has suggested that some of the organised elements are pollen grains that have contaminated the meteorite either during passage through the atmosphere, or some time during its storage on Earth. This view has received support from several sources. Fitch and Anders (1962) have presented strong evidence that several of Claus and Nagy's reported organised elements are ragweed pollens and fungal spores. This view is supported by Siegel (1962), an allergist. Similar opinions have been expressed by Durham (1962), who is chairman of the American Academy of Allergy's Pollen and Mold Committee.

This view that contamination of the meteorites has occurred from atmospheric pollens and spores led Briggs (1962d) to prepare specimens of Orgueil, Murray and Mokoia within a sterile glove-box containing filtered air. Under these conditions no objects resembling microfossils were detected, though small scraps of fluorescent organic matter were present in each meteorite.

At the present time a reasonable conclusion concerning the presence of indigenous microfossils in carbonaceous meteorites is ‘not proven’.

It is perhaps an interesting point that such difficulty should be experienced in deciding whether or not microstructures within an organic-containing rock are or are not microfossils.

Origin of Petroleum

It is widely agreed (Meinschein, 1959) that the source material for terrestrial petroleum deposits is biological debris of marine sediments. However, the discovery of petroleum-like hydrocarbons within carbonaceous meteorites has raised an interesting problem. If the meteorite hydrocarbons are not the remains of an extraterrestrial life-form, but are abiotic compounds formed in space, this observation immediately raises the question of whether any of the organic constituents of terrestrial petroleum are compounds brought to Earth by meteorites.

Organic matter has now been demonstrated in two classes of extraterrestrial debris; the carbonaceous chondrites and metallic flakes derived from meteor showers (Parkin et al., 1962). As yet there is no evidence about the chemical nature of the amorphous organic attachments of the meteor fragments, but their survival, together with physical properties, suggests that they may be a type of hydrocarbon polymer. As described above, the synthesis of this type of material in space by radiation is quite probable.

It is difficult to estimate the total amount of organic substances added to the Earth by meteoric matter. Thus if only the carbonaceous chondrites are concerned the 20 known meteorites of this class in collections contain some 6 × 10³ grams of organic matter. However, page 13 most meteorites fall unobserved. Taking the estimate of Brown (1961) that the average rate of meteorite falls is of the order of 1.1 falls per year per 10⁶ km², and assuming that carbonaceous chondrites constitute 3% of all meteorite falls, it is clear that the average influx of carbonaceous meteorites over the whole surface of the Earth is about 17 per year. These carry in with them several kilograms of organic matter. When considered over the whole Earth, this amounts to an average of about 10 micrograms of organic matter added to each km² per year. If meteorite falls have been reasonably constant throughout geological time,^* a total of the order of 10¹³ grams has been added to Earth's surface since the origin of the planet. This is several kilograms of organic matter per km².

So far only the carbonaceous chondrites, on which reasonably reliable data are available, have been considered. Yet these are comparatively rare objects. Much more abundant is meteoric dust. As mentioned above, organic attachments have now been detected on a high percentage of fragments collected from meteor showers. It is not possible as yet to estimate accurately the amount of organic material added to the Earth's surface by these fragments. However, if the presence of this material proves to be a general phenomenon for all types of meteor fragments, it is possible to make certain assumptions and derive a very approximate first estimate.

As there is a similarity between the hydrocarbon constituents of meteorites and those of terrestrial petroleum deposits, it is clear that meteorites could well be at least a partial source material for petroleum.

A possible objection to meteorite hydrocarbons being a source material for petroleum is the fact that the C¹⁴-age of hydrocarbons from marine sediments is about 3,000 years. However, it has recently been shown (Suess and Wänke, 1962) that meteorites contain C¹⁴, and the isotopic abundance of C¹⁴ is even greater than that found in modern terrestrial organisms. Hence, C¹⁴ determinations on sediment hydrocarbons cannot be used as a criterion of terrestrial biogenic origin.

It is perhaps significant that micropaleontologists have described many unusual small organic fragments found in sediments as microfossils of unknown origin and affinities (Jones, 1956). At least some of these could be polymeric hydrocarbons derived from meteoric fragments.

Most of the estimates given above are based on rather inadequate data, and some may be in error by several orders of magnitude. Nevertheless the realisation that organic material is present in meteoric matter would seem to be of possibly great importance to an understanding of the origin of petroleum.

Conclusions

The discussion above has reviewed the present controversy on whether meteorites contain biological materials. As yet no conclusion either way is possible. The issues raised by this problem seem to be of fundamental significance to the paleontologists, and also to proponents of a biological origin for petroleum.

Acknowledgments

Some of the previously unpublished work presented here would not have been possible without the generous gifts of meteorites from the following: Dr. W. A. Walters of the N.Z. Geological Survey; the Curator of the Wanganui Museum; Dr. E. P. Henderson of the Smithsonian Institute; Prof. Edward Anders of the University of Chicago; Dr. H. G. Macpherson of the Royal Scottish Museum; the Director of the Chicago Museum of Natural History.

This work was supported in part by a grant from the Victoria University of Wellington Research Committee.