ABSTRACT

The theory discussed by Joseph (2009a) is in general consonance with the panspermia models on which the late Sir Fred Hoyle, the present author and our colleagues have published extensively over the past three decades. Whilst Joseph suggests that a dying planetary system dispersed by a supernova explosion provides the source material for life on the Earth, we ourselves have argued more generally in favour of comets serving as amplifiers and carriers of life. More recently a group of us at Cardiff have argued that genes of “evolved” life could also be dispersed throughout the galaxy by impacts of comets on life-bearing planets like the Earth. Joseph (2000, 2009b) has also proposed a genetic model for evolved life and gene transfer, based on concepts of panspermia. While primitive microbes may be introduced into any new potential habitat via comets, horizontal gene transfers mediated by impacts on inhabited planets could lead to Darwinian evolution occurring on a galaxy-wide scale.

Keywords: Panspermia, Comets, Origin of life

1. Introduction

“Our argument suggests that it is to the comets that we must look for life, rather than to other planets or satellites of the solar system, Overwhelmingly it is in the interiors of comets where we should expect life to persist. Comets are transient, fast-moving vistors, and to dig down into one of them would be a technically difficult operation. But so great have been the advances of space research over only twenty years that perhaps we may look forward to the day when this potentially significant contribution to our understanding of the origins of life will be accomplished” -F. Hoyle and N.C. Wickramasinghe (1978)

“Because of their immense reproductive power, microorganisms can afford to take enormous losses. Only so long as some survive in a viable state the population can be renewed almost instantaneously whenever favourable conditions present themselves. The principal hazard for a microorganism in space lies in the destructive effect of soft X-rays of cosmic origin. As a protection against this hazard, microorganisms are known to possess an astonishing repair process operated by a battery of enzymes…..” -F. Hoyle (1982)

I concur with Joseph (2009a) that the only secure fact relating to the problem of life’s origins is encapsulated in a dictum eloquently enunciated by Louis Pasteur Omne vivum e vivo—all life derived from antecedent life (1857). If life is always derived from antecedent life in a causal chain that is clearly maintained throughout the fossil record, the question naturally arises as to when and where this connection stops. The continuation of the life-from-life chain to a time before the first life appears on our planet and before the Earth itself formed implies the operation of panspermia. Joseph draws attention to the earliest evidence of life on Earth at the time of Late Heavy Bombardment some 3.83 to 4 Gy ago that supports an external origin (Mojzsis et al, 1996, 2000). Evidence also points decisively to comets rather than asteroids being the main cause of impacts during this time (Jorgensen et al, 2009). A large fraction of the Earth’s oceans were brought by impacting comets, and it stands to reason that life could also have been most plausibly introduced at the same time. Viable cells from a nearby planet may also have been transported to Earth, as suggested by Joseph, initially, and also at later times. However, the precise mechanism or mechanisms by which such transfers might occur is worthy of more careful scrutiny.

2. Panspermia Models

Panspermia implies the widespread distribution of the “seeds of life” in the cosmos and similar ideas have had a long history (Arrhenius, 1908). Critics of panspermia have often claimed that such theories are of no value because they do not address the fundamental question of origins. Nevertheless the question of whether life originated in situ on Earth, or was delivered from the wider universe constitutes a scientifically valid line of inquiry.

Whilst theories of directed panspermia transfers the problem of origin to another site, possibly invoking intelligent intervention (Crick and Orgel, 1973), we have attempted to expand the domain in which cosmic abiogenesis may have occurred, focussing in particular on comets (Hoyle and Wickramasinghe, 2000). Like Crick and Orgel (1973) and Joseph (2009) we were impressed by the super-astronomical improbabilities involved in arriving at the highly specific complexity that we identify as life. Devising schemes to bridge this improbability gap presents a monumental intellectual and technical challenge.

Compared with the diminutive scale of a “warm little pond” on the Earth the totality of comets, replete with organics and liquid water interiors, offer significant advantages for an origin of life somewhere within the cosmos (Hoyle and Wickramasinghe, 1978, 1981, 1982, 1985, 2000). The first origin of life may have required the combined resources of billions of galaxies in a connected domain of an early Universe. Evidence of a spectral feature attributed to biochemicals in a galaxy at redshift z=0.8 is consistent with an origin of life some 4 billion years after the presumed Big Bang when the first galactic discs were formed (Wickramsinghe et al 2005; Speigel et al, 1997). For life’s subsequent spread we, like Joseph, derived encouragement from the growing body of evidence relating to the remarkable space hardihood of bacteria. Unlike Joseph, however, we placed greater emphasis on the transport of individual clumps of cells dispersed by radiation pressure of starlight, than the transference of living cells in debris scattered from a nearby source. The latter process may well serve as an adjunct to the more general dissemination of primitive life via comets.

3. Panspermia and Radiation Damage

Claims to limit the operation of radiation-driven panspermia (radiopanspermia) are in our view misguided; they are based on highly uncertain extrapolations from laboratory irradiation experiments to space conditions (Secker, Wesson and Lepock, 1996 Mileikowski et al, 2000 ). Ultraviolet damage is the easiest to shield against with only a thin (< 0.02µm) thick layer of carbonised material protecting the interior of a clump of cells (Wickramasinghe and Wickramasinghe, 2003; Wickramasinghe et al, 2009; Tuleta et al, 2009). In the case of ionizing radiation which is a potential hazard the limiting doses determined from laboratory studies invariably involves high fluxes delivered over short timescales could be very misleading (Wickramasinghe et al, 2009), Clearly it is not possible to mimic space conditions in the laboratory in regard to low fluxes delivered over astronomical timescales. But data such as cited by Joseph, where microbes are known to have survived ~ 10⁶-10⁸ years under terrestrial conditions with a low background flux of ionising radiation, clearly contradicts the oft-cited argument for rejecting radiopanspermia (Cano and Borucki, 1995; Vreeland et al, 2000; Loveland-Curtze et al, 2009). Moreover, in cometary panspermia models survival remains an exceedingly weak constraint. The overwhelming bulk of iterant cells could suffer break up and degradation, and only a fraction less than 10^-22 is required to survive in a viable state for a panspermic feedback loop to be completed (Hoyle and Wickramasinghe, 2000). That is to say only ~ 1 in 10²² biologically derived particles entering an embryonic comet cloud needs to retain viability so as to be exponentially amplified within the interiors of primitive comets. All this goes in the direction of offering support to the Joseph model in which microbial spores escaping from a dying planet could be propelled to reach the cloud that became our solar nebula.

More recently in collaborations with Bill Napier, Max Wallis and Janaki Wickramasinghe we have discussed models of panspermia in which comet impacts on inhabited planets like Earth inevitably distribute microbial life over a galactic scale (Wallis and Wickramasinghe, 2004; Napier, 2004; Wickramasinghe et al, 2009; Valtonen et al, 2008). In the case of our solar system close passages to molecular clouds occurring with a frequency of ~ 40My lead to comets from the Oort cloud being perturbed into orbits that bring them into the inner solar system with a consequent increase in the rate of collisions with the Earth. A fraction of unshocked, life-containing debris is thus expelled from Earth to reach nascent planetary systems in the approaching molecular cloud on timescales well under a million years. In this way even the products of local evolution of life (genes of higher life) could be transported across the galaxy (see also Joseph, 2000, 2009b).

4. Supernova connection

For several decades astronomers have argued that the formation of the solar system required the action of a shock wave from a nearby supernova (Boss and Foster, 1998; Boss, Ipatov and Keiser et al, 2008). The shock wave triggers rapid collapse of a dense cloud of interstellar gas and dust leading to the condensation of a protoplanetary disc from which planetissimals, comets and eventually planets form. Nucleosynthesis in the supernova produces radioactive nuclides such as ²⁶Al and ⁶⁰Fe, and this material would become incorporated in the bolides that form within the protoplanetary disc. Both these nuclides have relatively short half-lives ~ 10⁶ yr, so the discovery of their decay products in meteorites points decisively to nebular cooling and condensation on much shorter timescales. Moreover presolar grains comprised of SiC, graphite and diamond have been discovered in meteorites, and these may well be indicative of a carbon-rich giant star being the supernova progenitor.

Joseph’s model involves the speculation that such a carbon-rich red giant star, somewhat more massive than the sun (3-4 solar masses) once harboured life on an orbiting planet. Prior to the onset of the red giant phase bacterial spores could be expelled by radiation pressure to reach the interstellar cloud from which our solar system would later form. Stellar evolution then progresses from red giant to planetary nebula to a white dwarf. A supernova explosion demanded in the model requires accretion on to the white dwarf from a binary companion. Such a Type I supernova could trigger collapse of a nearby dense cloud - the cloud that became the solar nebula. However, I wonder whether a planetary system around a star that is a member of a binary system would have been stable enough to support life. Notwithstanding this reservation I agree that Joseph’s proposal is novel and interesting and certainly worthy of further investigation.

5. Microfossils in meteorites

The topic of microfossils in carbonaceous chondrites has sparked bitter controversy in ever since it was first suggested in the mid-1960’s (Claus, Nagy and Europa, 1963). Since carbonaceous chondrites are generally believed to be derived from comets, the discovery of fossilised life forms in comets would provide strong prima facie evidence in support of the theory of cometary panspermia. However, claims that all the micro structures (organised elements) discovered in meteorites were artifacts or contaminants led to a general rejection of microfossil identifications. The situation remained uncertain until early in 1980 when H.D. Pflug found a similar profusion of “organised elements” in ultrathin sections prepared from the Murchison meteorite, a carbonaceous chondrite that fell in Australia on 28 September 1969 (Pflug, 1984). The method adopted by Pflug was to dissolve-out the bulk of minerals present in the thin meteorite section and examine the residue in an electron microscope. Structures with uncanny resemblances to known microbial species were found in great profusion. These studies made it very difficult to reject the fossil identification. More recent work by Richard Hoover (1997) and his team leaves little room for any other interpretation of these structures than that they are microbial fossils. I agree with Joseph that this work is to be respected and taken seriously, although my own interpretation differs from his in that I think it most likely that the fossilisation took place within a comet.

In conclusion I would remark that whilst an external origin of life is now strongly indicated from many different standpoints, the particular model discussed by Joseph must remain one of several options.

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