Note: Because some of the information in this article may be outdated, it has been archived.
Terrestrial vs. cosmic origin
Life exists in extreme places on Earth, such as this black smoker, a hydrothermal vent, in the Atlantic Ocean. Photo: P. Rona, OAR/National Undersea Research Program (NURP)/NOAA
Dismissing the “warm organic soup” (presumably a reference to endogenous creation of life, either the Urey/Miller experiments or Darwin’s warm pond) is unjustifiable. The proponents of the primordial pond are simply attempting to answer a different question than the authors. After all, even if life on Earth derived from bacteria from comets, those bacteria must have evolved sometime somewhere. The authors cannot object to endogenous theories on the grounds that life only comes from life and never from non-living materials, unless they are ready to advocate that life was present from the big bang. If the authors agree that life came from non-life at some point, then the people developing these theories will at least help us to understand how the first life arose elsewhere, if not on Earth.
The authors seem to be (either consciously or not) running up repeatedly against ideas of evolutionary biology. The cause is the conflict between Pasteur and Darwin. Do the authors really advocate that “every plant or animal is preceded by a generation of the same”? This would appear to prevent speciation and life would never get from bacteria to everything else if, strictly speaking, this were true. Even harder is the step between chemicals and life.
That the extreme hardiness of bacteria is somehow “unworldly” is not convincingly argued. Indeed the authors appear either to not appreciate or to be deliberately ignoring the standard evolutionary explanation of how such traits arise solely on earth. I strongly disagree with the authors’ contention that this hardiness could not arise from purely terrestrial natural selection. They describe in their paper the wide range of environments on earth where bacteria thrive - thermal vents, Antarctic ices etc., and the evolutionary biology community believes that this terrestrial range of environments is adequate to select for organisms that thrive under these conditions. In fact, even a resistance to ultraviolet radiation could be explained by terrestrial selection. One could interpret it as a remnant of selection back at a time (before the ozone layer) when radiation on Earth was more prevalent. This kind of argument is used to explain the capacity of organisms to deal with variations in temperature, why not radiation resistance?
The fact that “microorganisms that are normally sensitive to ultraviolet light can, through repeated exposures, be made just as insensitive as the more resistant kinds” is not necessarily an “unearthly property.” It is also what would be predicted by evolutionary biologists assuming that these microorganisms had an entirely terrestrial heritage. This means that, right or wrong, the authors must admit that the consensus of scientific opinion is that natural selection can explain these traits, and that they are not necessarily “unworldly.” They may be consistent with panspermia, but are believed to be consistent with evolution on Earth too.
The authors conjecture that, in space, bacteria are subjected to a “process … analogous to coalification and graphitization of living material,” producing organics “similar” to those seen by astronomers (and presumably in meteorites too). It is not obvious that these processes will produce similar material (or what is similar for that matter). In space this is a low temperature, low pressure radiative process. This could result in very different compounds than those seen resulting from a high temperature high-pressure process of the kind seen on Earth. Has anyone actually performed this simulation experiment? There are French scientists who also make the comparison between astronomical data and the spectra of coals, and the same objections apply to them. Yes, it looks similar, but organics produced by plasma processing of methane or acetylene in processes intended to simulate the outflows of carbon-rich stars produce materials that fit the spectra. It may well be that the authors are correct, but for now the similarity of spectra merely makes their hypothesis ‘consistent’ with the observations, but so is the work by Sakata and Wada on plasma-generated materials in the lab1 and Greenberg’s “Eureka” material.2
When the authors ask rhetorically “Might not the same processes operate in the case of interstellar (IS) organic molecules?” (referring to the degradation of bacteria), the answer (and objection) is similar to the previous one regarding evolution. They do not do justice to their peers and underestimate their theories when they say “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.” These other theories also reasonably explain the presence of extraterrestrial organic materials. These other scientists have actually performed experiments, and having made these materials under low pressure conditions, have measured IR and mass spectra that appear to be consistent with what is seen on the telescope and in meteorites and IDPs too. The authors may still be correct, but they should give these other ideas the credit they deserve.
For example, Wdowiak’s plasma processing fits IR spectra really well, and not just old short wavelength spectra either (as is explained in the paper, Greenberg’s also does a great job of that) — it fits across the whole mid-IR.3 Furthermore, others are now also showing mass spectra that are consistent with what is seen in meteorites and IDPs.4 So as far as I can tell, these theories are superior in the following ways. They:
- are supported by experiments (Greenberg’s was actually flown in space)
- fit a wider range of data
- start from simple molecules known to be abundant in space and formed by simple processes
Microorganisms vs. organic compounds
By stating that IS grains “are of a size that would be typical for a bacterium, a micrometer or less” do the authors mean to imply that most interstellar grains are free floating individual bacteria? This was one of the “not so good” things about the Arrhenius theory. The authors have argued that bacteria are hardier than originally thought, but unshielded bacteria travelling interstellarly is no six-year jaunt on the space station. That they never return to this and perform a back of the envelope calculation (say based on LDEF survival rates) to convince us that they should survive is disturbing. Isn’t the multi-million year lifetime of clouds in the ISM, and the fact that most grains are thought to go through many clouds before incorporation into a forming stellar system, going to be problem for individual bacteria?
It’s possible that a cloud of bacteria would have some interesting polarization affects since they are made from chiral amino acids. So, the authors could make a statement about light polarization as well as scattering. Would bacteria-grains show the behavior in magnetic fields that some IS grains show? In addition, Grains in the dense ISM (where ices form) appear different from the grains in the diffuse medium. The authors should make that clear if it is solely to the dense medium grains to which they refer.
Do the spectra of desiccated bacteria fit more recent, longer wavelength spectra of GC-IRS5? Do they fit other similar objects? A fit based only on this narrow region of the spectrum is hardly unusual enough for words like “no alternative.”
Do “explanations of this sequence on the hypothesis of coalifying graphitic material (as is currently being done)” really involve “a violation of basic thermodynamics?” It is certainly possible to end up with products that are higher energy under kinetically controlled conditions.
Biological action is the “simplest” way to make a large quantity of small organic particles only in that biological growth unhampered by lack of resources is geometric. But it’s very complicated in that the conditions conducive to bacterial growth may not be common in space. In contrast, soot is made very efficiently from oxygen poor flames, and not only from the burning of biological organics. It has been shown that a wide range of organic molecules are produced efficiently in the outflows of carbon rich stars and this explanation is simpler in many ways than is the biological explanation in that it doesn’t require making the bacteria in the first place. This segues into the next comment: What are “…appropriate conditions for replication?” Is there adequate food for bacteria to reproduce so rapidly in the ISM, or in a comet?
It seems more likely that IS bacteria are the source of cometary bacteria than visa versa, since comets are believed to be formed from interstellar ice grains. What percentage of cometary grains is believed to escape the solar system?
While there is discussion of putative microfossils, and of intractable organics, conspicuously absent from this work is an explicit discussion of whether the many ‘individual’ organic compounds are consistent with their theory. We know considerably more about meteoritic organics than interstellar organics, including identities of many individual compounds analyses of structure (branched vs. linear, etc.) isotopic enrichment and chirality. There is ample evidence that some of these meteorites contain fairly pristine material (Murchison, for example) based on the fragile organics that were observed.
How do the authors know that the bacteria on Earth from the beginning were “ordinary”? That this represents the consensus of the community is doubtful. Given that the conditions on the early earth were very different from today (i.e., there was no oxygen) would we really expect that the same bacteria that were common then, would be common today? My understanding is that the population that we have around today is different. If the authors are correct, and the last common ancestor is falling from the sky every day then why don’t we see it? Would they like to be more specific as to what this “ordinary” kind of bacteria is like i.e. to what kind of bacteria do they refer? Eubacteria? Archea?
It is commonly believed in the scientific community that the current biological framework was preceded by one in which the nucleic acids reproduced in the absence of protein enzymes. This RNA world presumably was inhabited by nucleic acids that had the capacity to function both as catalysts and information carriers at the same time. This would obviate the need for the amino acid derived enzymes discussed by the authors.
By attributing the origin of life on Earth to bacteria from space without mentioning how those bacteria arose, the authors are proposing a solution that is equivalent to creationism. The authors are not really driven by the data, but rather by a desire to resolve such a conflict, or to avoid an explanation and that is why they had their eyes on (by their own admission) “…a possible biological origin from the outset.”
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