A conversation w/ Dr. Max Bernstein (NASA) about the origin of life

[stickdog]

Bernstein: I just returned and saw your letter, about whether these molecules (galactically and universally ubiquitous tar-like polycyclic aromatic hydrocarbons that exists on Earth as the "grit on your grill" or "the black on your toast") are pre- or post-biotic. You are right that these materials are simply assumed to be pre-biotic, well non-biotic really. But there is very good reason to think that. Organic compounds are widespread across not only our galaxy but others as well, and the manner in which they form is well understood and doesn't require any biotic processes. So, while it is possible that some of it has come from living systems, we begin by assuming the simpler much more likely scenario, that it's non-biotic. This principle, that the simplest answer should be assumed at the outset, is often called Occam's Razor. The other notion that you put forth, that life was actually seeded living extraterrestrial bacteria is called panspermia. While perhaps there is a remote possibility that it is what happened, I think that you overstate the plausibility. And personally I think that it's a cop out, just moving life's origin to a remote venue without answering the more interesting and tough questions. Don't get me wrong. It is possible that life originated elsewhere, and I don't personally discount the idea out of hand, but let's be honest - what we know is not consistent with that notion. And as scientists our job is to put every new theory to certain minimum tests, and panspermia has not even come close to passing these first tests, let alone the harsh scrutiny to which serious theories are subjected when the appear in the literature.

OK, I can see I have some 'splaining to do here.

1) Scientists have been surprised and are still being surprised almost weekly by the continually expanding limits of recently discovered Earthly microbial life. We have only recently begun to look for signs of life in all kinds of places previously assumed to be sterile. And everywhere we do look closely for life -- from the deepest hole to the highest balloon -- we find it.

2) It was not so long ago that astrochemists considered the notion of any interstellar organic matter (of biotic OR non-biotic origin) completely laughable.

3) Current theories that posit a terrestrial origin of life certainly do not pass any scientific "minimum tests" either. While your "cop out" argument certainly applies (in much the same way it applies to the Big Bang theory), Occam's Razor does not. Scientists who espouse an Earthly origin of life must face up to a logical conflict in their own reasoning. For example, if life is such a chemical imperative that it arises anytime and anyplace the right conditions exist, then why is our entire tree of life explicitly assumed to be 100% genetically connected? My point is that the same intuitive principles which lead us to assume a strictly generational view of evolution after any putative Earthly genesis of life also inherently lead us back to pre-Earthly times if we remain self-consistent.

Of course, the common objection to viewpoint -- and, IMO, the only viable objection -- is the lack of a pr oven mechanism of safe transport.

[stickdog]

1. continued ...

Bernstein: Basically, if you are going to ask an organism to seed life on Earth by traveling between the stars it will require a series of unlikely events above and beyond that first still not understood event that produced life. First of all you have to get it off the planet where it started and that would be probably be from a large impact of the kind that brought us the martian and lunar meteorites. Surviving that ejection event will be no picnic. Now even if it does, it is highly unlikely that it will survive the harsh conditions in interstellar space. And even if it can somehow, what are the chances that this organism would find itself on another habitable planet? Even if it did find the right planet, it would have to arrive at the right time because, as you know, current estimates of the earliest life indicate that it arose very shortly (in geological terms) after the planet became habitable. Even then, it would have to survive re-entry. Of course, if all the conditions are just right, your organisms might make it. But the chances are now vanishingly small.

Are you keeping up with all the recent discoveries of the expanding confirmed limits of microbial life? Earthly chemotrophic (getting their carbon strictly from inorganic sources), chemosynthetic (getting their energy strictly from mediating chemical, often geochemical, reactions) microbes can thrive in just about any conditions in which water exists in a liquid (or even "near liquid") state. Furthermore, many of these very same microbes can almost certainly survive the swiftest imaginable interstellar journeys -- theoretically limited only by the chance of being directly hit by a cosmic particle (for very long travel times, at admittedly high probabilities for any specific microbe). However, only one cell of trillions need survive and take root. And isn't a single mother cell the standard Earthly origin assumption? If not, then why the explicitly connected genetic tree of life?

And once one solar system planet, moon or other large terrestrial body becomes globally infected, it would be expected to rapidly infect all other nearby inhabitable bodies, would it not? For example, consider the Martian meteors that litter the Earth. So extraterrestrial microbes would be expected to spread throughout the galaxy just as they spread throughout a Petri dish speckled with agar -- by first infesting the closest speckle, then the next closest speckle, then the next closes speckle, then ...

[stickdog]

2. continued (2) ...

Bernstein: As I see it, the only way to make panspermia even remotely plausible would be if there were an awful lot of these organisms out there, and they were falling in all the time. You should be able to make a rough estimate of how many there might be in the interplanetary or interstellar medium. Now consider all of the meteorites and interplanetary dust particles that we have collected in the upper atmosphere, and the probes that have collected interplanetary dust. I think that you will find that we should have found evidence of this space bacteria.

So it takes only one 4 billion year old spontaneously generated cell to start all Earthly life in your scenario, but it takes trillions of "space bacteria" raining down on Earth to the present day to accomplish the same task in mine?

And please note that evidence (controversial evidence, of course) of "space bacteria" has also been found whenever and wherever anyone carefully looked for it:

http://www1.timesofindia.indiatimes.com/cms.dll/articleshow?art_id=4476520&sType=1

http://xxx.lanl.gov/ftp/astro-ph/papers/0310/0310120.pdf

http://www.panspermia.org/kissel.htm

http://www.panspermia.org/marslife.htm

http://www.panspermia.org/hoover.htm

http://www.panspermia.org/magneto.htm

http://www.panspermia.org/zhmur1.htm

http://www.panspermia.org/chiral.htm

http://www.panspermia.org/dayal.htm

To me, the crux of this (currently inherently speculative) argument is what you deem most likely:

1) that matter spontaneously generated into an unfathomably potent and complex evolutionary system during the first few hundred million years of the Earth's tumultuous formation

OR

2) that among (conservatively) millions of nearby planets, moons, other large terrestrial objects and even the denser potions of interstellar clouds over billions of evolutionary years and septillions of potential microbial generations, some microbial life form developed that was evolutionarily suited to space travel over long years in great numbers.

To me, the recently confirmed extremophilic characteristics of many varieties of Earthly microbes have been tipping the scale more and more in favor of the latter with each passing year.

[stickdog]

3. continued (3) ...

Bernstein: Although its not my area, it seems to me that if the Earth were still experiencing a continuous rain of bacteria of the same kind that started life here long ago then we would see very old organisms that would be identical to if not very similar to what is observed today. If we all evolved from this organism, it would be really easy to root the tree of life as the last common ancestor would be an extant (or recent) organism. This is definitely not the case.

No.

The most feasible model is that Earth was infected by some other, once vital, terrestrial body in solar orbit. After four billion years, this body could very well be dead or at least comparatively dead (although Io certainly appears to be very much alive and kicking right now).

Most remaining non-Earthly microbes are now either in comets (some of which are possibly the broken off watery crust of previously thriving planets) or beneath the surfaces of various other solar satellites. Remember that in this putative model the original contaminating microbes are chemotrophic -- and generally subsurface -- archea and bacteria (which are typically placed very near the root of the our current genetic tree of life, BTW). These organisms survive simply by eating rock, and thus would appear to have little or no regard for solar proximity.

Because of special photosynthetic conditions on Earth, a large surface population has managed to thrive here in a long term symbiosis with subsurface lif. However, Earth's long term surface/subsurface photosynthetic/chemosynthetic system would be the exception and not the rule in this model. The vast majority of infected bodies would consist almost exclusively of chemosynthetic subsurface microbial populations, at whatever (reasonably near surface) depth you'd expect to find liquid water considering the temperature/pressure gradients of each body. In fact, I wouldn't be at all surprised if we soon discover that (extra)terrestrial geologic life and subsurface microbial life are typically very closely intertwined.

Now I understand that this might be considered an extremely "radical" idea by some. But let me ask you what the solar system was created from in the standard model. The remnants of one or more previous stellar systems, perhaps? Certainly, a giant dust and gas cloud with some larger pieces. And you're quite sure there wasn't any microbial life in that system? Or is it that you're certain that not one spore of it could possibly survive the formation stages in a "freeze dried" state?

The idea that all Earthly life began on Earth is certainly compelling, just not nearly as likely. Turning your argument around on you, if life was such a chemical imperative, why haven't we come any closer to creating it in our labs despite generous grants and countless experiments? And why do we have no evidence of any new, genetically unrelated forms of life being generated on Earth since our putative genetic "mother cell" first developed about the same time that Earth became inhabitable?

[stickdog]

4. continued (4) ...

Bernstein: You claim that we already have some indirect evidence of dead and destroyed organisms in the form of organics from carbonaceous chondrites (meteorites) like Orgueil, Ivuna, Allende and Murchison, but that does not stand up. The organic compounds in Murchison (for example) are not at all what we would expect to see if they came from organisms like us. Even if damaged or destroyed bacteria should leave molecular fragments that are fairly distinct. (If they are our progenitors then they are like us, right?) But what we have seen in the meteorites is inconsistent with this notion. Most of the molecules have nothing to do with life. Even those that may appear to at first (even the chiral diastereomers measured by Cronin) are really more consistent with non-biotic synthesis. Not only are many of the amino acids non-biological but the distribution of deuterium and carbon-13 in the natural ones is not what one would expect from a biology that was anything like ours.

Your "numbers" argument is not convincing. In many eras, especially many early eras, our fossil record is woefully incomplete. Do we therefore assume there was no life in these times and that it picked up spontaneously and mysteriously at some later point? The sample size of meteorites that we have seriously examined for evidence of microbial life is infinitesimal and some of these actually even show promise. And we know that the early Earth was bombarded with conservatively thousands of times the debris that we see today. Today's Earth is protected from new infestations by both a stronger magnetic field and a thicker, far more reactive atmosphere -- almost completely due to a long and complex history of Earthly microbial activity. Furthermore, micro-comets and micro-meteorites are far more likely to survive impact without sterilizing their genetic passengers. Tell me, how many of these objects have we managed to capture for careful biological examination? Haven't we always simply assumed that any biological evidence we observed was necessarily the result of Earthly microbial contamination?

While I am personally fairly certain that extraterrestrial microbes or convincing evidence of such will be relatively easily found in the near future, the lack of any conclusive evidence among our current minute sample is surely not particularly damaging to a modest theory that proposes only that at least one healthy cell made a safe journey to Earth some four billion years ago.

[stickdog]

5. the merciful finale

Bernstein: So in the end what we are left with is this: For these theories to work many many things must be just right, and you must explain a lot of data that is not consistent. The alternative, that life developed here, requires only the conditions on Earth must have been hospitable, and is completely consistent with what we know. In the words of Carl Sagan incredible claims require incredible evidence so until that evidence is presented I'll stick with life starting here, although I happily admit that this is far from proven. One of my co-workers, who shall remain nameless, actually thinks that it is much more likely that life started here as a result of a deliberate program by intelligent space faring aliens, than that it happened by panspermia.

Directed panspermia is certainly possible, but it's not consistent with the evolutionary history of the Earth which is currently our one and only confirmed model of life. Microbes thrived here almost immediately and were followed by multicellular surface life only 3 or so billion years later -- after their intense climatic, oceanic and atmospheric activity set the stage for this (seemingly) much less likely form of life.

Could intelligent beings have seeded the Earth with microbes o' so many years ago in order to make it habitable billions of years later? Of course, and we may yet try to do the same to Mars or some other nearby body. However, the mechanisms required for undirected panspermia are simply the extremophilic quasi-immortal traits already evidenced by Earthly microbes combined with a plausible method of transport (impact events, extreme Io-like volcanism, or even planetary breakup) or recycling (surviving long periods of dormancy in initial formation clouds). No warp speed needed.

To me, saying that the most parsimonious explanation of Earthly life is that it originated on Earth is like saying that the most parsimonious explanation for a hermit's new strain of influenza is that the virus spontaneously generated itself from the readily available components of his body. To me, the infinite mystery is how life ever began in the first place, and I'd sure like to see a "parsimonious" explanation for this. However, once this quasi-immortal, exponential process appears -- however, whenever and wherever -- we must logically expect a steady contamination of the cosmos for that place outward and that time forward. For those who don't, I have a simple question: How do you propose that you will sterilize the Earth to keep it from contaminating other terran bodies in the future? Even after the sun becomes a blazing red giant, even after the sun goes out completely, (perhaps long dormant) subsurface Earthly microbes will patiently wait for the next inevitable impactor catastrophe to liberate them to journey to new, more vital hosts. As far as the gene pool of these subsurface chemotrophic microbes is concerned, if Mars crashed into Earth and broke it to little pieces, it would be one of the best things that could ever happen.