Thursday, 5 April 2012

The Origin of Life.

An article appeared in The Independent newspaper entitled ...

(by John von Radowitz)

... summarising recent research lead by Dr Jennifer Blank (Nasa/Ames Research Centre) showing that comets could have delivered amines to the surface of the newly created Earth.

My take on this was that while interesting, the bigger question is whether bacterial cysts could also make the journey because the climatic variations in the environment of a small dying planet, losing its atmosphere and subject to harsh seasonal extremes, would lead to evolution by natural selection preadapting bacterial cysts for survival in deep space, as stated below. In addition it seemed worth discussing the chemistry of self replication since the more we know about it (eg Philipp Holliger et al's recently publicised XNA experiments at the MRC molecular biology lab) the less of an inexplicable or unpredictable event the origin of life appears to have been.

In 1989 Thomas Cech and Sidney Altman were awarded the Nobel Prize for Chemistry for their discovery of the catalytic properties of RNA. Since RNA has both self replicatory and enzymatic powers this makes it, or something like it, a good candidate for having been the living molecule which preceded the evolution of the cell. Since pyrimidines (the building blocks of RNA) can be synthesized from amines and amines are known to be synthesised from the simple molecules which abound in the universe and collect on the surface of planets, this implies that in the huge expanse of cosmological time preceding the formation of the Earth it is very credible that life evolved elsewhere first and raises the possibility that some of this may have seeded Earth before life had time to evolve here of its own accord. So I joined in the comments section and reproduce my arguments here with a few embellishments.

In short it seems possible to me that given the relatively short time between the hypothesised planetary collision which created the Earth-Moon system and the beginnings of life, the Earth was seeded by an intact organism which first evolved on a planet in a different star system. 


Life origin theories still dogmatically avoid any consideration of undirected panspermia as once favoured by Hoyle and Wickramasinghe. I would have thought it was not impossible for comets to first preserve and then disperse entire bacterial cysts when passing between the Earth and the Sun. Cysts might enter the interstellar medium from, impacts with or directly from the outer atmosphere of, dying planets slightly smaller than Mars, gradually losing their atmosphere and water, with harsh seasonal extremes, where bacteria would naturally evolve durable cysts and become preadapted for surviving space travel on the stellar winds. A cyst moved by the solar wind and cushioned by the magnetosphere has a vanishingly small chance of reaching the Earth's atmosphere intact at a speed which would allow it to fall to Earth and live again, but billions of such cysts would make that event an inevitability and it only needed to happen once.

Fellow commentator 'derekcolman' asked me this...

Nice theory, but it begs the question, how did that bacterial life evolve on those other planets?

So I replied...

Aha ! Polypeptides show no replication ability in isolation, its not in their nature, whereas the sense-antisense structure of RNA makes it intrinsically capable of replicating its own polymeric sequences which may themselves have enzymatic capabilities as in the ribosomes today. So RNA is the best candidate for protolife. Amines alone cannot make life, but they can react with each other to make pyrimidines and these can make RNA like molecules in which circumstances RNA sequences might evolve to use amines in their environment and begin the process of evolution. This can happen anywhere the chemistry and energy balance is right, for long enough or in large enough quantities.

The question is why would one consider that more likely to have happened elsewhere. One reason is timing. "Other planets" existed long before the Earth did, so if living molecules can spontaneously arise at all, they were likely to arise there first. Judging from the estimated age of universe 13.75 bn years (assuming for now this isn't a Kelvinesque underestimate) compared to the estimated time it took for life to be established on Earth by whatever means ie approximately 1 bn years (and one can argue for a time period much less than this), the other planets had at least ten times as long to evolve life as Earth had so the probability is that at least one planet somewhere in the vicinity of Earth did based on time alone. But the chances of life beginning on other planets first are astronomically higher than the likelihood that life arose first on Earth, due to the enormity of the number of other planets.

The very large number of candidate planets implies a huge diversity of chemical environments. e.g. planets akin to warm Titans with massive amounts of hydrocarbons and ammonia and water on the surface with strong tidal heating to provide volcanic energy to compliment the light of its sun may have had ideal chemistry to act as an incubator for living molecules in the first generation of chemically diversified solar systems following the earliest supernovas long before the Earth existed.

The likelihood that life from one of these seeded the early Earth depends in part on how many life generating planets exist or have existed close enough to allow physical debris from them to have drifted to this solar system. Allowing for speeds with a magnitude in the range of the solar wind speed of 400km/s over 10bn years the upper range of over 13.3 million light years easily includes all of the local group of galaxies. Whether a bacterial cyst could stay viable for that long is another matter but that depends on the preadaptive evolution of the phenotype of the cyst and its travelling environment, which are highly variable factors. All known bacteria are adapted to the Earth environment and one cannot dismiss the possibility of seeding based on their vulnerabilies as a bacterium evolving on a dying planet would develope very different qualities more suited to space travel and lose them again once it arrived somewhere safer to live.

If one contends life evolved on Earth one has to accept it is more than likely that it evolved elsewhere first. The question is whether it can travel from there to here, between star systems and for the reasons given in my previous post I would consider it possible. What is wonderful about the Earth is that it has remained hospitable to life for so very long compared to its neighbours, due to plate tectonics and the constant tidal massage received from the moon in orbit which has (just barely) maintained an atmospheric balance sufficient to allow creatures like us to evolve who will one day carry life to other planets and whatever we evolve into will do so for as long as the universe will support life I am sure. So the Earth is a champ with great staying power and we owe it everything we have as well as our deepest respect for our own sake and the sake of life in the future, but it seems likely to me that it was given life from somewhere else even if NASA haven't dug anything up on Mars...yet!

Someone else mentioned Zecheria Sitchin as though he was a credible authority at which point I felt obliged to put a different perspective, while discussing the nature of the very early Earth just after the formation of the Moon...

That would not be my choice of reference for the impact theory.

Reginald Aldworth Daly (Harvard) proposed the impact hypothesis for lunar formation in 1946 and it was more widely discussed in scientific circles in 1974-5.

Sitchin began his pseudoscience publishing career in 1976 and like Von Daniken et al, simply incorporated elements of current progressive science into a mythical, mystical, cosmological mishmash to give his books superficial credibility and entertainment value in order to make money.

Nevertheless planetary collision has been widely accepted as a bonafide hypothesis which explains real observations of identical isotopic signatures for the Earth's and Moon's surface compositions. The only problem with it as a mechanism for the propagation of life (from one planet to another via collision) is the surface temperature of Earth after the collision. Pahlevan et al 2007 (CalTech) suggest that the collision was so energetic that in the aftermath there was an atmosphere of high temperature silicon vapour over a molten surface. This kind of temperature would vapourise any bacterial life no matter how hardy it had become and would have sterilised the Earth and Moon.

So it would not be possible for planetary collision to be a means for the propagation of viable organisms. This might only have occurred after the surface solidified and atmosphere cooled sufficiently to permit liquid water.

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