Panspermia

[taetae]

Just came across this deal.


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Directed panspermia

A second prominent proponent of panspermia was the late Nobel prize winner Professor Francis Crick, OM FRS, who along with Leslie Orgel proposed the theory of directed panspermia in 1973. This suggests that the seeds of life may have been purposely spread by an advanced extraterrestrial civilization. Crick argues that small grains containing DNA, or the building blocks of life, fired randomly in all directions is the best, most cost effective strategy for seeding life on a compatible planet at some time in the future. The strategy might have been pursued by a civilization facing catastrophic annihilation, or hoping to terraform planets for later colonization. Later, after biologists had proposed that an "RNA world" might be involved in the origin of life, Crick noted that he had been overly pessimistic about the chances of life evolving on Earth[6]. See: Francis Crick.

Other proponents of panspermia believe that life never evolved from inorganic molecules, but that it has existed as long as all other forms of matter. This is an extension of panspermia called cosmic ancestry.

Theoretically, by humans traveling to other celestial bodies such as the moon, there is a chance that they carry with them microorganisms or other organic materials ubiquitous on Earth, thus raising the curious possibility that we can seed life on other planetary bodies. The same can be said for unmanned probes manufactured on Earth. This is a concern among space researchers who try to prevent Earth contamination from distorting data, especially in regards to finding possible extraterrestrial life. Even the best sterilization techniques can not guarantee that potentially invasive biologic or organic materials will not be unintentionally carried along. So far, however, in the limited amount of space exploration conducted by humans, "terrestrial pollution" does not appear to be a problem although no concrete studies have investigated this. The harsh environments encountered throughout the rest of the solar system so far do not seem to support complex terrestrial life. However, it should be noted that matter exchange in form of meteor impacts has existed and will exist in the solar system even without human intervention. As evidence, some argue that anomalies found within Martian meteorite ALH 84001 indicate that bacteria could travel from planet to planet without intelligent help.

There exists speculation on a connection to the Titius-Bode Law, argumenting that earth may have received seeds of life by directed panspermia, because the extraterrestrial senders knew that earth belonged to a sun system with stable Titius-Bode structure. See: External Link "Directed Panspermia and Titius-Bode"

(Francis Harry Compton Crick OM FRS (8 June 1916 – 28 July 2004) was an English molecular biologist, physicist, and neuroscientist, who is most noted for being one of the co-discoverers of the structure of the DNA molecule in 1953.)

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What do you all think about this?

 

[taetae]

I'm particularly interested in thags opinion and interpretation on niss one.

 

[ThaG]

I should probably start with saying that there's an unwritten rule in biology that Crick is never wrong

If you ask me about panspermia supporters claiming that life have existed forever and never evolved from inorganic compounds, I can tell you that they are undoubtedly wrong and that's as sure as it's sure that evolution happened and we are products of it

The real question is whether life arrived on Earth in the form of primitive cells on meteorites or prebiotic evolution happened here.

We have a fairly good idea what the conditions needed for prebiotic evolution are and we can also make some educated guesses about how the fundamental molecular mechanisms evolved (translation being the crucial one)

The current view which most scientists support is that the first molecules capable of self-reproducing were RNAs or RNA-like heteropolymers. There are some problems with the chemistry of RNA-polymerisation in an abiotic system (2'-5' and 5'-5' bonds being formed in addition to the cannonic 3'-5') but there's been significant progreess towards demonstrating that certain minerals can catalyze the formation of 3'-5' bonds exclusively.

Many RNA molecules can catalyze their own synthesis and these molecules can be very short. This has been demonstrated in lab conditions using random sequences. RNA molecules can also catalyze a wide range of other reactions, including protein synthesis. In fact, every known organism uses RNA-catalysis for protein synthesis as the ribosome is actually one huge protein-assisted ribozyme. RNA can also catalyze DNA synthesis and there are RNA-dependent-RNA-polymerases which can replicate RNA molecules.

Anyway, given these observations and molecular fossils and many others I didn't list because I don't have time, we can say that the RNA world existed with great confidence.

What probably happened is that protein synthesis evolved in the RNA-world and since proteins are much better at catalyzing biochemical reactions, they were selected for. DNA appeared later and substituted RNA as a carrier of genetic information and RNA took the role of a messenger and regulatory molecule with some catalytic functions.

Some of the big questions remaining are "How exactly the genetic code appeared", "How did the ribosome evolve?", "Why exactly these 20 amino acids and how they were selected?", "What did the Last Universal Common Ancestor look like", etc.

We kind of know why all amino acids are L-oriented - certain minerals catalyze only the synthesis of L-enantiomers of amino acids so life probably evolved on such rocks.

What we don't know for sure is whether the RNA world happened here on Earth or somewhere else

You can see that there are many events in the prebiotic and early biotic evolution that had to happen in order for the first cells to develop. We know this happened, the problem is time. There is good evidence that bacterial cells already existed about 3.5-3.7 billion years ago.

So we're left with less than a billion years for all the geological and chemical evolution - formation of oceans and early atmosphere, building up sufficient amounts of the chemical compounds necessary for prebiotic evolution, and evolution of life itself. The Earth itself is 4.56 BY old. According to some estimates, the RNA world can't have lasted for more than 100 million years and must have happened around 4 Ya, which doesn't seem to be enough for all crucial steps in evolution to occur. Of course, this doesn't mean it can't have happened, but it would be much nicer if the Earth was, say, 6 Billion year old and the first cells appeared 3.8 billion years ago.

This is one of the main reasons some scientists think that prebiotic evolution happened somewhere else and life came here already at the cellular stage. There is one additional observation pointing towards these conclusions

There is a bacteria called Deinococcus radiodurans. It can survive 500Gy radiation and lives happily in nuclear reactors. For those that don't know radiation causes breaks in DNA and this is why it's so lethal. Deinococcus radiodurans has the same DNA as every other organism on Earth but it has some interesting and very efficient mechanisms for repair of damaged DNA, this is why it survives radiation doses much higher than the lethal dose for any other known cell. (we use radiation to treat cells in my lab and I can tell you that all human cells are dead at 15-20Gy and much lower doses are sufficient to cause extensive damage).

The question is why this bacteria exists - there is no way it appeared after the invention of nuclear reactors because as I already said it has developed intricate mechanisms for repair of DNA, which can't have evolved so quickly. There is absolutely no way that anywhere on this planet selective pressure directed towards the evolution of a life form able to survive 500Gy ever existed. But these are exactly the traits needed for a bacterial cell to survive in space, which is food for thought, to say the least...

But then another question arises - if panspermia happened once, ~4 billion years ago, why did the lineage leading to Deinococcus radiodurans retained the ability to survive high doses of ionizing radiation, when all other cells didn't? The selective pressure hasn't been there for nearly 4 billion years....

I can't give you an answer to this question, neither I can tell you whether panspermia hypothesis are true or not, but if you ask for my opinion, I would say I am more like 60:40 pro-panspermia (even if it's just because of the rule I mentioned in the beginning)

When we find extraterrestrial life, all these questions will be answered, maybe even on the first planet where we find it - if the cells we find there share the same genetic code and fundamental molecular mechanisms, and there ribosomal RNAs are homologous to ours, we can be sure we share a common ancestor with them

 

[taetae]

Thanks for the response.

 

[ThaG]

Forget about the Deinococcus, this is explained already...

(should have thought about it)

http://scienceblogs.com/notrocketsc...he_worlds_most_radiationresistant_animals.php

Category: Animal behaviour • Animals • Evolution • Invertebrates • Sex
Posted on: March 24, 2008 5:00 PM, by Ed Yong

Blogging on Peer-Reviewed ResearchBdelloid rotifers are one of the strangest of all animals. Uniquely, these small, freshwater invertebrates reproduce entirely asexually and have avoided sex for some 80 million years. At any point of their life cycle, they can be completely dried out and live happily in a dormant state before being rehydrated again.

Proseola.jpgThis last ability has allowed them to colonise a number of treacherous habitats such as freshwater pools and the surfaces of mosses and lichens, where water is plentiful but can easily evaporate away. The bdelloids (pronounced with a silent 'b') have evolved a suite of adaptations for surviving dry spells and some of these have had an unexpected side effect - they've made the bdelloids the most radiation-resistant animals on the planet.

Radioactive danger

Ionising (high-energy) radiation is bad news for living cells. Far from granting superpowers, it damages DNA, often completely breaking both strands of the all-important molecule. If you think of DNA as a recipe book for the various parts of a living thing, the double-stranded DNA breaks that are caused by ionising radiation are like tearing the book up into small chunks.

Absorbed doses of radiation are measured in Grays and ten of these are more than enough to kill a human. In comparison, bdelloids are a hundred times harder. Eugene Gladyshev and Matthew Meselson from Harvard University found that two species shrugged off as much as 1,000 Grays and were still active two weeks after exposure.

At this dose, their egg-laying capacity took a large hit and fell to 10% of previous levels, but even so, they weren't sterilised completely. Their daughters (who are all identical clones of their parents) also showed a similar lack of ill effects due to the radiation. These figures make the bdelloids the most radiation-resistant of all animals so far tested. Even other rotifer groups show similar levels of sterilisation at a fifth of the doses tolerated by bdelloids.

Their closest animal rivals are the tardigrades or 'water bears', impossibly cute aquatic animals that are quite possibly the hardest animals alive. Like the bdelloids, they can also enter a dormant, dried-out state where they can withstand extreme heat, temperatures close to absolute zero, poisonous gases and extreme radiation. As I've blogged about before, they may soon be revealed as the first animals to have survived the open vacuum of space. But even these hard-cases have been sterilised by the 500-1,000 Gray doses tolerated by the bdelloids.

Genomes in pieces

Bdelloid.JPGThis resistance is all the more amazing because radiation affects the DNA of rotifers in the same way as other animals - it shreds it. Gladyshev and Meselson measured the size of the remaining pieces in one species, Adineta vaga, immediately after being exposed to radiation. They found that a 560 Gray burst broke the animal's genome in over 500 different places, and the 1,000 Gray doses that they contended with so well created over 1,000 double-stranded breaks.

The fact that the bdelloids survive and their offspring are fertile is a clear sign that they have an extraordinary ability to repair these breaks, or to protect the proteins that do so. But most places on earth, including the habitats frequented by rotifers, have very low levels of background radiation and without intense sources, there is no impetus for an animal to evolve extreme resistance. How then could it have evolved?

Other species provide a clue. Only bacteria can give the bdelloids a run for their money in the resistance stakes and one in particular, Deinococcus radiodurans, has a name that literally means "terrifying berry that withstands radiation". Like the bdelloids, it can reassemble its genome after it has been torn asunder into tiny fragments. In general, bacteria that are resistant to radiation also tend to be resistant to prolonged bouts of dehydration, a connection that the tardigrades also share.

It turns out that both drought and radiation pose similar challenges including the production of damaging reactive oxygen molecules and frequent DNA breaks. So Gladyshev and Meselson believe that the ability to shrug off killer doses of radiation is a happy side-effect of adaptations to dry-living.

Coping without sex

The resistance to drought may have given the bdelloids a competitive edge over parasites, predators and other rotifers that aren't so hardy. It may also have ensured their success when they first started to adopt an asexual way of life, by mitigating some of the more harmful side effects of this strategy.

Without the genetic shuffling that accompanies sex, asexual reproduction is often viewed as a poor long-term strategy that leaves a species unable to adapt quickly to new challenges. But some groups have argued that the process of shattering and reconstructing their genome may provide the bdelloids with genetic benefits that compensate for this drawback.

The bdelloids repair their broken DNA by using a duplicate piece as a template for copying the lost information at the site of the break. If this template strand contains a gene with a new beneficial mutation, the animal would suddenly have two copies, and a positive change that might otherwise have been genetically overlooked could more easily spread within a population.

The genomes of asexual animals are prone to invasion by genetic parasites called transposons, selfish bits of DNA that can jump around a genome. Our genomes are rife with these parasites and their spread is predicted to go unchecked in asexual lineages to the point where they do so much damage that the species goes extinct. Frequently repairing their DNA may give bdelloids a route for cutting out unwanted genetic parasites and indeed, these animals have remarkable streamlined and transposon-free genomes.

Images by Diego Fontaneto and David Mark Welch.

Reference: doi:10.1073/pnas.0800966105. This paper will be published some time this week in PNAS.

 

[Hutch]

DNA repair mechanisms utilized to tolerage high doses of radiation can also be used, and most likely evolved, to tolerage extreme desiccation, which has been correlated with increased breaks in DNA. As you mentioned before, the fact that these microorganisms have existed on this planet for so long without shedding radiation tolerance genes suggests that they evolved on Earth. That, and the fact that they respire, are most often found in environments rich in organic compounds and contain several copies of theie genome (allowing for redundancy), provides further evidence against an extraterrestrial origin.

Panspermia is an interesting concept, I doubt it's true but one can't rule it out completely though. The world stopped paying attention to Crick after he deciphered the DNA code and lost his marbles :S