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  #1  
Old 01-29-2017, 06:01 PM
Leah S Leah S is offline
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Default Life on Other Planets – A Biologist’s Viewpoint

At the PAS meeting on January 19, Dr Lauren Edgar shared some very interesting information about the discoveries of the rover “Curiosity”, including the possibility that there might have once been a measurable amount of water on Mars. A PAS member asked whether they had found any signs of life, and added, “maybe primitive life, like amebas…”

I would like to comment on this as a biologist (M.Sc., Zoology).

While it is known that water is essential for life, the presence of water does not necessarily guarantee the development of living organisms. The equation doesn’t work both ways. Just as – you need to buy a ticket in order to win the lottery, but having a ticket won’t guarantee that you will win the jackpot.

Even “primitive” organisms, such as protozoans and bacteria, are incredibly complex. They must carry out all functions of a full organism, with only one cell to do it! For example, they must be able to ingest food, digest it, and metabolize it to release energy, in a way that the energy can be utilized afterwards to power other activities. They must be able to eliminate waste products, and to manufacture proteins – both structural proteins and enzymes (which are proteins that carry out various reactions). They must be able to detect stimuli and respond to them, e.g. moving towards food or away from danger. And they must be able to duplicate their DNA and undergo mitosis (cell division), which is quite a complex choreography.

In “higher” organisms, specialized cells (arranged in tissues and organs) carry out various functions. But a one-celled organism must do it all!

Scientists have not yet found an explanation for how life was spontaneously generated on our own planet, much less on others. The logistics are against it, even in a nurturing environment such as Earth, which has had a moderate temperature and large amounts of liquid water for billions of years. Just to give one example: a protein is a chain of hundreds of amino acids, but not every series of amino acids is a functional protein, just as not every series of letters is a real word. The specific order of various amino acids in the protein chain determines its 3-dimensional structure, which in turn will determine whether it is a functional protein, and what it will be able to do. And a living cell, even the smallest bacterium, requires many different kinds of functional proteins, and many identical molecules of each one. Even if we imagine a pond or lagoon that is full of amino acids ready to randomly organize themselves into protein chains, it would be highly unlikely for even one functional protein molecule to spontaneously appear, much less several different kinds, with several identical copies of each – just as we do not usually find one of Shakespeare’s sonnets (or even one logical sentence) spontaneously appearing in a bowl of alphabet soup. The “pond full of amino acids” is theorized for the purpose of justifying the spontaneous generation of the first life, but there is no sign that any such pond ever existed. Also, besides the mystery of how the first proteins arose, the origin of the genetic code in DNA is another mystery.

So the jump from inanimate matter to life is quite large, even under ideal conditions. If it were easy for life to be generated, we would expect to see life arising on Earth several times in its history; instead we see it appearing only once, and all living things today are descended from that one event.

Some people point to extremophiles (organisms that can survive in hostile environments, such as high temperature, pressure, or acidity) as evidence that life can be spontaneously generated even in extreme environments. However, it is important to realize that the ancestors of today’s extremophiles originally lived and developed in a much more nurturing environment. After living cells already existed, some of them were able to adapt to extreme conditions as well. But they did not originate there.

I believe that NASA and other organizations encourage people to believe that life can easily be spontaneously generated on any planet that has water, or even other solvents (such as moons with an “ocean” of liquid alcohol or methane); and for a very prosaic reason. Suppose NASA wants to ask for funding for a space probe to explore the geography of a planet or moon. As an astronomer, I would find that fascinating, but the response of the general public is: “Didn’t we already launch a space probe once? Why should we spend money on another space probe when we can spend it on another baseball stadium!!!!” But if NASA says, “we want to send a space probe because we think we might find *water*! And you know that if there’s water, there’s got to be life – at least primitive life!” – people open their wallets and ask, “how much do you want?”

I think that even if this is what they are “selling” to the public, as astronomers we should at least be aware of the true probabilities regarding the spontaneous generation of life on other planets.
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  #2  
Old 02-01-2017, 02:47 PM
Mark Johnston Mark Johnston is offline
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Here's my view:
First of all, we have only direct access to one world with water, so clearly with a sample size of 1 it's hard to extrapolate. But I believe the likelihood is high that simple life will arise (has arisen) on 'wet' worlds.
I would cite the Miller experiment, first performed in 1952, where the gases present in Earth's early pre-life atmosphere were mixed together with water, sparks added (to simulate electrical discharge of lightning) and dozens of amino acids, fatty acids and urea were spontaneously created. This experiment has been repeated dozens of times with the same results. These molecules and sugars are also routinely found in meteorites, which suggests they form naturally and easily.
Further tests have shown that given time, when such organic molecules are left alone with an energy source, they begin to link together to form more complex structures. Certain molecules can self assemble into orderly structures - hollow spheres similar to the membranes of cells, and long chains that are suggestive in structure to DNA. (look up "chemical evolution")
In 2016 fossils were found in Greenland that were 3.7B years old, suggesting life arose immediately on Earth as soon as it was formed and the heavy bombardment phase was over.
Neil de Grasse Tyson has an excellent video on how the eye evolved from a crude mass of cells that could not differentiate bright from dark to the complex and elegant structures that are now common.
I would be willing to bet we'll find evidence of microbial life in our own solar system (e.g. Europa) or detect molecular oxygen in an exoplanet's atmosphere (indicating life) within 20 years. Sadly, I doubt we'll spot alien space ships, or even fat creatures doing the backstroke in Titan's seas, but do think simple life is common and will soon be discovered.
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  #3  
Old 02-01-2017, 03:55 PM
Eric Steinberg Eric Steinberg is offline
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Default Some thoughts

I think a large part of the problem is DNA itself. It's not just a complex molecule, but an astonishing multiplicity of functions that work together in a choreography that is nothing short of mind-bending. Just the mechanism for separating the strands prior to copying is a wonder, never mind messenger RNA, transfer RNA and the functions of the ribosome.

Check this out: https://www.youtube.com/watch?v=OjPcT1uUZiE

There have been some attempts to run the probabilities of the basic DNA structure and function "arising", even assuming self-organizing qualities. The time scale tends to end up in the 10's to 100's of trillion years, problematic in a 14 billion year old universe.

In fact, off the record many evolutionary biologists will admit that there really isn't a clue how the DNA system came into existence. Again off the record some very solid scientists have suggested seeding from somewhere else....

This is just the basic mechanics of the DNA storage, templating and reproduction system. It says nothing about the actual information encoded in the DNA that prescribes living organisms and how it came to be.

I think it's easy to skip over the underlying molecular mechanics of DNA or for that matter the mechanisms of how a completely new species could arise, given the what make for different species are in fact, profound differences in biology. To do so however is, I believe, a gross oversimplification that leads to unsupportable assumption.

I'm inclined to agree with Leah that life is much more special and rare than some would like to think....
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  #4  
Old 02-01-2017, 06:11 PM
Peter Turner Peter Turner is offline
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Default To DNA or Not To DNA. That is the question...

It may be that using the word "life" should be modified to "life as we know it". As Mark noted, we are a sample size of one. To say that life forms must come from a water world with organic molecules, using a DNA type structure, seems to constrain things too much.

And, while DNA may potentially take a long time on average to come to life, who's to say that DNA based life is the only way to create life? Would we even be able to evaluate non-DNA based life?

However, extrapolating the vast quantity of exo-planets and exo-moons that appear to exist in our corner of the galaxy to the universe as a whole, means there are potentially Quintillions of planets. This is based on roughly three thousand planets found from searching 3,000 stars. There are probably far more in those 3,000 stars, however using our current technology it would take 500+ years to completely survey them all. We've effectively only looked at planets that are inside the orbit of Mars on these 3,000 stars. If we were on another star we couldn't have found Jupiter, Saturn, Uranus or Neptune as the planet must pass across or behind the star three times to become an exo-planet candidate. Neptunes orbit takes 165 years.

There are 200 billion plus stars in the Milky Way and 100 billion plus galaxies in the observable universe. That equals over 200 Quintillion stars, plus however many exo-moons. At 100 trillion to one for DNA to form, that means it could have formed 2 million times on planets alone. (This analogy is a little like the 9 woman + one month = one baby scenario. But you get the idea.)

Searching just the stars in our neighborhood we have no way to even survey these exo-planets / moons for life. In the next 10 - 20 years we should be able to determine if they have water in their atmospheres. In that same time we may have sent some robot explorers to some of the moons of Jupiter and/or Saturn to look for signs of life. So unless ET comes and knocks on our front door, or we get lucky with some of our searches, I wouldn't hold our breath that we will find life anywhere for a long, long time. But the search will be fun - and expensive.

Last edited by Peter Turner; 02-01-2017 at 06:17 PM.
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  #5  
Old 02-02-2017, 06:09 AM
Leah S Leah S is offline
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Eric - thanks for the support.

Mark - just to point out that the Miller experiment was based on a mistaken view of the composition of Earth's early atmosphere. it's been repeated successfully with those same starting materials, but that's apparently not what the Earth's early atmosphere contained; and it doesn't work with other starting materials which reflect our current understanding of Earth's early atmosphere. as for "self-assembling molecules" - snowflakes and crystals can self-assemble but they aren't alive. regardless, a short polypeptide chain or even a longer random chain of amino acids is not a functional protein. and my point was that even "primitive" life is not really that "primitive" when we examine all the activities and *functional* complex molecules that it requires.

Pete - I didn't speculate on the possibility of some kind of life arising somewhere else in our galaxy or in another galaxy, if an Earth-like planet exists with the proper components and the proper distance from a suitable star. I was talking just about our own solar system where there is only one Earth-like planet. the part that bothers me is the automatic assumption that "primitive life" will automatically arise whenever there is liquid water, or some other solvent, such as alcohol or methane. the complexity required, and the statistical probability of that happening, are amazing enough on our own planet, which has had moderate temperatures and liquid water for billions of years. and yes I know that the first bacteria appeared within half a billion years after the Earth cooled down enough to have liquid water, as Mark mentioned. that just makes it more amazing, it doesn't mean that it happens automatically.

there is a joke about an immigrant who got off the steamship in New York and saw a silver dollar on the pier. he was about to pick it up and then he said to himself - why should I bend over just for silver? the streets in America are paved with gold!!!

regardless, if it were "easy" and "automatic" for life to arise, then it would have happened more than once on Earth. so I think it's unrealistic to extrapolate to other planets or moons in our solar system with less-than-ideal conditions. e.g. even if Mars might have had water at one time, it wasn't in the same quantity or for the same length of time as on Earth. and even if Europa might have some kind of "ocean" under a thick coat of ice, it gets much less sunlight due to the distance, and temperatures are extremely cold - which makes it difficult for chemical reactions of any kind to take place.

as for the possibility that some other molecule might fulfill the function of DNA - it doesn't matter what molecule fulfills the function of storing and transferring the information, as long as it is suitably complex for the task; the question is how a method can arise where one molecule (such as, but not limited to, DNA) can *code* for another one. i.e. not only do we need a functional polymer (protein or otherwise) with suitable variable units, but also a method with instructions to repeatedly assemble it. and a random chain of amino acids is not necessarily a functional protein.

it's nice to dream about finding extraterrestrial life, but I really doubt if any of the other planets or moons in the solar system has the conditions for it. in any event, the claim that any presence of water, even for a short time, will inevitably lead to "primitive life" is highly unlikely.
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  #6  
Old 02-02-2017, 08:59 AM
Peter Turner Peter Turner is offline
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Leah,

I agree that the chances for other life in our solar system are remote. My point was to say that other life may not be DNA / protein based. And, using earth as an example for where life may be found, seems to be a false assumption.

Even with DNA, our knowledge is less than 65 years old. The serious study of genetics is much younger. What looks complex to us now may be child's play in 100 years. As an example: In 2012 scientists created an alternative to DNA, called XNA. They are using it to shed light on how molecules first replicated and assembled into life billions of years ago as well as understanding how better to do gene manipulation. XNAs form a double helix with the potential to be more stable than DNA because they are made up of six coding blocks rather than DNA's four.

If we can make XNA 59 years after discovering DNA, is it possible that something like XNAs formed naturally? This is an unanswerable question at this point. However, as technology strips away the complexity of DNA we should be in a better position than we are now to say that other life is possible in our solar system. Be it DNA based or otherwise.
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Old 02-02-2017, 02:31 PM
Sam Insana Sam Insana is offline
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While it may be difficult to spontaneously generate life on a single planet, once life is present, in the form of spores, they might be carried unintentionally aboard spacecraft, or meteorite rock fragments, and end up on other worlds. Spores can be very resistant to extreme temperatures and radiation and might someday germinate if the other world's environment becomes supportive of life.
I also think the Drake equation of over 50 years ago, while the individual variables are difficult to quantify, does demonstrate that the numerical odds of life existing on some of the trillions of other planets is pretty good. I won't argue "intelligent" life since if we are equating our own life with intelligence, then that life may not be long lived with our propensity to destroy our own environment.
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Old 02-02-2017, 06:13 PM
Leah S Leah S is offline
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Pete,

I'm not focusing on a specific molecule but on the principle.

yes, theoretically there can be other polymers that fulfill the functions of DNA and protein. that's not the question. the molecule would just need to be a polymer made of similar but non-identical units. (i.e. the units in the polymer would be analogous to amino acids in a protein, or to the 4 bases in DNA, or to letters in a word.) the genetic polymer (analog of DNA) would need this to be able to store information; the functional polymer (analog of protein) would need it to be able to assume different 3D shapes and other properties.

what I am saying that the logistics of producing *any* molecule of this type, regardless of its identity, would face the same difficulties. a planet or moon that has water (or other suitable solvent) for only a short time, e.g. less than a billion years, or in only a small quantity, would not be able to do it. and if the temperatures are too cold, it would be difficult for chemical reactions of any type to take place. (I teach chemistry, I can tell you all about reaction rates.)

and that's just regarding the molecules. as I described in my original post, a living cell is more than just a molecule or two. it needs to perform a wide variety of functions, with different chemical participants for each one. that is a big jump from a few amino acids (or whatever other monomer) assembling into a short chain.

theoretically, among the trillions of stars that might have planets in our galaxy and other galaxies, maybe there is one with suitable conditions for development of life. but I don't think that any of the planets or moons in our solar system, other than Earth, has the right conditions. it is unrealistic to say that life is automatically generated wherever there is even a small amount of water for even a short time. it just ain't so.
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  #9  
Old 02-02-2017, 06:29 PM
Leah S Leah S is offline
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Sam,

as I've described in my previous posts, I'm not talking about the question of life on a planet of one of the many trillions of stars outside our solar system.

I was specifically talking about the solar system - and the way that we are repeatedly told that if we can find water on any of the other planets or moons in our own solar system, even in a small quantity, for a short time, then life will automatically develop. my point is that this is unrealistic because of the logistics, and because even "primitive" living things are much more complex than most people realize.

as for spreading of spores - that isn't what people usually mean when they talk about life on other planets. e.g. nobody is talking about "life on Mars" that is only due to contamination by a spacecraft from Earth. and the possibility of spores coming from an extrasolar planet would be *really* remote. (first, there would need to be life on that planet; then the spores would need to be disseminated into space and somehow reach one of the planets or moons in our solar system. and that is really remote, due to the distances involved and the enormous amount of empty space. that's why we often find meteorites that originated in our own solar system but not from extrasolar planets.)
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