In a long but well-written article posted today, Robert Shapiro (professor emeritus of chemistry and senior research scientist at New York University) describes The current state of origin-of-life research. Summary: "The sudden appearance of a large self-copying molecule such as RNA was exceedingly improbable. Energy-driven networks of small molecules afford better odds as the initiators of life." A Simpler Origin for Life (21 Feb 2007 at Scientific American.com)

Answerpudlians know that Darwinism is a fequent topic of discussion and debate here at AP. The theory of evolution, however, does not address the origin of life itself -- i.e., how prebiotic chemistry gave rise to self-reproducing molecules and simple organisms. This topic has been discussed here before, though the particular discussion I recall a year or two ago seems to have eluded my search of the archives. Had I found it, I would have simply continued the thread.

At any rate this is a great article summarizing what is known and unknown in this intriguing area of research. Herewith are some excerpts:

quote:

Which came first, the chicken or the egg? DNA holds the recipe for protein construction. Yet that information cannot be retrieved or copied without the assistance of proteins. Which large molecule, then, appeared first in getting life started--proteins (the chicken) or DNA (the egg)?

A possible answer came...

quote:

...two decades earlier with the discovery of ribozymes, enzyme-like substances made of RNA. A simple solution to the chicken-and-egg riddle now appeared to fall into place: Life began with the appearance of the first RNA molecule.

This became known by theorists as the 'RNA world'.

quote:

The hypothesis that life began with RNA was presented as a likely reality, rather than a speculation, in journals, textbooks and the media. Yet the clues I have cited only support the weaker conclusion that RNA preceded DNA and proteins; they provide no information about the origin of life...
...
The analogy that comes to mind is that of a golfer, who having played a golf ball through an 18-hole course, then assumed that the ball could also play itself around the course in his absence. He had demonstrated the possibility of the event; it was only necessary to presume that some combination of natural forces (earthquakes, winds, tornadoes and floods, for example) could produce the same result, given enough time. No physical law need be broken for spontaneous RNA formation to happen, but the chances against it are so immense, that the suggestion implies that the non-living world had an innate desire to generate RNA. The majority of origin-of-life scientists who still support the RNA-first theory either accept this concept (implicitly, if not explicitly) or feel that the immensely unfavorable odds were simply overcome by good luck.
...
Many chemists, confronted with these difficulties, have fled the RNA-first hypothesis as if it were a building on fire...Nobel Laureate Christian de Duve has called for "a rejection of improbabilities so incommensurably high that they can only be called miracles, phenomena that fall outside the scope of scientific inquiry."

Shapiro goes on to describe a new class of theories of the origin of life:

quote:

The theories employ a thermodynamic rather than a genetic definition of life, under a scheme put forth by Carl Sagan in the Encyclopedia Britannica: A localized region which increases in order (decreases in entropy) through cycles driven by an energy flow would be considered alive.
...
Systems of the type I have described usually have been classified under the heading "metabolism first," which implies that they do not contain a mechanism for heredity. In other words, they contain no obvious molecule or structure that allows the information stored in them (their heredity) to be duplicated and passed on to their descendants. However a collection of small items holds the same information as a list that describes the items. For example, my wife gives me a shopping list for the supermarket; the collection of grocery items that I return with contains the same information as the list. Doron Lancet has given the name "compositional genome" to heredity stored in small molecules, rather than a list such as DNA or RNA.

But details, we want details! :

quote:

Over the years, many theoretical papers have advanced particular metabolism first schemes, but relatively little experimental work has been presented in support of them.

Shucks.

He finishes off with a couple of great quotes:

quote:

In the words of the late Jacques Monod, "The universe was not pregnant with life nor the biosphere with man. Our number came up in the Monte Carlo game." The small-molecule alternative, however, is in harmony with the views of biologist Stuart Kauffman: "If this is all true, life is vastly more probable than we have supposed. Not only are we at home in the universe, but we are far more likely to share it with unknown companions."

I hope others find this as fascinating as I do.

The French biologist/philosopher Bergson (b. 1859) speculated that wherever matter/energy is present, life will emerge (arise by inherent attributes) if conditions are favorable. Trouble with that sort of speculation is you can't demonstrate it.

I have a problem with expressions such as "improbabilities so incommensurably high that they can only be called miracles, phenomena that fall outside the scope of scientific inquiry."

I think he is confusing 'improbabilities' with 'impossibilities', and that is clearly an error.

I find the odds of winning the lottery so daunting that I never buy a ticket; but I do not deny that it is possible to win the lottery.

If we agree to define 'miracles' as things so highly improbable they are outside of the range of scientific inquiry, that's fine by me. In this sense, we could say that the pyrammids were miraculous, since we can't prove how they were constructed, and in fact it seems difficult to imagine how it could have been done if our understanding of the state of engineeering in the 4th to 6th millennia before ours is good.

But I'm afraid that to use the word 'miracle' at all, with its connotations in our language, encourages the notion that there must have been a designer, since nothing can arise out of nothing. Of course those who hold that notion don't realize that they are not evading the problem, but just re-stating it a different way.

If nothing can come from nothing, then that-which-exists is eternal. And who is to say what can happen in eternal time? Improbabilities that are incommensurably high, perhaps?

Still I like the more comprehensible small-molecules model .

Babthrower on Christian De Duve's quote: "I think he is confusing 'improbabilities' with 'impossibilities', and that is clearly an error."

There are improbabilities and then there are improbabilities. Winning the lottery, while tantamount to impossible for an individual player, still occurs often -- there are winners every day.

How about the possibility that every air molecule in the room you're sitting in will suddenly rush into one corner and stay there long enough for you to asphyxiate? Impossible? No, improbable, given that gas molecules collide randomly. Or the probability that you can walk right through a brick wall, given that all the quantum states might randomly align to allow it. Or even the proverbial monkey at a keyboard reproducing "Hamlet." Randomness is quirky.

Those latter probabilities, however, are so low that such an event would not be expected to occur once in every, say, gogolplex lifetimes of the universe. Most people would just dismiss it as impossible. You can't walk through brick walls.

Yet here we are! Life on earth has occurred. Even if not actually likely, the origin of living systems might have been improbable in the first sense (like winning the lottery) rather than improbable in the second sense (like walking through a brick wall). For the latter kind of improbable event to have occurred, de Duve implies, would have been miraculous and unprecedented in scientific theory -- a signal that the hypothesis is wrong.

In this view the probability of life arising could not have been too exceedingly low.

An interesting question: in infinite time, would not every possible thing happen?

(Probably?)

Just curious: In all or any of these theories, is it simply an assumption that life began on this planet, and not somewhere else? I'm not suggesting anything but I would hope researchers in this field don't limit themselves to earthly environments, or to only organic possibilities.

No, answering for myself, no assumption that it began here and not somewhere else. Maybe it begins whereever matter/energy exists by the very nature of the actions of matter/energy. Maybe it began elsewhere and migrated here in the form of viruses (a questionable example of 'life' anyhow) or protein molecules in space dust. Maybe it exists only here. But that's doubtful.

"An interesting question: in infinite time, would not every possible thing happen?"

To the extent that we can speak meaningfully of 'infinite time', any random process will yield all possible results. Eventually the chimp types the complete works of Shakespeare.

In a similar way, the decimal digits of the pi (or any irrational number) are certain to contain any specified sequence of digits, such as one million consecutive zeroes.

Physicists have a related saying: "That which is not forbidden is mandatory."

Cool!

Kendor, I'm not sure it's assumed that life began on Earth, though that is the simplest explanation for our existence. If life were seeded from elsewhere (called panspermia or exogenesis) it merely reframes the problem of biochemical origins one step: How did that life arise?

As to non-organic life (not carbon-based), I don't think anybody knows. I've heard speculation about silicon-based life (Si being Carbon's closest relative in the periodic table) but carbon is uniquely versatile in its bonding flexibility. I defer to the chemistry experts to explain further.

In any case, we know only one kind of life and it's organic. Moreover, among basic nutrients, the sugars are mostly dextro-isomers while amino acids are mostly levo-isomers. If we ate food consisting of mirror-reversed molecules, our bodies would have a hard time metabolizing L-sugars and D-amino acids and we'd starve to death.

Thus these molecular selectivities, however arbitrary they may have been when they first arose, are self-sustaining. A form of symmetry-breaking, I guess.

Would we say time ended if there were no longer any way (theoretically) to measure it? (If motion stopped.)

If motion stopped we wouldn't be able to speak and would say nothing.

Hypothetically, I meant. If motion stopped would it be irreversible?

?!

quote:

Originally posted by babthrower:
Hypothetically, I meant. If motion stopped would it be irreversible?

What does that mean? That it wouldn't start again?

Right, Fab. The sequence of speculative statements:

1. We'll define 'the external world' including but not limited to 'the world of the senses and the extended senses' as 'the universe': that which exists, including us.

2. Either the universe had a beginning or it didn't. If it had a beginning then there is something outside of the universe -- the cause. But we already have defined the universe as 'everything that exists'. So the universe is eternal, since only something external could have caused it, and there is nothing external.

3. Since the universe is eternal then time is only a measure of physical change within the universe.

4. If due to the eventual even distribution of energy the universe becomes 'without events' might we (now) describe such an imaginary state as 'outside of time' (The term 'time' has lost its application?)

5. Can change occur in a universe which is without events? It would seem it couldn't because where would the impetus come from, since there is nothing outside of the universe?

Or is my statement #4 just plain wrong, because change is always implicit in any state of the universe?

We already know that our minds are not designed to grasp these matters, because we reason our way into a contradiction:

The universe either had a beginning or it had no beginning.

A. Nothing exists without a cause. So the universe had a beginning.

What caused the universe to begin? Nothing can cause itself, so something else. But since nothing can cause itself then what caused 'something else'? And so on. Infinite regression. Never do we reach a First (uncaused) Cause.

So: Not A.

Stating both A is true and A is false is a contradiction.

or

B. The universe is without a beginning because it includes everything, by definition. Nothing could have caused it.

But it's impossible that something could be without a beginning.

So it's false that the universe is without a beginning.

So: Not B.

Stating both B is true and B is false is a contradiction.

So since the options are exclusive and both are contradictory then we can't reason our way out of the dilemma.

Using this kind of elementary reasoning the ancient Greeks decided that the universe is eternal. That was what they opted for. They couldn't prove it, nor can we. So our human reasoning is basically flawed. Because the universe exists. So the contradiction is an illusion, and at least one of our common-sense assumptions is wrong.

(Sorry I took this off topic, Perf, but the time thing interests me.)

These are obviously metaphysical questions that stand outside science at the present time. In the future we may have a better handle on concepts like time, space, and causality that will make this discussion seem quaint. Who knows?

On the possibility of silicon-based life:
From NASA Astrobiology:

quote:

Carbon, the MVP in all known biological molecules from sugar to DNA and even squid ink, is unique in that its bonding versatility allows it take on many forms: long side chains that make up fatty acids and cell membranes, ring structures that compose hormones and sugars, and even simple gaseous molecules like methane (CH4) or carbon dioxide (CO2). Can silicon compete?

The short answer is probably not. Silicon simply doesn’t have the moves. While carbon is perfectly comfortable in a variety of different structures (rings, long chains, multi-ring chains, and double-bonded carbon catenations), silicon’s analogous structures are comparatively unstable and sometimes highly reactive. Additionally, such analogous silicon compounds may never occur in nature; the largest silicon molecule ever observed had only six silicon atoms. In contrast, some carbon-based molecules can have tens of thousands!

Silicon also has the formidable disadvantage of being less abundant in the universe.

The article, however, gives silicon a possible role in the origin of carbon-based life:

quote:

One of the unsolved mysteries in the origin of life is why life came to employ one chiral version of a molecule (left vs. right) in its reactions and not the other. Some chemists believe that the chiral selection process in the pre-biotic “soup” might have been aided by a “handed” silica (SO2) surface. Both left- and right-handed molecules could have interacted with the chiral surface, and were aligned according to handedness. In this manner chiral molecules were separated and sorted in preparation for pre-biological selection.

A Wikipedia article on the subject:

quote:

One main detraction for silicon-based life is that unlike carbon, silicon does not have the tendency to form double and triple bonds...Silicon's inability to readily form long silane chains, multiple bonds, and rings severely limits the diversity of compounds that can be synthesized from it. Under known conditions, silicon chemistry simply cannot begin to approach the diversity of organic chemistry, a crucial factor in carbon's role in biology.

So E.T. is probably not made of glass.

Not that there's anything wrong with metaphysics...