Evolution of Life: Exploring Probable Pathways

[sage]

how did life evolve out of inanimate organic molecules is a question that has vexed scientists for decades now and no consensus has yet emerged on the issue. however there are many competing theories including RNA world hypothesis, khauffman's models and a bizarre proposition of an initial evolution of proto life from clays. curious to learn what you yourself think as probable pathways by which life could have evolved, whether it is a rare event in the history of our universe or can happen often under right conditions.

 

[Nereid]

Hi sage, very good question!

[nitpick]The origin of life is more in the arena of abiogenesis than evolution; the latter's domain of applicability starts once there is life![/nitpick]

While there is no shortage of rich brews of organics in early solar systems (and other places) - formed by increasingly well understood physical processes - so the opportunities for life to get started seem to be many, how often it did start independently has become even more difficult to determine! Why? Because it's also becoming clearer that a) there's quite a lot of material which is exchanged between planets and proto-planets (within a solar system, and maybe between star systems), b) some forms of life could easily survive a transfer from one planet to another (deep inside a rock, for example), so c) it may be that once life starts (arrives?) in a (proto-) solar system, it will likely spread to all habitable planets and moons, and maybe aborting independent beginnings.

 

[sage]

but neried what do you think is the most likely way that life originated. I've heard about panspermia hypothesis, but that simply shifts the question to some other place in the universe.also i believe that one should look into that alternative last when it is established that there is no way that life could have abiogeneted on earth.i believe it is too early to say that. there are many ecosystems that we had hardly studied. not much is known of abyssal ecosystems , nor about subtarranean ones deep within the crust.anyway given that we do know what we do how do you think life could have originated here or anywhere on this universe. perhaps we can have a poll on this!

 

[Nereid]

[joke]I believe that the whole universe is actually an escaped über-virus, from a 128-dimensional computer game, and that carbon-based life merely an epiphenomenon, inevitable given the mutated code of the 128,1289-dimensional nature of the über-virus! [/joke]

Seriously, the exciting thing is that we are beginning to be able to find some answers to these sorts of questions, and even formulate tentative hypotheses. The sad thing is that I'll be long gone before even halfway decent answers will be found

So, what's to be done? Well, why not develop in-principle tests? examine implications of hypotheses to see if there are other ways to test them? check consistency with well-established theories? Plenty of work for aspiring astrobiologists!

 

[Grev]

I like your joke. :D

And I'm just an observer BTW, just trying to be more wise. :)

 

[Widdekind]

Here are some relevant threads, all dealing (directly or indirectly) w/ the Panspermia Theory:

• https://www.physicsforums.com/showthread.php?t=1935
• https://www.physicsforums.com/showthread.php?p=1998489
• https://www.physicsforums.com/showthread.php?t=278815

 

[KAckermann]

As if the odds were not long enough...

OK, there is strong evidence that nearly all life on Earth originated from a single progenitor, and that is the encoding of amino acids from nucleotides. In mRNA, a nucleotide can contain one of the 4 bases A,C,G, or U, and nucleotides triple up to form a codon, which codes for a specific amino acid in a ribosome. A codon can assume one of 4³=64 states, but there are only 20 amino acids, so many of the amino acids are coded for by several of the 64 values. The fact that the exact mapping from codon states to the same amino acids in virtually all life strongly suggests a common progenitor.

Now here is where things get dicey, in my opinion:

The mapping could have been anything, but it wasn't. It so happens that the redundancies in the coding significantly lower the chances of a harmful mutation occurring. It is arranged so the 3rd nucleotide in the group of 3 is highly tolerant of a mutation. It is statistically unlikely to have happened by chance.

 

[Andy Resnick]

There's 21 amino acids- you forgot selenocystine. And three codons are 'stop' codons. But yes, there is redundancy in the 3-nucleotide code generating amino acids. Some amino acids, like methionine and tryptophan, are only encoded by a single 3-nucleotide sequence, while others like serine and leucine are encoded by 6 sequences. I don't know if the number of different coding sequences corresponds to any functional assay, like frequency of occurence.

Why do you claim that the coding of 3-nucleotide sequences to amino acids is statistically unlikey to have happened by chance, when neither survival nor reproduction occur by chance alone?

 

[BoomBoom]

The mapping could have been anything, but it wasn't. It so happens that the redundancies in the coding significantly lower the chances of a harmful mutation occurring. It is arranged so the 3rd nucleotide in the group of 3 is highly tolerant of a mutation. It is statistically unlikely to have happened by chance.

Well, keep in mind that the only reason there is a 3-base codon is based on the structure of the transfer-RNA. It matches up with 3 nucleotide bases and 1 amino acid. It does wind up working out that way that a SNP many times will result in no difference in amino acid, but that has more to do with the t-RNA than it does any sort of genome regulation.