I had no idea

[james_davis_nicoll]

As pointed out in email, JEP on evolution.

Comments

[daveon.livejournal.com]

I still think, to borrow a phrase from upthread, deep time and LOTS of chemical sorting could account for that.

[neowolf2.livejournal.com]

Maybe, but the odds of getting something like life as we now know it, just be random combinations of molecules, are extremely low, so low that we wouldn't have expected it to happen anywhere in the observable universe. The exponential decline in probabilities as molecules get larger kills this approach pretty quickly.

Biologists should look for very low complexity "life", in my opinion. Billions of atoms is too much.

BTW, I saw an interesting graph plotting complexity of life vs. time. Complexity has been gradually increasing, but if you plot backwards, the intercept with the horizontal axis is about 10 billion years ago. This suggests that life actually predates Earth, and Earth was seeded with a simpler kind of life that then continued to evolve. This might be another explanation for the "Great Filter" of the Fermi problem: such transfers of life to newly formed planets may be necessary for evolution to continue, but are also very uncommon.

[tandw.livejournal.com]

Biologists should look for very low complexity "life", in my opinion. Billions of atoms is too much.

OTOH, I would expect very low-complexity "life" to have been eaten by the more complex stuff as it comes into existence. Essentially, the conditions that were favorable to the development of protolife/less complex self-replicators have long since been changed beyond recognition by the presence of life here.

[mme-hardy.livejournal.com]

Yes, but the odds of getting another working combination are also unimaginably low, and another, and another, and the set of all theoretical combinations of working molecules could -- we don't know -- sum to something very large. The existence of one, precisely specified, kind of life is a single data point, and you can't generalize unless you've got lots of datapoints, which in this case we have not got.

"The odds of my being hit by a car at this particular corner at this particular moment are staggeringly low!" Fine, but the odds of somebody being hit by a car at this corner seem to be about 20%, and the odds of your being hit by a car somewhere, at some point in your life, are quite substantial.

[daveon.livejournal.com]

Well, that was Hoyle's preference. It might be, although I suspect that life, for various values there of, probably is pretty damn common . Intelligent tool using life, much, much less common.

[heron61.livejournal.com]

Given that either life evolved essentially as soon as Earth cooled sufficiently to have liquid water, and there's at least arguable evidence that Mars had or has life, either panspermia is correct or life evolves quite rapidly (at least on a Deep Time scale). Of course, rapidly on a Deep Time scale basically means less than 100 million years. I'm also expecting the universe to be teaming with non-intelligent life.

[neowolf2.livejournal.com]

The alternative explanation for the rapid origin of life on Earth is that the conditions for which OoL were possible only last a short time. So, if life is to originate at all, it must happen quickly.

If Mars has life, we can't rule out the possibility (the probability, actually) of contamination from the early Earth (or vice versa). There were lots of impacts in the early solar system, which would have sent lots of rocks into solar orbit to carry simple cells back and forth.

[rpresser.livejournal.com]

Please forgive me, but I am unreasonably amused by your remarkably apropos typo ("teaming" instead of "teeming"). All the universe's non-intelligent life teamed up against us ... quite a picture.

[graydon saunders (from livejournal.com)]

We _didn't_ get life as we know it.

Life as we know it, as we notice, as our intuitions expect, is macroscopic eukaryotic multicellular bilaterally symmetric segmented metazoans. That's probably five big steps -- original replicators, original cells, step before eukaryotes where there's enough environmental specialization to get cells that can club together to form eukaryotes, eukaryotes, and then embryology so you can have a substantial organism -- from where life arose.

Even figuring out what the environment was like back then is tough; not much rock, few temperature proxies, etc. The idea of catalytic surfaces and then chemical sheets has been getting a bit of traction, at least at the level I can follow along at.

Also, measures of complexity are very, very tough, and defining complexity is tougher. It's very easy to produce something delusive when you do. (what's the complexity of a diplodocid sauropod? why is this less than that of a minke whale?)

[neowolf2.livejournal.com]

By "life as we know it" I mean any life. The simplest living cells we've found are staggeringly complex. Eukaryotes are more complex still.

Catalytic surfaces really don't address the meat of the problem, which is how one puts together a system in which the replication of information is accurate enough for natural selection to occur (where the information not to be eroded away by excessive random error.)

[carloshasanax.livejournal.com]

What tandw said. That sort of system will look like food to modern life.

I don't think there's that much of a gap. Working from the top down, catalytic RNA has been known for over thirty years. Working from the bottom up, ribose forms in condensation reactions from formaldehyde; adenine forms in condensation reactions from hydrogen cyanide. The bottleneck step, to my mind, would be the formation of reaction conditions which favor the nucleoside linkage.

The pairing of replication with metabolism and membrane division are separate things, of course, but there are signs of early lock-in.

[neowolf2.livejournal.com]

That might work if you have pure ribose, and pure bases. I doubt such things can work at all in the more random glop formed by condensation reactions,without enzymes to catalyse precisely the ones you want.

The scam in origin-of-life research is to show you can make chemical X at some low concentration in some mixture with other stuff, and then start the next round of experiments with pure X purchased from a supply house.

[carloshasanax.livejournal.com]

Actually, not so much. There's a lot more possibility in chemical replication than what we see today, because modern life has converged on a highly efficient solution. You don't need exactly ribose -- DNA is conclusive evidence of that -- and the sugars that work are thermodynamically preferred in Butlerov-style synthesis. Even more obviously, you don't need exactly adenine. There are five big bases that are used in the current set-up, but there are dozens which have been identified as occurring.

There are also plausible natural circumstances which concentrate organic compounds with different properties, in processes roughly analogous to ore formation, or the separation procedures undergraduates perform in chemistry lab.

It's the specific problems -- nucleoside linkage, the origin of chirality, the coupling to phosphate energetics, etc -- that are the real pain in the ass. The "random glop" argument is a red herring.