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Comment by ealloc | Hacker News Reader

Personally I think the idea that life/prebiotic soup came to earth on a comet is suspect, for the reasons explained in these blog posts and the links therein:

http://sandwalk.blogspot.ca/2011/11/nasa-confusion-about-ori...

http://sandwalk.blogspot.com/2009/04/can-watery-asteroids-ex...

Two problems with the 'organic asteroid' theory are the 'racemization problem' (how to explain why life only uses L isomers rather than both the L and D isomers present in meteorites), and the fact that the amount of organic matter in meteors is tiny. Compare that to the opportunities for life to arise on earth in its diverse chemical environments.

See the calculations by Jeffrey Bada discussed in the second link.

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It's possible that there's no reason life chose L or D isomers, but choosing one and sticking to it was advantageous (I use the word "choose" loosely here).

Abiogenesis is one of those things where there is way more speculation than evidence. Some speculation says that it is because the molecular machinery maintenance to handle both would be twice as taxing for organisms.

Of course, having the reverse isomer molecules would be advantageous against predators (who couldn't digest it) and a boon to natural selection.

Like I said, plenty of speculation to be had.

Not being digested doesn't stop you getting killed.

By predators it does. Unless you're chomping on some raw grass right now -> then I stand corrected.

I just ate some bread

Sounds digestible and non-sequitur.

I believe what they're getting at is that it's possible to do things to other species which leaves them digestible when they weren't initially. For example, human cooking, bird gizzards, cows chewing their cud, or rabbits eating their feces to give a second pass of digestion.

You talk about cooking meat, but we still have enzymes in our stomachs (e.g. pepsinogen) that only digest L-peptide chains. As a predator, you do not digest D-peptide chains.

- It is analagous to eating raw grass.

- It is not analgous to eating processed/cooked grains, hence it is non-sequitur.

- It is a matter of biochemistry, not culinary arts.

I gave the original speculation as an example of there being too much speculation around abiogenesis and evolution. This discussion proves it.

A few points

I also eat plenty of insoluble dietary fibre.

Prey animals would need to signal their non digestability to their predators.

Animals without predators would overwhelm the biome.

Disease is a predator.

It renders predation on you maladaptive, because the predator doesn't get back the energy expended in hunting, so over time it does actually make you less likely to get killed.

And then something evolves that can eat you, because there's an unfilled niche.

This is a common misconception.

Evolution is directionless and unguided. It's not like a startup, under the guidance of humans, pivoting for a niche market.

You're correct, and I didn't mean to make it seem as though it was an intentional process.

But historically, when one aspect of the ecosystem goes unchecked, something usually comes along to take advantage of plentiful biomass, one notable exception being humans.

We have lots of rats and roaches. Not directly feeding off our bodies, but on our extended phenotype.

Don't forget domesticated cats. It's still a wild animal, but one that has adapted itself to live off humans in quite clever ways (essentially by mimicking human children).

Great point!

Yes, except even herbivores occasionally show evidence of eating meat. Deers are occasionally seen eating birds, similar has been seen in Cows (IIRC Lal the cow was documented eating its owners chickens from the coop) and in fact all Bovid species occasionally show eating birds.

If there's a world of untapped food, this exceptionally rare behaviour would rapidly become advantageous, especially when the prey isn't adapted to the unusual predator.

Evolution has no guide, but all evidence shows it evolves into almost every available niche and quite specifically that any large available resource will be tapped.

So what if some herbivores will occasionally eat meat? You're assuming that the organism should look, act, and behave strictly within the boundaries of an English description.

Again, evolution is still unguided and to assume that something will evolve into an open niche is a gambler's fallacy.

I'm not even going to attempt to argue this, because I sense I'm simply going to be banging my head against the wall because you're going to be locked into semantics and obsessing over "unguided".

Convergent evolution, and history argue against you. C4 photosynthesis evolved independently 60 times. There's no guarantee it will, and I'm not arguing it will. However, the weight of evidence argues that if there's a readily available niche it is a matter of time.

If you argue I won't win the jackpot, you're likely right - in fact I'd put money on my probability of losing. However, if you argue no one will win the jackpot in the next 100 years you're an idiot.

You're the one trying to use the semantics of "herbivores" to make a point in biology. By assuming that there is even a "jackpot" to inevitably be won is making the gambler's fallacy. There is no "jackpot".

Quit projecting and just stop already, electromagnetic. You can't even bow out and admit you're wrong gracefully, so you skip straight to name calling.

Bravo.

You don't even know what the Gambler's Fallacy is! Gambler's Fallacy is betting according to a limited collection of statistics (red came up 5 times in a row, bet on black!) and not to true probability.

I didn't intend to refer to you personally as an idiot, but turing a phrase however that appears very lost on you.

I'm certain you're just intentionally being a troll, because you're unwilling to make a single basis of an argument or even support your reasoning and can't even use your terms correctly, and now you're attempting to bait me with condescension.

> Of course, having the reverse isomer molecules would be advantageous against predators (who couldn't digest it) and a boon to natural selection.

This, of course, doesn't really mean much. Something being advantageous doesn't imply that it must show up as a product of natural selection nor does it imply that such an adaptation must survive into perpetuity.

This is the kind of change you really don't expect to have an evolutionary pathway. Random genetic changes to genes won't suddenly start a full system of reverted isonomers out of the blue, and there's way to slowly get there (it's negative all the way through than a little positive afterwards).

I gave it as an example of there being too much speculation.

There is absolutely no evidence for it and it is not meant to be taken as as serious as some of you are taking it. If anything, it proves there is too much speculation -_-

Of course, having the reverse isomer molecules would be advantageous against predators (who couldn't digest it) and a boon to natural selection.

Why would the reserve isomer not be digestible? The human body can metabolism racemic amino acids.

> advantageous against predators

Only until your species divergently evolves and one of the descendent species evolves into predators. Then it's game on again!

Then again, if reverse isomers = indigestible, what is your species going to eat? I suppose this strategy might work for plants.

It could be a 'frozen accident', where either one works but once a positive feedback loop starts one version crowds out the other.

I confess ignorance about this theory, but I see it a lot and I don't understand. It just seems bizarre to think that life began on a comet. Sure, we want to know the origin of life. What does pushing it off the Earth solve? I guess my question is, What are the positive reasons for thinking life arrived from a comet? I can't think of any. As a layman, the theory is so strange I'm not even sure why it needs to be refuted. Can someone explain why this idea is popular?

One reason for its appeal is that the earliest fossils / signs we have of life on earth are pretty close to the ground not longer being molten lava. An origin of life in comets would give more time.

Well, the first life that existed was maybe some kind of rna-like molecule with very little to no protection. Not even a cell. I do not think any of these could leave any traces, even a fossil.

Yes. And that puts the origin of life even closer to the molten lava stage.

I see it more along the lines of being a hypothesis. We do not have nearly enough evidence to put it off the table, even though it is not the most likely explanation.

Asimov wrote a great essay on this - see A5 in this. http://www.arvindguptatoys.com/arvindgupta/asimov-electron.p...

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