Comment by pierrec | Hacker News Reader

Comment by pierrec | Hacker News Reader

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Sure, you could start with metabolism first, and then get structure later. But you can narrow it down even further by saying this: you start with self-replication, and everything else comes after. Evolution stems from self-replication. This is why RNA, of all the things we know, is the best candidate for the origin of life. The smallest self-replicating RNA is relatively small and simple, and can arise spontaneously and mark the beginning of evolution, given the right environment. The separation of structure and metabolism doesn't even exist yet.

The fact that a self-replicating RNA could have arisen given what we know of primordial earth is extremely convenient. Something different and unknown could have come first, but it would have to fit the bill of "can arise spontaneously" and "can self-replicate and pass down incremental modifications", which is very difficult to pull off. We also have good certainty that LUCA (last universal common ancestor) featured RNA.

I will also try to answer: what did the first organism eat? The concept of food/energy source doesn't apply so well to the first self-replicating RNA. The environment features constantly interacting bases, and the competition between self-replicating RNAs stems from this. The distinction between individual organisms and the environment isn't as clear-cut as what we know now. Through this competition, one of the first things to be acquired was probably a membrane. Eating can become possible and/or useful after you have a membrane, which also allows you to extend the environment in which you can thrive.

I went through several different ways to approach responding to your comment before writing this version. I think this is most succinct.

Self-replication can itself be viewed as a metabolic process. You really can't have replication before metabolism, because replication is metabolism. The question is which metabolic process came first.

Since replication is anabolic and requires an energy source, it seems like it makes sense to deal with energy first.

You can have reactions that harness energy to do nothing useful -> reactions that harness energy to cause self-replication -> life.

>I went through several different ways to approach responding to your comment before writing this version.

I also edited it quite a bit (but now I'm done, promise...) One of my points is that replication does not require a separate energy source, in the right environment. RNA is highly reactive given the right temperature and conditions, this is why all modern life does not use it as permanent storage medium (this also makes it very difficult to work with in the lab). Only replication of a DNA-based life form strictly requires a control mechanism and energy source.

>> One of my points is that replication does not require a separate energy source, in the right environment. RNA is highly reactive given the right temperature and conditions,

I actually don't know what a flask full of RNA will do when left to its own devices. My science education has gaps. However, really basic chemistry says that you can't create big molecules from small molecules without some sort of energy gradient, because the big molecules have less entropy than the small ones.

When you have a flask full of stuff, what it does spontaneously is go to thermodynamic equilibrium. If you want a self-sustaining reaction that creates specific big molecules over and over, you need to keep adding energy.

Fortunately, Earth is located near a huge fusion reactor, and comes equipped with its own auxiliary internal fission reactor. There was plenty of energy gradients in the environment then, the same way there are now.

Why isn't the following chain plausible?

Chemical compound soup, natural reactions -> natural reaction produces the first reaction which is self replicating (I'd assume RNA here) -> self replication reaction dominates natural reactions (to what order, who knows) -> self replicating reaction mutates into a form that better harnesses energy from environment (your metabolism) -> metabolic empowered reaction dominates -> etc

From a primatives perspective, it seems reasonable that the first "evolutionary" advances would have all been incredibly preferential. In the sense that they had no competition with similar capabilities.

Imagine the first organism able to metabolise something (sunlight?) for its own purposes. Against "organisms" that were still waiting for chance to bash them against the right compound.

I wrote out the mechanism that I'm proposing down lower, and I'm going to paste it again up here so that people see it.

My idea of how Metabolism First would work is something like this:

1. You get a lot of weird chemistry happening, say, in pools of water. This chemistry starts creating large organic molecules that are like sludge building up in the pools. These molecules are dead, but they are rich in energy and perhaps interesting building blocks. Maybe they are made of amino acids.

2. Now you have this unbelievably rich energy source, and at some point there start appearing chemical chain reactions that start "eating" the sludge.

3. Life originates from these chemical chain reactions.

I suppose it kind of flips the prevailing hypothesis around. Rather than have lightning and such build our proto-organism directly, instead it builds up a sludge that the proto-organism eats.

With the planet being sterile, this organic sludge could build up forever until something figures out how to eat it.

> The smallest self-replicating RNA is relatively small and simple, and can arise spontaneously and mark the beginning of evolution, given the right environment.

Do you have a source? If by small you mean 100+ bases it has little chance of arising spontaneously. The article speaks only to the possible spontaneous formation of building blocks, not of them assembling into complex forms.

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