3 ms·

> Are self-replicating RNA sequences more common than we believe? Almost certainly yes. > If so, why haven't we encountered more of them in nature? Because t

by lisper 4y ago

> Are self-replicating RNA sequences more common than we believe? Almost certainly yes. > If so, why haven't we encountered more of them in nature? Because they can't compete with modern life. Nearly all of the biomass on earth is in use. A primitive replicator, even if it arose today, would have to compete for resources against all the existing life on earth. It wouldn't stand a chance. This is the reason there is only one universal common ancestor. It's not necessarily that abiogenesis only happened once, it's that only the descendants of one common ancestor survive today. All traces of all other abiogenesis events have been obliterated. You have to compute the abiogenesis odds for an environment that, by definition, does not yet include life. Here is a rough back-of-the-envelope approximation: the biomass of earth is about 2^52kg. Avogadro's number is 2^76. So there is the potential for the early earth to have 2^120 nucleotide bases and amino acid molecules floating around. If the complexity of a simple replicator is around O(100) bits then the odds of finding one by chance when you are rolling that many dice in parallel approaches 1 in just a few million years. After that, evolution takes over.

Calavar 4y ago
First, the majority of the biomass of the earth is water, not organic molecules, and big fraction of what's left is lipids and carbohydrates, not nucleic or amino acids. So a simple multiplication of biomass by Avogadro's number is likely off by at least an order of magnitude. Second, yes, today's world has an enormous amount of amino acids and nucleic acids, but that is after billions of years of enzyme-catalyzed synthesis inside trillions of living cells. The prevalence of amino acids/nucleic acids prior to abiogenesis would have been orders of magnitude lower in a world where those enzyme pathways didn't exist yet and biologic molecules with catalytic activity were still in the process of being bootstrapped. Third, does your back of the envelope calculation account for the very short half life of RNA? That is really the biggest issue. If RNA didn't decay, then forget about hundred base sequences; the primordial earth might have been filled with million base sequences.
- lisper 4y ago
  > the majority of the biomass of the earth is water The figure I cited is the carbon biomass. https://www.visualcapitalist.com/all-the-biomass-of-earth-in-one-graphic/ https://www.visualcapitalist.com/all-the-biomass-of-earth-in... > that is after billions of years of enzyme-catalyzed synthesis inside trillions of living cells Doesn't matter. What matters is the size of the search space that could be explored by the chemistry, and if even a single replicator exists in that search space. The extent to which that first replicator resembles present-day life is irrelevant. The only reason to refer to that at all is to get some idea of what the information content of a minimal replicator is. > Third, does your back of the envelope calculation account for the very short half life of RNA? Again, it doesn't matter. As long as the half-life of the replicator is more than the time it takes it to produce >2 offspring, that's enough to get the ball rolling.

by lisper 4y ago

Calavar 4y ago

First, the majority of the biomass of the earth is water, not organic molecules, and big fraction of what's left is lipids and carbohydrates, not nucleic or amino acids. So a simple multiplication of biomass by Avogadro's number is likely off by at least an order of magnitude. Second, yes, today's world has an enormous amount of amino acids and nucleic acids, but that is after billions of years of enzyme-catalyzed synthesis inside trillions of living cells. The prevalence of amino acids/nucleic acids prior to abiogenesis would have been orders of magnitude lower in a world where those enzyme pathways didn't exist yet and biologic molecules with catalytic activity were still in the process of being bootstrapped. Third, does your back of the envelope calculation account for the very short half life of RNA? That is really the biggest issue. If RNA didn't decay, then forget about hundred base sequences; the primordial earth might have been filled with million base sequences.

lisper 4y ago
> the majority of the biomass of the earth is water The figure I cited is the carbon biomass. https://www.visualcapitalist.com/all-the-biomass-of-earth-in-one-graphic/ https://www.visualcapitalist.com/all-the-biomass-of-earth-in... > that is after billions of years of enzyme-catalyzed synthesis inside trillions of living cells Doesn't matter. What matters is the size of the search space that could be explored by the chemistry, and if even a single replicator exists in that search space. The extent to which that first replicator resembles present-day life is irrelevant. The only reason to refer to that at all is to get some idea of what the information content of a minimal replicator is. > Third, does your back of the envelope calculation account for the very short half life of RNA? Again, it doesn't matter. As long as the half-life of the replicator is more than the time it takes it to produce >2 offspring, that's enough to get the ball rolling.