New Scenario for Early Earth History and the Origin of Life


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This Science magazine News article discusses a new scenario for early earth history that is claimed to explain the earth's surface metal distribution and that it could effect conditions affecting the origin of life.

A new scenario suggests that some 4.47 billion years ago—a mere 60 million years after Earth took shape and 40 million years after the moon formed—a moon-size object sideswiped Earth and exploded into an orbiting cloud of molten iron and other debris.
This was before the end of the heavy late bombardment (timeline diagram in article) when many large impactors are thought to have hit the earth.

This is then claimed to result in a hydrogen rich atmosphere due to:
The metallic hailstorm that ensued likely lasted years, if not centuries, ripping oxygen atoms from water molecules and leaving hydrogen behind. The oxygens were then free to link with iron, creating vast rust-colored deposits of iron oxide across our planet's surface. The hydrogen formed a dense atmosphere that likely lasted 200 million years as it ever so slowly dissipated into space.
Reducing conditions are often considered favorable for life origin conditions because they favor the production of increasingly more complex organic molecules which life depends upon.

The article then elaborates on how this hydrogen atmosphere might have lead to the earlier production of RNA components and lead to an early "RNA world" which has been hypothesized to be the precede the current situation where DNA predominates as the information carrier of most life forms. Because RNA molecules can have both biological information properties as well as enzymatic properties (ribozymes), they are thought to be good candidates for early informational molecules. They would embody their own enzymatic properties instead of having to rely upon a complex system of making proteins from RNA sequence to produce the early enzymatic properties that life is thought to have required.
I'm guessing a more reduced atmosphere would also favor the production of other kinds of organic molecules also, like lipids, amino acids, sugars.
They also discussed alternative possible explanations: early stages of life could also have been based upon a combination of RNA's and proteins (RNA/protein world hypothesis?), and that some the early establishment of metabolism would have been important to initiating living processes (metabolism first hypothesis).

There are several different issues of the origin of life problem, among which are:
  • When and under what chemical conditions did life first form?
  • Where did the origin of life "action" took place (warm pond, hot ocean vents, warm ocean vents (different chemistry), salt water laces that could under go wet-dry cycles, mineral rich fresh water places that could undergo wet-dry cycles, ocean bottom, on pyrite crystals, or in clay, and probably others)?
  • What were the physical/chemical drivers of the proto-cell's metabolism?
  • What was the molecular basis of early enzymatic activities (RNA, proteins, surfaces of crystals or ceramics, nickle/iron/sulfur/?? compounds)?
  • What was the source and importance of the establishment of the cell membrane?
  • What was the order in which these different traits evolved?
Different scenarios have been proposed which deal with these issues in different ways.
These are different competing hypotheses with potentially partially overlapping explanatory components which makes comparisons between the alternative scenarios difficult.
Although interesting, some of the explanations in this article for certain aspects of the overall explanation are not the only proposed hypotheses to deal with the issues listed above.


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Here is a (free) related article from Scientific American (2007) by Robert Shapiro.
This article was mentioned in the article linked to in the OP, and I found it for free (Google search!)!

Shapiro gives a short clear explanation of what a metabolism first scenario (he calls it "small molecules", as opposed to RNA (a big molecule)) might be like.
He says these kind of scenarios would require the following:
  • a boundary to separate living from non-living (lipid membrane or similar), Freeman Dyson's "garbage-bag world"
  • an energy source to drive the events that keep life organized (energy from redox reactions)
  • a coupling mechanism to link energy production with processes that maintain life (ATP production and use?)
  • metabolism established as a network of chemical reactions
  • ability to grow and reproduce
Nick Lane has a much longer, more developed, and up-to-date presentation on metabolism oriented scenarios in his (not free) book The Vital Question (2016).
(You can get free reprints of most of his papers at his website.)
He presents somewhat detailed scenarios, many involving specific chemistries,
and he presents an interesting possible site (alkaline hydrothermal vents, where hydrogen rich water warmed from reacting with malfic rocks like olivine upwells through cracks in the rocks to eventually interact with sea water (different pH etc.).
Chemical deposition in these slower flowing vent occurs where the different solutions come together creating a finely porous structure with lots of little internal spaces. (Many separate compartments, with the different solutions are separated by finely precipitated walls of the internal spaces.)
The two solutions with contrasting redox potentials are hypothesized to interact via iron/sulfur catalysts embedded in the precipitated walls to generate organic compounds.


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Jack Szostak, another origin of life researcher who has done a lot of work on examining the likely materials composing proto-cells and on prebiotic nucleic acid chemistry, also has a Scientific American article (from 2009) on the topic (discussing hydrothermal vents as a potential area for protocell replication): papers/ricardo-szostak-sa2009.pdf

I saw Szostak give a seminar on the topic a few months ago, and he discussed some of the new ideas described in the Science news piece. It's a very fascinating field with some very neat results coming down the pipeline.


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2018 Award
Something the scenario that Nick Lane is pushing is that sometime early in the formation of life there were little bags (lipid vesicles; and before that the pores of deposited rocks at the vents) where proteins (or something) containing Fe/S/maybe Ni, embedded in the membrane, were able to catalyse redox reactions to make new organic molecules inside the vesicle.
This provides a power source for the rather large energetic needs of life processes and would presumably result in high concentrations of organic molecules inside the vesicle.
The energy source and high concentrations of organic molecules would be important for driving other reactions needed to create increasingly more and different molecules as life processes elaborated.
In contrast, it is hard for me to imagine how life could really get going if there were only molecules floating around unrestricted in solution without an energy source and high reactant concentrations to drive further reactions.

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