The emergence of life


The Oparin Ocean hypothesis : Primordial Soup

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The RNA World hypothesis deals only with the 'information' of life - the software. Metabolisn, the hardware of life, is a separate issue. In fact, there is reason to beleive that the metabolism and information-processing components of life may have originatd independently, and merged to generate what we think of as life.

The first realistic suggestion about how metabolism could have originated was the Oparin Ocean hypothesis. The idea was that chemical reactions and meteoric influx concentrated organics in the ocean, creating an ocean (or at least tidal pools) of nutrient-rich organic 'soup'. Spontaneous and random polymerizations created self-replicating molecules that eventually evolved into metabolism and life.

There were two sources of organic material in the early Earth: in situ synthesis and meteoric influx.

In situ chemical synthesis from primal atmospheric components

Irradiation or electrical sparking of mixtures of methane/ammonia/water/hydrogen/cyanide and other possible atmospheric mixes yields lots of organic compounds, including adenine and amino acids. This is the classic Miller-Urey experiment:

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Miller-Urey apparatus (source unknown)

The implication of this is that that lightening and cosmic irradiation of an early Earth atmosphere could have generated signoficant amounts of organic "stuff" from which life may have emerged.

Meteoric influx

Comets and asteroids are rich in carbon and organics. In fact, comets are similar in composition to life!

Element Halley's comet Mr. Halley
H 55.2% 63%
C 11 9.5
N 2.4 1.4
O 27.6 25.5
water 50 75

Metabolism was seen to be the result of sequential exhaustion of required substrates, followed by the utilization of handy precursors instead to make what was needed, i.e. when compound X is exhausted from the primordial soup, compund Y was used by converting it to compound X

Y --> X

Then when compund Y was exhausted, a related compound Z was used by converting it first to compound Y, then to compound X:

Z --> Y -->X

This process, repeated over-and-over, generating the metabolic pathways we know today.

The Oparin ocean hypothesis seems generally plausible, but like any notions about the origin of life is pretty vague and, at least for now, untestable. And it must be said that there are some signifcant weakness of the hypothesis that need explaining, for example:

  1. An entire ocean of primordial soup? (but it could have been a tide pool or evaporation pond)
  2. Activated precursors are unstable, especially at high temperatures, and so wouldn't likely have accumulated to any significant concentration
  3. Biochemistry is too complicated stereochemically - there are too many isomers of everything. The useful compounds end up at trivial concentrations, and overwhelmed by other isomers that would compete for reactions
  4. Biological polymerizations are unifirmly dehydration reactions, and so are strongly disfavored in aqueous solution (55M H2O). Accumulation of polymers is not going to happen.

So the Oparin Ocean is not a great theory, but it was the best available until ....

Wäctershäuser's hypothesis of surface metabolism

The basis behind this hypothesis is that life originated in an organic scum adhered to mineral surfaces, rather than in solution. The hypothesis comes with a very elaborate set of chemical features that solve most of the problems with the Oparin Ocean hypothesis:

  1. Surfaces can concentrate very low concentrations of solution organics to very high local concentrations by adsorbtion
  2. polymerization is favored on surfaces (low effective water concentration)
  3. surface chemistry is stereospecific
  4. activated precursors aren't required to drive reactions

Pyrite and similar minerals binds anions, especially organophosphates, but let them diffuse around in two dimensions on their surfaces. This is consistent with the observation that the most fundamental metabolic pathways use phospho-intermediates. The peripheral pathways are not composed of phospho-intermediates, and were probably invented later.

These surface metabolic reactions may have been catalyzed only by cofactors before the invention of enzymes (either protein or RNA). In this view, metabolism arose before(or independetly of) biological information-processing (RNA or DNA), and even before biological catalysis.

Cellularization, in this scenario, is thought to have occurred when lipid by-products of metabolism later enclosed the surface reaction centers, creating a kind of simple 'cytoplasm' enclosed on one side by a lipid membrane and on the other by the mineral surface. As metabolism became increasingly complex and independent of the mineral surface, the escape of these surface organisms from their mineral substrate could have happened.

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The RNA World (if it existed) may have originated within this proto-biotic metabolism, or it may have originated independently, and then the two merged to create some of the arliest forms that we might recognize as living.