JTW's Evolutionary Origins - Author: Wood, John T.

Heterotrophic Vs. Autotrophic Origins

Theories of Heterotrophic Origins (THOs)

Traditionally, heterotrophic origins have been assumed on the basis that the organization of heterotrophic metabolisms are generally simpler those of autotrophic metabolisms.

This assumption was consistent with the notion of a prebiotic broth rich in high energy organic compounds.

Such a broth was predicated on the idea that the chemistry of the early Earth atmosphere was of a highly reducing nature, i.e. rich in methane (CH₄), ammonia (NH₃), and hydrogen (H₂).

Theories of heterotrophic origins (THOs) circumvent the problem of finding a specific energy source, instead invoking the organic synthesis of energy rich compounds thru a variety of non-specific, geo-physical mechanisms under reducing conditions.

In THOs a commonly used approach to modelling biochemical pathway evolution is that of "Retrograde Evolution" introduced by the American biochemist Norman H. Horowitz in 1945.

In "retrograde evolution" pathways start with substrates (potential metabolites) available within the "broth" and evolve in a manner that seems to be backwards relative to the sequence of reations within the pathway.

As depletion of key metabolites from the "broth" proceeds, some mutant chemical networks - i.e. those capable of converting other available compounds into the initial metabolites of, or other intermediates within, pre-existing chemical networks - are favored and spread rapidly thoughout the chemical environment upon depletion of the original metabolite.

An additional hypothesis of Horowitz involved a symbiotic fusion of two variant chemical networks, when - due to simultaneous depletion of key compounds within each network - the two pathways are capable of complementing each other by synthesizing the depleted components of the other network, thereby sustaining each other by shifting the early prebiotic networks from reliance on the prebiotic stock of those depleted molecules to reliance on some other precursor molecule still available.

In 1974 Ycas proposed a theory of "Patchwork Evolution" - based on class reactions and broad specificity enzymes - as an alternative theory of pathway evolution.

This theory was further refined by Jensen in 1976 by adding the notion of "substrate ambiguity".

Both approaches presuppose some sort of proteineacous catalyst and gene duplications capable of producing multiple copies of an enzyme in real time, as well as generating divergent classes of enzymes in evolutionary time - the raw material of pathway construction.

Multiple simultaneous copies of a catalyst allow the network to differentially amplify low propensity reactions relative to other reactions that might otherwise be favored.

While this will not effect the equilibrium concentrations of the reaction, it will allow low-propensity reactions - that form key intermediates - to funnel these intermediates to the next step in the reaction sequence, thereby sustaining the network and generating novel chemical pathways and compounds.

It is generally assumed in THOs that glycolysis/gluconeogensis was one of the first biochemical pathways to emerge and that the early prebiotic chemistry would be dominated by phosphorylated triose and hexose sugars.

A number of chemical contradictions are encountered in this assumption, and generally the problem of early chiral sugar evolution has been considered to be some what intractable.

With the advent of photosynthesis and autotrophy this process would eventually "bottom out" and a stable and sustainable ecological cycle would be established via the interplay and exchange between heterotrophs and autotrophs.

Theories of Autotrophic Origins (TAOs)

With the advent of new data - concluding that the early Earth possessed an atmosphere that was at best slightly reducing, if at all - this position became untenable.

It is now believed that the early atmosphere contained much more carbon dioxide (CO₂) than that assumed by the likes of Oparin, Urey, Miller, and others at the incipient beginnings of the field of prebiotic chemistry.

This type of atmosphere cast doubt on the viability of prebiotic "broth" scenarios, there may have been very little pre-existing high-energy organic compounds on which heterotrophs could feed themselves.

The emergence of heterotrophy would have to wait for the advent of autotrophy, autotrophs make a reliable food source - i.e. they "taste" good!

Additional support for theories of autotrophic origins (TAOs) has emerged from the study of Woese's rRNA-based Universal Phylogenetic Tree.

An examination of the tree reveals that it's deepest branches are occupied by anaerobic, sulfur-dependet, hyper-thermophilic autotrophs - giving reasonable cause to suspect a autotrophic origin.

TAOs require that the constituents of metabolism be constructed from scratch.

Based on the current understanding of the prebiotic atmosphere - the most readily available primary carbon building block unit is generally considered to be carbon dioxide (CO₂).

Since the process of carbon fixation is energetically costly, an appropriate and reliable source of energy must be found capable of sustaining the required reactions.

For this reason TAOs have been difficult to formulate, but when attempted tend to focus on two possibilities: photo-autotrophic (PAT) metabolisms powered by sunlight and chemo-autotrophic (CAT) metabolisms powered by inorganic chemical processes.

The most obvious source of energy is sunlight, but there are inherent contridictions in the use of sunlight to run the primordial reactions system in the abscence of the complex structures and mechanisms that capture photons and protect existing biomolecules from degradation by UV.

There are reasons to believe that the regions accessible to sunlight were poisoned by the presence of alkaline oceans - due to high soda content (Na₂CO₃) - and a very small, sufficient amount of atmospheric oxogen (O₂).

PAT theories primarily focus on the evolution of conjugated double-bond systems, polycyclic aromatic hydrocarbons (PAHs), porphoryins, and other light harvesting molecules as well as on the emergence of some form of Calvin Cycle capable of attaching CO₂ to larger molecules using the energy of the harvested light.

Organisms exhibiting PAT metabolisms are usually highly complex and are not considered to be indicative of the metabolic nature of the progenote.

CAT metabolisms, on the other hand, exploit inorganic chemical processes and are considered to be the main contenders for a energy source in TAOs.

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