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The Krebs Cycle, also known as the Citric Acid Cycle (CAC) or Oxidative Citric Acid Cycle (OCC), is generally encountered running in the oxidative direction and dismantles acetate into carbon dioxide while using the energy derived to reduce coenzymes and ultimately produce ATP. It has a counterpart, the Reductive Citric Acid Cycle (RCC) that runs in the reverse direction, assembling carbon dioxide and water into larger complex molecules with carbon backbones.
It appears on the basis of available facts that the circumstances surrounding the origin of life on Earth are inextricable from the origin of the solar system and Earth itself - the biosphere rising off the geosphere is the one continuous biochemical reaction that has persisted from the beginning of life on earth to the present.
The emergence of a metabolic network capable of autocatalytically building and decomposing the monomeric components required for macromolecular polymerizaion, and cellular encapsulation, marks the crossing point from the inanimate to the animate. The emergence of cellular encapsulation would facilitate the global spread of that metabolic network - thereby, effecting a transformation of the geosphere into bioshpere as the chemistry of the planet changed in response to the input and output of materials derived from the geological substrate (as well as the hydrological and atmospheric environ) through the metabolic networks of the globally distributed cellular populations.
The primary setting for the origin point is postulated to be hydrothermal vent systems forming just after the last major extraterrestrial impactor to boil off the Earths oceans had struck, ~3.9 bya at the end of the Great Bombardment period. As the water vapor condensed and rained down, the first stable terrestrial oceans emerged. Although subsequent impactors would strike that were capable of boiling the oceans, by 3.9 bya the enduring body of water we know as the ocean had been established and would persist long after the last of these impactors struck, up until this day. The planetary dynamics sought equilibrium in the aftermath of planetary accretion and extraterrestrial bombardment. Through the fissures and hydrothermal vents that formed on the surface of the early Earth flowed an up-welling of organic material moving across the mineral surfaces of the vent systems giving rise to a complex environmental chemistry.
Following the work of Graham Cairns-Smith and Günter Wächtershäuser, I will depart from the primordial soup theories of Alexander Oparin, Stanely Miller, Leslie Orgel and others -and, take the position that the metabolism was ignited in the form of a surface-catalyzed reaction system that evolved into a dissipative structure synergistically organized as an autocatalytic Surface Metabolist on the sides of the hydrothermal vent systems of the newly formed Earth. Although it appears that Wächtershäuser (and others) appropriately focused on the reductive citric acid cycle (RCC) as the starting point for the auto catalytic core of the metabolism, I will deviate from Wächtershäuser in that I feel Thomas Cavalier-Smith has appropriately identified Archea and Eukaryotes as sister domains and of relatively recent origin (950 mya), not ancient and primordial (3.5 bya) as had been originally proposed by Carl Woese - an idea that gained widespread acceptance but appears to be a flawed interpretation of the molecular and fossil data.
The goal, ala Elie Metchnikoff, is to elucidate a series of mechanistically continuous and physiologically viable transitions between each stage in the evolution and diversification of life.
Phylogenetic classification of animals and their unicellular relatives.[b][2]Choanoflagellate and choanocyte
Animals are multicellular colonies and pattern formation is at the heart of embryology.
One of the great problems that evolutionary developmental biologists have been trying to solve since the time of Darwin is - how unicellular organisms living in a colonial environment made the transition from cells being selected as individuals within the population of cells - to, the entire multicellular colony being selected as an individual, an event that marks the transition to multicellularity. This threshold has been achieved at least three times in evolution of the eukaryotic domain,[c] yielding three of the seven major biological kingdoms[d] - plants, animals, and fungi, each with its own historically contingent outcome.
We can rephrase the problem for animals as - how unicellular organisms living in a clonal colony, along with other potential microfluoral and faunal symbionts, made the transition from cells being selected as intracellularly-digesting phagocytic individuals within the population - to, the entire multicellular colony being selected as a single extracellularly-digesting individual functioning as a unitary whole; an event that marks the transitional endpoint in the origin of metazoans as a firmly established biological form of organization.
For animals with their unicellular ancestral roots in the relatively sophisticated choanoflagellate protozoa, the problem becomes one of understanding how they made the transition to Porifera, Cnidaria, Bilateria, Deuterostome, and Chordate - and, onward to vertebrates and mammals where Edelman will pick up the story. Edelman investigates the problem of pattern formation from the perspective of evolutionary developmental morphology, but enters the game with the vertebrates.
Beyond the Realm of Experience the Universe is inherently Mysterious...
Animal Nervous systems operate fundamentally in terms of adaptive pattern recognition rather than logic. They are finite structures with a finite resolution on reality and their experience. They cannot know an effectively infinite reality in it's totality, therefore they don't attempt to do so because it would be impossibly costly in terms of time and resources. Instead, animal nervous systems evolved a strategy of adaptive pattern recognition that allow for the environment to be cognitively modeled or "imagined" based upon the finite nature of it's experience. The nervous system is capable of constructing a seemingly infinite variety of cognitive approximations that are of greater or lesser degree of correspondence to the actual features of reality that they are experiencing. They are not primarily interested in those models with the closest correspondence to reality, but rather to those models meeting their primary adaptive needs at any particular point in time and space. Animal nervous systems are not logic devices, nor are they Truth-seeking devices. They are primarily engaged in adaptive pattern recognition, albeit with Humans, the degrees of creative freedom made available by language, and the constraints of logic, reason, and empiricism; can all be harnessed as techniques for fine tuning the correspondence between the cognitive construct and the outlines of existential reality. But there always remains a limit of validity for the cognitive model, beyond which paradox and contradiction will arise. At best we can expand the range of validity for our cognitive models by reexamining our existing models and modifying their underlying assumptions to be more consistent and coherent with the actual contours of the phenomena we seek to model.
↑Subject to major change, revision ,and/or retraction at any moment.
↑Original description: "Figure 1. Phylogenetic classification of animals and their unicellular relatives. (a) A timeline of different events during early animal evolution. The transition to animal multicellularity, and therefore the origin of the first animals, occurred sometime at the end of the Tonian period, according to molecular clock estimates. The oldest fossil or geological evidence of recognizable animals dates back to the Ediacaran period, with molecular clocks extending the emergence of different animal phyla back to the Cryogenian. Time units are million years ago (Ma). (b) Cladogram representing the major clades of the tree of animals and the major groups of unicellular relatives of animals: choanoflagellates, filastereans, ichthyosporeans and corallochytreans-pluriformeans. Coloured nodes indicate different ancestors that we can reconstruct and that are important to understand the transition to animal multicellularity; the highlighted internal branch (from the Urchoanozoan to the animal LCA) indicates the animal stem. Uncertain positions within the animal tree and within Holozoa are represented with polytomies."[1]
