Biologists have long recognized that cells could be divided into two major types: Prokaryotic (bacteria) cells with no nucleus and Eukaryotic cells with a nucleus.
The discovery of extremophilic prokaryotes whose cell structure differed significantly from that of bacteria lead to the splitting the Prokaryotes into two domains, Eubacteria and Archaea.
During the 1970's Carl Woese proposed that all biological life could be divided into the three-domains Archaea, Bacteria, Eucarya - primarily based upon his seminal work in developing rRNA phylogenies based upon comparative small subunit rRNA sequences.
Interestingly, these results revealed that the prokaryotic Archaea and eukaryotes (Eucarya) were much more closely related to each other than to the Bacteria - the two domains sharing many similarities in their small subunit rRNA sequences.
Based upon the fact that the domain Archaea contains many extremophilic species capable of living under the conditions that prevailed during the period between 8.5 and 8.9 bya, it was assumed by many that Archaea were the most likely representive of the last universal common ancestor(LUCA) of biological life existing today.
Traditionally, Eukaryotes were thought to have emerged approximately 1.6 to 2.1 bya (possibly as far back as 2.7 bya if sterane biomarkers are considered to be diagnostic of eukaryotes).
Eukaryotes may have emerged as a result of an endosymbiotic merger between a α-proteobacterium and archean host as suggested was first suggested by Ivan Wallin - later, Lynn Margulis provided convincing supporting cytological and molecular evidence.
More recently, comparative analysis of the molecular machinery of life, as well as a detailed re-examination of the microfossil paleontological data, has lead Thomas Cavalier-Smith to suggest his Neomuran Theory which proposes that Archea and Eucarya are very late-evolving (only 850 mya!) sister domains under the superdomain Neomura - the ancestors of Neomura being highly evolved Bacteria.
The origins of the eukaryotic metabolism and organelles remains a hotly debated topic with many problems to be resolved, hence these ideas are subject to revision - but, it is hoped that we will continue to obtain a greater and greater resolution and clarity on the evolutionary relationships amongst the domains as time goes by.
Traditionally, organisms have been classified as unicellular or multicellular, but a closer examination of the eukaryotic domain reveals three primary modes of eukaryotic organismal organization:
During the lifetime of a eukaryotic organism an individual may pass thru one or more of these forms of organization.
Cavalier-Smith suggests that the primitive Eukaryotic life cycle was composed of four stages:
He further suggests that "Early life-cycles evolved by the temporal coupling or partial uncoupling of these four processes." (Cavalier-Smith, 2002)
The origin of the animal bodyplan has it's roots in the capacity of it's free living unicellular choanoflagellate protozoan ancestors to form multicellular colonies.
Today there are approximately 20,000 known species of protozoa.
Protozoans have traditionally been grouped into three major categories:
T. Cavalier-Smith (1993) offers a refined definition the Protozoans as "Predominantly unicellular, plasmodial or colonial phagotrophic eukaryotes, wall-less in the trophic state. Primitively possessing mitochondria and peroxisomes (unlike Archezoa); when mitochondria and peroxisomes are both secondarily absent (Parabasalia, Entamoebia, Lyromonadea, and anaerobic ciliates only), hydrogenosomes and/or Golgi dictyosomes are present instead. Ciliary hairs are never rigid and tubular (unlike most chromists); haptonema absent (excludes nonphotosynthetic haptophytes). Chloroplasts, when present (some euglenoids and dinoflagellates only), contain neither starch nor phycobilisomes (unlike in Plantae), have stacked thylakoids, and usually have three, rather than two, envelope membranes. Chloroplasts are located in the cytosol, never within a smooth periplastid membrane inside either the lumen of the rough endoplasmic reticulum or a fourth smooth membrane (unlike Chromista); ejectisomes never of the double-scroll cryptist type (this excludes the cryptist Goniomonas); the few multicellular species have minimal cell differentiation and altogether lack collagenous connective tissue sandwiched between two dissimilar epithelia (unlike Animalia)."
T. Cavalier-Smith describes Choanozoa as "Uniflagellate unicellular or colonial protozoa; mitochondrial cristae with flattened nondiscoid cristae; ciliary root a symmetric cone or radial array of single microtubules... Trophic cells with a single cilium surrounded by a collar of microvilli (supported internally by actin filaments) that are used to catch bacteria prior to their phagocytosis. Free-living. Unicellular or multicellular."
At the origin of the first multicellular animals, the individual cells of the metazoan body retained most (if not all) of the hereditary patterns of activity of their once free-living protozoan ancestors.
All of the following choanoflagellate traits will be utilized in the transition to multicellularity and the emergence of the animal bodyplan:
Protozoa usually reproduce asexually (mitotic division) either by budding, fission, or multiple fissions.
Protozoans are also capable increasing their genetic diversity through occasional sexual reproduction (meiotic division), exhanging genetic material via conjugation.
A key step in the formation of animal bodyplan formation will involve controlling and repressing this natural tendency on the part of individual cells to immortally reproduce - a task that is primarily achieved thru diffusible signaling molecules and cell surface interactions between neighboring cells or the extracellular substrate.
The ancestral protozoan line leading to animals is believed to lie within the Flagellates, specifically, the Choanoflagellates.
The choanoflagellate protozoan ancestors of metazoans were highly evolved unicellular organisms with a diverse suite of molecular and biochemical tools geared to interaction between themselves collectively and the environment as a whole.
The individual cell, and especially the Choanoflagellate ancestors of the first animals, is an sophisticated organism with it's own set of biological imperatives and potentials - Animal bodyplans are multi-generational multi-cellular colonies (similar to civilizations) possessing a range of specialized and differentiated cellular societies within which these imperatives and potentials are realized on an individual and collective basis.
Indeed this can be most clearly seen in motile cellular individuals within the vertebrate bodyplan such as granulocytes (including neutrophils, basophils, and eosinophils), monocytes (including macrophages), and lymphocytes; as well as in the germ line.
With the emergence of colonial protozoa many innovations essential for the transition to multicellularity are acquired.
Within the cell colony, cell membrane interactions between individuals and diffusable secretion gradients across the colony come to play an important roles in the coordination of the colony as a whole - regulating metabolic and reproductive rates.
More than a century and a half of observation and experimentation has clearly demonstrated the validity and utility of the theory of Natural Selection as a descriptive model for a broad range of phenomena within the biological domain.
The principles of population biology that underpin the theory of Natural Selection allow for a bottom-up description of a wide range of phenomena that occur at all scales of biological organization.
The organization of the animal bodyplan has been shown to emerge in a historically contingent and adaptive manner from the bottom-up and not from a preconceived notion of structural-functional design implemented from the top-down perspective.
The physical requirements of the Theory of Natural Selection are:
Indeed, an important 20th century realization concerns the utility of Somatic Selection and the concept of Somatic Selective Systems in understanding physiological systems such as the immune system (as in Clonal Selection Theory of Neils Jerne and Frank MacFarlane Burnet) or the nervous system (as in the Gerald Maurice Edelman's Theory of Neuronal Group Selection).
The concept of a somatic selective system may be extended to include embryological development and physiological function in general.
Somatic Selection represents an extension of the Darwinian principles used to describe the evolution and development of species within the ecology over geological timeframes; to the description of the evolution and development of cell populations within the body over the lifetime of the organism.
The requirements of a somatic selective system are three-fold:
The homeotic genes are particularly important because they code for homeodomain proteins that bind to DNA and serve as transcription factors required to facilitate the expression of genes downstream in the unfolding developmental genome.
Homeodomain proteins are composed of a variable region that determines the specific activity of the protein, as well as a hinge region and a 60 amino acid homeodomain region comprised of four alpha-helixes.
The third helix of the homeodomain region is responsible for recognizing a specific DNA sequence within other genes in the genome - as well as engaging in regulatory protein-protein interactions.
Homeodomain transcription factors act on genes that contain the specific DNA motif recognized by the trascription factors homeodomain region.
Homeotic genes are key components in the reciprocally connections between cell surface cell adhesion molecules and the genome - playing a key role in transitions between epithelia and mesenchyme during development.
The ability to recognize and adhere to a substrate requires specialized cell-surface molecules to mediate the interaction.
The extracellular matrix of animals contains several proteoglycan components:
Additional, non-proteoglycan components of the extracellular matrix in animals include:
A key step in the development of multicellularity was the acquisition and regulation of cell adhesion molecules that allowed cells to become attached to each other.
By up-regulating and down-regulating the presence of adhesion molecules on the surface, populations of cells are able to cycle thru an epithelial/mesenchyme cycle that allows for the formation of tissues and migratory cells depending on the particulars of the developmental context.
Adhesion molecules are derived from four major gene families:
The cell and substrate adhesion molecules are believed to be linked to various transcription factors in such a way that each co-regulates the activity levels of the other.
These molecules are also believed to be coupled to the genenome and metabolism via the cells second-messenger systems and cytoskeletons.
All eukaryotic cells possess the equivalent of a 'cellular nervous system' in the form of second-messenger systems.
The second-messenger systems function by coupling cell-surface events with the activity involving the genome and metabolism of the cell, thereby allowing the cell to adapt and respond to it's immediate environment in an intelligible manner.
The major second-messenger systems are:
The CAMs are believed to be linked to various transcription factors in such a way that each co-regulates the activity levels of the other.
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The emergence of a dynamic cytoskeleton enabled single-celled eucaryotes and protozoa to form a diverse array of adaptive cell morphologies in response to events within the lifecycle and the ecology of the organism.
The three major cytoskeletal systems are the:
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