Speculations on Life

[Widdekind]

Bacterial (Prokaryote) lateral-transfer (Conjugation) of DNA genes (Plasmids) represents "proto-Sexual reproduction"

 

Plasmids are pieces of DNA, that bacteria incorporate into their own genome:

 

In the process of Conjugation, bacteria transfer genetic material to each other, through extra-chromosomal pieces of DNA, called Plasmids. Bacteria are normally haploid, but bacteria that receive Plasmids become diploid for any genes contained on the Plasmids (Teaching Company Biology, lect. 27).

Such seems superficially similar, to (lysogenic) bacteriophage virus DNA, which enters bacterial cells, and is incorporated into the cell's internal functionings. Perhaps Plasmids underlie the Eukaryotic transition to diploid-ism, representing some sort of "proto-sexual reproduction" ??

 

 

 

Archaea associated with extreme environments

 

The vast majority of Prokaryotes are bacteria, and archaea now tend to live in extreme environments, such as hot springs, or extremely salty water (Teaching Company Biology, lect. 24)

Archaea are similar to bacteria in appearance, but their molecules are organized in a different way. It is thought that Archaea may represent the link between Prokaryotes [Archaea & bacteria] and the group that contains all other plants, fungi, & animals -- the Eukaryotes domain... Archaea are common in lakes, seas, & oceans, and occur in odd places, such as undersea vents, hot springs, and in salt pans. They may be the most [geographically] widespread organisms on the Earth ([Life on Earth] First Life, p.10-11).

 

 

Bacteria breed rapidly

 

Bacteria are pared down to a minimum compatible with a free-living lifestyle. They are ruthlessly streamlined, everything geared for fast replication. Many keep as few genes as they can get away with; they have a propensity to pick up extra genes from other bacteria when stressed, bolstering their genetic resources, and then lose them again, at the first opportunity. Small genomes are copied swiftly. Some bacteria can replicate every 20 minutes, enabling exponential growth at astounding rates, so long as raw materials last (Lane. Life Ascending, p.90).

 

 

Core qualities of Archaea & Bacteria

 

The side-by-side cohabitation, of Bacteria & Archaea, two great microbial domains representing millions of species that often share similar niches, provides one example [of biochemical integration]... Gene swapping goes on all the time in Life, especially amongst microbes. But, in fact, Archaea & Bacteria seem to have jealously guarded certain very basic genes. So far as we know, Archaea have never shared with Bacteria (or Eukarya) their ability to metabolize by making methane, yet methanogenesis is widespread among Archaea, occurring in locations as diverse as deep-ocean vents, and the human gut. Conversely, photosynthesis has apparently never passed from Bacteria (or Eukarya) to Archaea. So it is clear that very different forms of microbes can compete in the same space, for many of the same resources, without one form ever eliminating the other (Davies. Eerie Silence, p.59).

Cyanobacteria are otherwise known as blue-green algae, because of their color when they grow together in their millions Their greenish color is the chlorophyll they use for photosynthesis, just like most other plants. Cyanobacteria grow in water with very low levels of oxygen but plenty of carbon dioxide. When conditions are favorable, they multiply to such an extent, that they form a green mat over the surface of the water, which is called algal bloom Animal-like bacteria [Eukaryotic Protista] are far more widespread than cyanobacteria. They live everywhere -- in water, on land, in air, on other organisms, inside other organisms, and on dead organisms ([Life on Earth] First Life, p.10-11).

Chloroplasts are one of the many different types of organelles in the plant cell. In general, they are considered to have originated from cyanobacteria through endosymbiosis (Wiki).

Thus, the fateful endocytosis, of the first cyanobacteria-cum-chloroplast, by a mitochondria-bearing Eukaryote, clearly occurred before the Earth's atmosphere was completely oxygenated, ~2.1 Gya.

 

 

Methanogenesis demands absence of Oxygen

 

Biochemistry of methanogenesis

 

Methanogenesis in microbes is a form of anaerobic respiration. Methanogens do not use oxygen to breathe; in fact, oxygen inhibits the growth of methanogens. The terminal electron acceptor in methanogenesis is not oxygen, but carbon. The carbon can occur in a small number of organic compounds, all with low molecular weights. The two best described pathways involve the use of carbon dioxide and acetic acid as terminal electron acceptors:

 

CO

₂

+ 4 H

₂

→ CH

₄

+ 2H

₂

O

 

CH

₃

COOH → CH

₄

+ CO

₂

However, methanogenesis has been shown to use carbon from other small organic compounds, such as formic acid (formate), methanol, methylamines, dimethyl sulfide, and methanethiol. The biochemistry of methanogenesis is relatively complex, involving the following coenzymes and cofactors: F430, coenzyme B, coenzyme M, methanofuran, and methanopterin.

 

Importance in carbon cycle

 

Methanogenesis is the final step in the decay of organic matter. During the decay process, electron acceptors (such as oxygen, ferric iron, sulfate, nitrate, and manganese) become depleted, while hydrogen (H₂) and carbon dioxide accumulate. Light organics produced by fermentation also accumulate. During advanced stages of organic decay, all electron acceptors become depleted except carbon dioxide. Carbon dioxide is a product of most catabolic processes, so it is not depleted like other potential electron acceptors.

 

Only methanogenesis & fermentation can occur in the absence of electron acceptors other than carbon [oxygen?]. Fermentation only allows the breakdown of larger organic compounds, and produces small organic compounds. Methanogenesis effectively removes the semi-final products of decay: hydrogen, small organics, and carbon dioxide. Without methanogenesis, a great deal of carbon (in the form of fermentation products) would accumulate in anaerobic environments (Wiki)

 

 

Pre-Eukaryan Archaea was a (fermenting) Endocytotic Phagocytic "micro-predator", stressed by Oxygenation of Earth's atmosphere >2 Gya

 

Eukaryotic cells first arose, about 2.1 billion years ago, from an Archaean ancestor. At some point in [Earth] history, cells began to "eat" each other, by engulfing other cells in their membranes (endocytosis). As competition for resources increased [necessity is the mother of all invention], endocytosis served as an efficient way to gain many resources at once... Before this could happen, however, cell membranes had to become more flexible, which also facilitated the evolution of other membrane-bound chambers within existing cells. This development may have given rise to the Eukaryotic nucleus, which enabled increasingly large & complex genomes to evolve... The evolution of Eukaryotic cells, around 2 billion years ago, caused an explosion of diversification, and evolutionary change sped up [environmental stress?], eventually creating multi-cellular organisms. Eukaryotic cells diversified into a range of single-celled Protists, which are abundant today. Eukaryotic cells, however, also gave rise to the evolution of multi-cellular organisms (Teaching Company Biology, lect. 24)

Edited January 11, 2011 by Widdekind

[Widdekind]

Symbiotic cooperation has repeatedly been behind major, and explosive, radiations of Earth Life into new ecological niches.

 

(1) Evolution of Eukaryotic "compound-cells", 2 Gya, through Endo-symbiotic absorption, of Mitochondria & Chloroplasts, created explosive evolutionary radiation of new Lifeforms, adapted to new niches, ultimately leading to yet-more-complex multi-cellular Lifeforms:

 

Eukaryotic cells first arose about 2.1 billion years ago, from an Archaean ancestor. At some point in history [on Earth], cells began to 'eat' each other, by engulfing other cells in their membranes (endocytosis [in-hiding]). As competition for resources increase, endocytosis began to serve as an efficient way to gain many resources at once.

 

The Endo-symbiotic theory of Eukaryotic evolution suggests that at least two important organelles developed from endocytosis that resulted not in cell death, but in a symbiotic relationship between the engulfing cell and the engulfed cell... Mitochondria & Chloroplasts are organelles specialized for cellular respiration and photosynthesis, respectively, and likely to have arisen as described by the Endo-symbiotic theory...

 

The evolution of Eukaryotic cells around 2 billion years ago caused an explosion of diversification, and evolutionary change sped up, eventually creating multi-cellular organisms. Eukaryotic cells diversified into a range of single-celled protists, which are abundant today. Eukaryotic cells, however, also gave rise to the evolution of multi-cellular organisms. Phylogenetic analysis suggests, that the first multi-cellular organism arose perhaps 1.5 billion years ago, but the first potential fossil is 1.2 billion years old. More convincing multi-cellular fossils do not appear until about 600 million years ago. Multi-cellularity permitted a huge range of diversification, as a result of cellular specialization. Selective pressures could act, on only parts of an organism, and specialized cells could become very well adapted to their tasks. Multi-cellularity also large, complex organisms to develop. Organisms that were large enough could develop internal environments that enabled them to survive in harsh conditions and external adaptations to those conditions (Teaching Company Biology, Lecture 24).

Cyanobacteria are otherwise known as blue-green algae, because of their color when they grow together in their millions Their greenish color is the chlorophyll they use for photosynthesis, just like most other plants. Cyanobacteria grow in water with very low levels of oxygen but plenty of carbon dioxide. When conditions are favorable, they multiply to such an extent, that they form a green mat over the surface of the water, which is called algal bloom Animal-like bacteria [Eukaryotic Protista] are far more widespread than cyanobacteria. They live everywhere -- in water, on land, in air, on other organisms, inside other organisms, and on dead organisms ([Life on Earth] First Life, p.10-11).

 

 

(2) Symbiotic associations, of Fungi & Cyanobacteria [Lichens] 600 Mya, and Fungi & Plants 400 Mya, allowed photosynthetic organisms to colonize the continents of Earth:

 

Some symbioses have an accessible fossil history... There is paleontological evidence for lichens 600 million years ago, and Arbuscular Mycorrhizal fungi 400 million years ago...

 

The roots of more than 75% of plant taxa are susceptible to infection by Mychorrhizal fungi, which generally promote plant mineral nutrition... The association with one type of Mycorrhizal fungi, the Arbuscular Mychorrhizal (AM) fungi, is very ancient, probably evolving c.400 million years ago, at the time of the origin of land plants [on Earth]... The symbiosis, and the resultant enhanced capacity of early plants to acquire nutrients from the substratum was a prerequisite for the evolutionary transition of plants from aquatic to terrestrial habitats... If associations had not evolved, then terrestrial landscapes would probably have been dominated by microbial mats & crusts, especially of cyanobacteria [and lichens ?] (Douglas. Symbiotic Habit, p.20,24).

 

 

 

(3) The development of more modern symbiotic associations, between plants' roots, and other carbon-fixing fungi, c.100 Mya, enabled land plants to exploit polar & patchy places:

 

Arbuscular Mycorrhizas (AMs) are the most ancient, probably evolving in early land plants about 400 million years ago, in the Devonian, and widely distributed today among the bryophytes (especially liverworts & hornworts), pteridophytes, gymnosperms, and angiosperms. [some] plants have subsequently captured alternative fungal partners, often accompanied by the elimination of AM fungi, giving rise to functionally and morphologically distinct Mycorrhizas...

 

The Ectomycorrhizal (ECM) fungi are predominantly basidiomycetes related to saprotrophic fungi, especially wood-decaying taxa. They probably arose at the time of the evolutionary origin of the gymnosperm Pinaceae (c.120 million years ago) and the agiosperm Fagales (c.100 million years ago) in the Cretaceous. The transition from AM to ECM may have been a slow process, involving plants colonized by both AM fungi and ECM fungi as evolutionary intermediates... The dominant ECM plans include many woody perennials growing in seasonal climates at high latitudes, on soils containing organic nitrogen. ECM fungi are generally better suited than AM fungi to exploit these conditions, because they form a substantive sheath around roots in which seasonally available nutrients can be stored.

 

[Meanwhile] the evolution of orchid Mycorrhizas is simple replacement of AM fungi by orchid Mycorrhizal fungi, coincident with the evolutionary origin of orchids about 100 million years ago... A key feature of orchids is their ability to exploit specialized and very patchy habitats, made possible by their production of dust seeds... very tiny (one to several micrograms) seeds devoid of any reserves to support growth of the germinating seedling This strategy is linked to the receipt of organic carbon from their Mycorrhizal fungi (Douglas. Symbiotic Habit, p.39).

 

 

(4) Symbiosis in animals has helped them expand across the continents of Earth. Mycetocyte symbionts, 'in insects and a few other arthropods (notably the ticks)', enable their hosts to inhabit niches offering only 'nutritionally poor or unbalanced diets'; Mammals benefit from symbiosis with their gut bacteria; & symbiosis saved scleractinian corals from a deep population bottlenech, after the KT Mass Extinction, 65-50 Mya (Douglas. Symbiotic Habit, p.39,42,49).

 

 

CONCLUSIONS:

 

(A) Since symbiosis is so successful, and, hence, widespread, upon this particular planet, similar "stable mutualisms", may exist, across the Cosmos, on many Life-inhabited worlds.

 

(B) Humans seeking to exploit space, as a new niche [for Earthlings], could engineer artificial symbioses, such as radiation-resistant "radiodurans tattoo ink", or "Live ink". Perhaps future human space-farers might get specialized full-body tattoos, a little like Crusader-clans' Enhanced Imaging (EI) interface, or the Maori warriors of Earth.