Origin of Life

Prokaryotes, Viruses and Origin of Life
from multiple web sites and BIOLOGY: The Science of Life by Wallace, King and Sanders 2nd Edition Scott, Foresman and Co. 1986

Hypothesis on the Origin of Life	Miller-Urey Experiment	Early Earth	Hypothesis Today: How strong is it?	Coacervates and Proteinoids	Earliest Cells	Prokaryotes	Origins
	Organization of the Prokaryotes	Prokaryote Characteristics	Archaebacterial Life	Eubacteria	Other Bacteria	Eukaryotes	Endosymbiosis Hypothesis

Hypothesis on the Origin of Life

Compelling evidence suggests that life arose spontaneously on the Earth, at a time when unique conditions were present
Haldane and Oparin were the first to speculate on the spontaneous origin of life. The Haldane-Oparin Hypothesis proposed that the precursors of life's molecules formed from inorganic sources and these underwent polymerization to form macromolecules. Such macromolecules included primitive versions of enzymes. Droplets, or coacervates, formed and were enclosed by protein or lipid shells. A next hypothetical step required the formation of genelike autocatalytic molecules (RNA world in BSCS textbook) that could assure the faithful reproduction of such enzymes.

Miller-Urey Experiment

Miller and Urey tested the Haldane-Oparin hypothesis with a device that simulated the primitive environment. Using a mixture of gases and applying an electrical discharge, they succeeded in synthesizing amino acids, aldehydes, and carboxylic acids, all known to be monomers of cells.

Early Earth

New knowledge of ultraviolet light has changed ideas on the primitive earth atmosphere. The atmosphere in which life arose was more likely to have consisted of water, carbon dioxide, carbon monoxide, molecular nitrogen, and some free hydrogen.
Energy sources in the primitive atmosphere may have included ultraviolet light, lightning, heat, and geological shock energy.
Estimates place the origin of life some 3.8 billion years ago; fossils of cyanobacteria are believed to be 3.5 billion years old.

Hypothesis Today: How strong is it?

For the hypothesis to remain viable, scientists must be confident about the following: proposals about early conditions, the spontaneous formation of monomers of life and their polymerization into the polymers of life, the spontaneous formation of cell-like bodies, and the formation of simple, self-replicating chemical systems.
Scientists are still debating over which came first, the nucleic acids or the proteins.
Recent experiments on the revised atmospheric conditions have been successful in producing the usual monomers and a few that were not formed in the Miller-Urey experiment.
The mass action law suggests that polymerization is not likely to have occurred in the sea, but more likely to have occurred in heated and highly concentrated pools of monomers. Treating amino acids in this manner, Sydney Fox produced polymers that aggregated into what he called proteinoids.
Experiments performed with certain RNA molecules indicate that it is able to self replicate due to its ability to act as enzymes that worked on themselves. These molecules were able to evolve in the test tube.
DNA and RNA are usually seen as carriers of genetic information, with the occasional function of RNA as a scaffold, as for example in the ribosome. This picture has changed with the discovery of self-splicing of certain RNAs, most notably that of the Tetrahymena group I intron by Tom Cech, and the active role of RNA in the RNase P in the process of maturation of tRNAs by Sid Altman. These observations laid the foundation for the concept of catalytic RNA for which the Nobel prize of 1989 was awarded to these two colleagues.

Coacervates and Proteinoids

Coacervates simulate life processes such as growth and reproduction. By taking in selected materials, they grow to a critical mass and then divide.
Experimenting with coacervates, Oparin constructed simple biochemical systems such as those containing replicating enzymes and nucleic acid elements. He managed to get the coacervate to duplicate some elementary biochemical mechanisms.

Earliest Cells

There is disagreement on the metabolic characteristics of the early protocells. Some suggest they were chemotrophic or phototrophic, while others claim they were simple anaerobic heterotrophs. The continued autocatalytic synthesis of monomers may have provided carbon compounds for the early heterotrophs (heterotroph hypothesis)
Keen competition for energy rich monomers may have encouraged variants that could extract hydrogen from inorganic sources, using light energy in simple photosystems. The use of water as a hydrogen source probably came later since it requires complex photosystems.
When the use of water in photosynthesis occurred, oxygen joined the gases of the atmosphere for the first time.
The buildup of significant amounts of oxygen in the atmosphere required an enormous time period because of the presence of oxygen sinks - elemental substances such as sulfur and iron that readily combined with the gas.
As cell populations grew, new modes of heterotrophic nutrition arose. Food chains emerged as cells began devouring cells through phagocytosis.

Prokaryotes
Origins

Among the earliest evidences of life are 3 to 3.5 billion year old column like stromatolites, laminated deposits produced by dense bacterial populations.

Organization of the Prokaryotes

Recent revisions of prokaryote taxonomy have divided the older kingdom Monera into two subkingdoms, Archaebacteria and Eubacteria. The two differ significantly in their cell wall chemistry (protein substances in Archaebacteria and peptidoglycan in Eubacteria) and in other aspects of biochemistry.

Prokaryote Characteristics

Prokaryote cells are smaller than those of eukaryotes, they lack membrane bound organelles, and their chromosomes are circular DNA molecules lacking protein. They divide by binary fission and occasionally undergo genetic recombination through conjugation (exchange of DNA). They often have slimy capsules, and the flagellum.

Archaebacterial Life

Archaebacteria commonly live in conditions reminiscent of ancient Earth. They commonly live in hot, acidic, and salty environments. Most must live in the absence of oxygen.
Examples of archaebacteria include methanogens - anaerobic methane generators, extreme halophiles, extreme thermophiles, and thermoacidophiles, bacteria that live in dense salt concentrations, very hot water, and hot, acidic conditions, respectively.
Where found, photosynthetic archaebacteria use the pigment bacteriorhodopsin for the capture of light energy used in building the chemiosmotic gradient and ATP synthesis.

Eubacteria

Eubacteria include phototrophs, chemotrophs, and heterotrophs (including decomposers and parasites).
Eubacteria occur as the rod shaped bacillus, the spherical coccus, and the corkscrew shaped spirillum.
Bacilli occur singly or in chains and commonly form dormant, resistant endospores. The coccus forms are arranged in pairs (diplococcus), chains (streptococcus), and clusters (staphylococcus). Spirillum forms - spirochetes - occur singly.
Eubacterial cell walls contain peptidoglycan, which is very thick in some and thin in others. Some are coated with a polysaccharide, while others have an outer lipid membrane.
Vital stains are used to color cells for microscopic viewing and for classification. Gram's stain is used for the later purpose; cells that retain the dark purple stain are Gram positive and those that don't are Gram negative. Gram positive bacteria are susceptible to penicillin, which inhibits wall synthesis.

Other Bacteria

Slime or gliding bacteria (Myxobacteria) us slime excretions in moving.
Chlamydobactera are the source of some antibiotics.
Mycoplasmas are the smallest known cells, lacking a cell wall and definite shape. All are animal parasites.
Cyanobacteria are found in all aquatic environments including the ocean. Photosynthesis is carried out in highly organized thylakoids containing light capturing pigments.

Eukaryotes
Endosymbiosis Hypothesis

The endosymbiosis hypothesis proposes tha cukaryotic cells arose from a number of prokaryotic cells that through some kind of invasions and incorporations became one.
The evidence to support this hypothesis is that many organelles are similar to bacteria. These include the presence of DNA and RNA in mitochondria and chloroplasts.