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Cryptochromes
According to the first model, magnetoreception is possible via the radical pair mechanism, which is well-established in spin chemistry  and was speculated to apply to magnetoreception in 1978 by Schulten et al. In 2000, cryptochrome was proposed as the "magnetic molecule", so to speak, that could harbor magnetically sensitive radical-pairs. Cryptochrome, a flavoprotein found in the rod cells of the retina in the eyes of birds, among which European robins, and in other animal species, is the only protein known to form photoinduced radical-pairs in animals. The function of cryptochrome is diverse across species, however the mechanism by which it functions is the same: exposure to blue light excites an electron in a chromophore which causes the formation of a radical-pair whose electrons are entangled. This quantum entanglement is essential for the precision in magnetoreception. Recombination of this pair produces a chemical product or signal of which the kind depends on the position of the birds head with respect to Earth's magnetic field. This magnetic field is only 0.5 gauss and radical pair mechanism is the only plausible way that weak magnetic fields can affect chemical changes.

Issues
Despite more than 50 years of research, a sensory receptor in animals has yet to be identified for magnetoreception. It is possible that the entire receptor system could fit in a one-millimeter cube and have a magnetic content of less than one ppm. As such, even discerning the parts of the brain where the information is processed presents a challenge.

The most promising leads – cryptochromes, iron-based systems, electromagnetic induction – each have their own pros and cons. Cryptochromes have been observed in various organisms including birds and humans. Magnetoreception through cryptochromes is extensively studied in the European Robin. Iron-based systems have also been observed in birds, and if proven, could form a magnetoreceptive basis for many species including turtles. Electromagnetic induction has not been observed nor tested in non-aquatic animals. Additionally, it remains likely that two or more complementary mechanisms play a role in magnetic field detection in animals. Of course, this potential dual mechanism theory raises the question, to what degree is each method responsible for the stimulus, and how do they produce a signal in response to the low magnetic field of the Earth?

Then there is the distinction of magnetic usage. Some species may only be able to sense a magnetic compass to find north and south, while others may only be able to discern between the equator and the pole. Although the ability to sense direction is important in migratory navigation, many animals also have the ability to sense small fluctuations in earth's magnetic field to compute coordinate maps with a resolution of a few kilometers or better. For a magnetic map, the receptor system would have to be able to discern tiny differences in the surrounding magnetic field to develop a sufficiently detailed magnetic map. This is not out of the question, as many animals have the ability to sense small fluctuations in the earth's magnetic field. This is not out of the question biologically, but physically has yet to be explained. For example, birds such as the homing pigeon are believed to use the magnetite in their beaks to detect magnetic signposts and thus, the magnetic sense they gain from this pathway is a possible map. Yet, it has also been suggested that homing pigeons and other birds use the visually mediated cryptochrome receptor as a compass.

Extra information:


 * the magnetite system looks ancestral while the radical-pair strategy (and neural processing foci) appear derived. (2008)
 * the magnetite system is said to work like a map, while the radical pair strategy functions as a compass, but magnetite allows the detection of the North and South pole which is exactly the function of a compass, whereas the radical pair strategy allows much more because it might enable the animal to see Earth's magnetic field.
 * The underlying magnetic sense is hypothesized to involve intra-cellular iron oxide, i.e., magnetite nanoparticles (Fe3O4), which would be sensitive to the horizontal polarity of a magnetic field, enabling these animals to distinguish between magnetic north and south, independent of light.
 * Cryptochrome, a flavoprotein found in the eyes of European robins and other animal species, is the only protein known to form photoinduced radical-pairs in animals. When it interacts with light particles, cryptochrome goes through a redox reaction, which yields radical pairs both during the photo-reduction and the oxidation. The function of cryptochrome is diverse across species, however, the photoinduction of radical-pairs occurs by exposure to blue light, which excites an electron in a chromophore. Magnetoreception is also possible in the dark, so the mechanism must rely more on the radical pairs generated during light-independent oxidation. (from quantumbiology)

Darwinian threshold or Darwinian transition is a term introduced by Carl Woese to describe a transition period during the evolution of the first cells when genetic transmission moves from a predominantly horizontal mode to a vertical mode. The process starts when the ancestors of the Last Universal Common Ancestor (the LUCA) become refractory to horizontal (or lateral) gene transfer (HGT) and become individual entities with vertical heredity upon which natural selection is effective. After this transition, life is characterized by genealogies that have a modern tree-like phylogeny.



Before the Darwinian Threshold
LUCA is often considered to be an already complex organism with a DNA-based genome, a complex informational flow and an efficient metabolism, but some authors, like Carl Woese did, believe instead that the LUCA was not a discrete entity but rather a diverse community of cells that survived and evolved like a biological unit.

Carl Woese indicated that most likely there existed high mutation rates and small genomes. Also present were small proteins and larger imprecisely translated "statistical proteins". Entities in which translation had not yet developed to the point that proteins of the modern type could arise have been termed “progenotes,” and the era during which these were the most advanced forms of life, the “progenote era”

These organisms or biological entities, these progenotes (or ribocytes), had RNA as informational molecule instead of DNA. RNA is capable of both catalysis and replication and could have been central to the origins of heredity and life itself. Compartimentalization with membranes was not yet completed and translation of proteins was not precise. Not every progenote had its own metabolism; different metabolic steps were present in different progenotes. Therefore it is assumed that there existed a community of sub-systems that started to cooperate collectively and culminated in the LUCA.

It has been proposed that the initial molecular events were carried out by Transfer RNAs (tRNAs). It is hypothesized that structured tRNAs could have provided amino acids during a process called self-translation of a single extended tRNA strand.

After the Darwinian Threshold
Most scientists place the LUCA at the root of the tree of life. From this root depart two Prokaryotic Domains: the Bacteria and the Archaea. Just after this first split, one of the branches, going towards the Archaea, splits again and gives rise to a third branch which is that of the Eukaryotes so that now we have all three Domains of life. Carl Woese thought that even during the era around the origin of the LUCA, the root and the first branches were very blurred since the cells were not very well defined yet and HGT was still quite important. Some authors retain LUCA was a mesophilic eukaryote. According to these authors the Domains that derived from LUCA through a process of reductive evolution or "streamlining" were Prokaryotes; mesophilic and thermophilic Bacteria and thermophilic Archaea. The term "prokaryote" should therefore be abandoned, since it suggests that "prokaryotes" preceded "eukaryotes" in their evolution from LUCA towards complexity.

Pangene

Fascinerend leven

William Bateson

Inheritance

De Vries

Zimmer

About Hippocrates

Pangene Blom

James Lovelock