However, with the deepening of research, an unexpected theory emerged: birds, the descendants of dinosaurs that have flown on the earth for tens of millions of years, actually "saw" the earth's magnetic field directly through electronic behavior at the quantum level. Quantum entanglement-Einstein called it "ghostly teleportation"-is the secret that birds can go home.
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How do migratory birds "sense" the geomagnetic field?
Abstract: The migration path is so long, but birds can reach their destination accurately. How do they determine their flight direction in the vast world?
The migration of birds is not only amazing, but also puzzles scientists: the migration path is so long, but birds can reach their destinations accurately. How do they determine their flight direction between heaven and earth? Historically, various hypotheses to explain the directional migration of birds, such as landform theory, memory theory and magnetic field theory, have emerged one after another. With more and more observations and experiments, the first two hypotheses gradually gave way to the magnetic field theory. Then, a new question arises: how do birds feel the earth's magnetic field to decide their flight direction? After all, the strength of the earth's magnetic field is only one twentieth that of a refrigerator.
However, with the deepening of research, an unexpected theory emerged: birds, the descendants of dinosaurs that have flown on the earth for tens of millions of years, actually "saw" the earth's magnetic field directly through electronic behavior at the quantum level. Quantum entanglement-Einstein called it "ghostly teleportation"-is the secret that birds can go home.
Red-bellied shorebirds, also known as robins, are widely distributed in the northern hemisphere, and their migration and orientation mechanisms are best known. (picture:)
The reason why we can "see" quantum entanglement is that birds have a delicate photosensitive protein, which was first found in plants. Recently, German scientists made an important breakthrough. They found that the reason why birds can perceive magnetic force is the contribution of the core coenzyme of this photosensitive protein. On February 3, 65438, their magazine published two papers in succession, pointing out that the molecule that plays a central role in the magnetic navigation of photosensitive proteins is the derivative of vitamin B2.
Bird migration, protein, geomagnetic field and quantum physics, four seemingly unrelated subjects, how are they related? This is an old and cutting-edge story.
Protein who feels blue light.
As early as Darwin's time, people noticed that plants would react differently to different wavelengths of light. For example, under short-wavelength blue-violet light or even ultraviolet light, the height of plant growth is significantly inhibited-which is one of the reasons why most alpine plants grow short. However, it was not until the end of the 20th century that what scientists called the blue light receptor was really identified. 1993, scientists first obtained the gene encoding blue light receptor from Arabidopsis thaliana, a commonly used experimental plant. The discovered blue light receptor was named cryptochrome (CRY). It is named because people think that this protein can help non-flowering plants (cryptoflowering plants) such as algae and moss to absorb blue light.
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CRYptochrome, or Cry protein, does participate in a series of physiological activities that plants need to feel short-wavelength light. However, only two years later, people were surprised to find that there were also genes encoding CRY protein in human body. Subsequently, cryptochrome genes were also found in Drosophila, mice and other animals. People finally realized that CRY protein is a photoreceptor protein widely distributed in eukaryotic cells. Of course, birds are no exception.
In animals, CRY protein is mainly concentrated in nerve tissue, especially in tissue cells related to light sensitivity. Retina is one of the tissues with the strongest expression of CRY protein. Obviously, it also participates in physiological activities regulated by light in animals. On the other hand, the migration and orientation of birds have long been observed to be related to light: during the day, birds fly with strong orientation, but at night, they are more likely to get lost. The traditional geomagnetic perception theory can't explain this phenomenon well. In 2000, Klaus Shuten, a professor at the University of Illinois in the United States, put forward a new theory, that is, CRY protein participated in the geomagnetic field induction of birds.
Light, Quantum Physics and the Earth's Magnetic Field
CRY protein in animal retina is mainly distributed in the layered structure formed by stacking inner membranes of visual cells. These lamellae are perpendicular to the long axis of photoreceptor cells, so they are parallel to the inner surface of retina, so that CRY protein appears in an orderly array on retina.
CRY protein can feel light because it contains a flavin dinucleotide (FAD) molecule. Fashion may sound out of reach, but it is closely related to our life. It is a redox coenzyme involved in many important reactions and plays a very important role in the metabolism and respiration of sugar, fat and protein. The core component of FAD is riboflavin, also known as vitamin B2. Without this essential vitamin, it will do us great harm.
FAD also plays an important role in the process of feeling light, and this molecule and the amino acid side chain around it constitute a subtle light reaction center. When a photon with a suitable wavelength (usually in the blue-violet and ultraviolet bands) is injected, FAD molecules can absorb the energy of this photon and enter the excited state. At this time, it will absorb a proton (H+) from the surrounding environment to form FADH+, and then take an electron from the tryptophan group belonging to CRY proteome. This series of charge transfer process makes the conformation of CRY protein change, which leads to the subsequent reaction.
Spatial structure of CRY protein and its bound FAD molecule. Photo: uiuc.edu.
If so, it is impossible to establish contact with the earth's magnetic field. However, the mysterious quantum effect suddenly became the key to the matter at this time. We know that a pair of electrons occupying the same orbit always have opposite spin directions, which is determined by Pauli exclusion principle, one of the foundations of quantum physics. But when one of the electrons is excited and "escapes", this restriction disappears-under normal circumstances, various random factors from outside will affect the spin state of the excited electron, thus breaking the rule that "pairs must be reversed".
However, in the CRY protein, these two pairs of electrons are still bound by each other's states after excitation-in other words, they can still remain entangled for a long time. And this is long enough to give the earth's magnetic field a chance. When the earth's magnetic field acts on the pair of electrons at a specific angle, it can change their spin state, thus affecting the time required for the electrons to return to the ground state orbit, which also affects the time for CRY protein to start the downstream reaction.
Therefore, when the parallel magnetic field passes through the curved retina, the direction of the magnetic field received by the CRY protein in different parts of the retina is different, which leads to the difference in the activity of the CRY protein in different parts of the retina, and then affects the perception of light. This difference in sensitivity is not only manifested in the azimuth difference, but also reflects the latitude position-because the included angle between the geomagnetic field and the ground plane at different latitudes is different. Therefore, from the perspective of birds, their vision not only includes the scenery they see, but also uses light and shade to express azimuth and latitude information. It can be said that this is a set of "head-mounted display system" in the eyes of birds, which is much more precise and earlier than the equipment used by the most advanced fighter pilots. Because FAD is excited by light, it essentially forms a pair of free radicals containing unpaired electrons, so this theory is also called "free radical pair model".
From the perspective of birds, scenes in different directions may be like this. Image source:
Still moving forward
In fact, in the 1970s, when Shu Teng first explained the mechanism of birds' perception of magnetic field with the theory of "free radical pair", the theory was once despised or even ridiculed-because no one believed that birds' eyes could have such an exquisite geomagnetic perception system. However, with the passage of time, more and more evidence is supporting this model. For example, it has been found that the navigation function of birds is destroyed when they emit extremely weak electromagnetic waves, but the frequency can just affect the direction of electron spin-this non-intensity determining phenomenon cannot be explained by traditional theories. In addition, the model also found that when the magnetic field intensity is close to the geomagnetic intensity (about 1 gauss), the influence of "geomagnetic visibility" is the most obvious; When the magnetic field intensity is too large, this effect will be weakened, which can explain the sensitivity of birds to weak magnetic fields.
Eurasian silver carp migrates long distances between Europe and North Africa every year, so this finch bird is often used in migration-related research. Picture:
The "free radical pair" model is one of the popular theories to explain bird migration's orientation mechanism. Recently, an experiment by the University of Frankfurt and the Max Planck Institute in Germany took the "free radical pair" model a step further: the FAD state that makes CRY protein active was determined.
In fact, people have been paying close attention to the state of FAD in cryptochrome, especially when CRY is active, because it determines which pair of free radicals CRY protein uses to feel the earth's magnetic field. Experiments show that two groups of "free radical pairs" can be produced in bird calls: first, the stimulated FADH and tryptophan form the first group of free radical pairs; Subsequently, the excited FADH is reduced to FADH-, and then oxidized by oxygen to the excited FADH. At this time, it and oxygen molecules (O2? -) forming a second pair of free radicals.
In previous studies, it was found that the transition of FAD from ground state to excited state and from excited state to reduced state required the participation of short wavelength light. But these two steps have different requirements for light-the former requires blue light and ultraviolet light with shorter wavelength, and the latter can be realized by green light with longer wavelength. Therefore, stimulating birds with different wavelengths of light can produce FAD in different states. In order to detect the active CRY protein, scientists have developed an antibody that specifically binds to CRY protein after conformational changes.
Domestic chickens were transferred to different wavelengths of light after being irradiated by white light, and scientists detected the production of active CRY protein in their retinas. However, if the chicken was first placed in the dark and then moved to different wavelengths of light, scientists found that the active CRY protein in the chicken retina disappeared after being irradiated by green light. In addition, if chickens exposed to white light are exposed to green light for a long time, the active CRY protein in retina will gradually decrease or even disappear.
Through this experiment, scientists show that in the presence of blue light and ultraviolet light, FAD can always cycle between four states. When only the green light exists, this cycle is blocked in the first step. When shifting from darkness to green light, birds can only use the previously excited FAD to produce the subsequent reduced Fadh-and the second group of free radical pairs-and then the CRY protein can be active. However, FADH- and the second pair of free radicals it produces will be gradually consumed with the progress of the reaction, and the ground state FAD will no longer be excited, so it will not produce active CRY protein.
In cryptochrome, FAD changes between four states under the action of light. Picture:
Subsequently, scientists also investigated the orientation ability of silver carp in Europe and Asia under different lighting conditions. Similar to the results in domestic chickens, the Eurasian silver carp exposed to green light after dark environment lost its orientation ability, while the Eurasian silver carp turned to green light after white light irradiation could initially determine its orientation, but with the extension of green light irradiation time, this orientation gradually disappeared. Together, these two experiments show that the existence of the reduced state of FADH- a triggers the activation of CRY protein, and the second pair of free radicals produced subsequently is the key factor to feel the earth's magnetic field.
Of course, the above work is only one of the advances in understanding the mechanism of birds' perception of the earth's magnetic field. The "free radical pair" model itself, like any discovery and progress in science, is still advancing in exploration, constantly discovering and answering new questions and doubts. There is still a long way to go to fully explain the phenomenon of bird migration and orientation. And this is a microcosm of human's continuous and in-depth exploration of nature.
(Source: china wildlife conservation association)