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The Magnetic Sense Is More Complex than Iron Bits

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Many unrelated animals, from bacteria to birds, have tiny particles of magnetite in their bodies. For many years, biologists assumed that these magnetized bits of iron were the key to understanding the geomagnetic sense in migrating organisms: cells sense the torque of these iron crystals when they align north, like tiny compass needles. This theory, however, might be only part of the story. The real key may rely on proteins that respond with intrinsic iron atoms of their own. At least, that’s the working hypothesis currently growing in popularity.

Cryptochromes are light-sensitive proteins in the retina that respond to blue or green light. They are thought to form pairs of free radicals in response, perhaps interacting with iron-rich proteins. This would link magnetoreception to vision and to circadian rhythms, which also involve cryptochromes. Kenneth J. Lohmann, who has long studied magnetic navigation in sea turtles, was intrigued by the hypothesis from Chinese physicists writing in Nature Materials last January. As we noted last November when the discovery was first publicized, they proposed that the long-sought mechanism is a “magnetic protein biocompass” involving cryptochromes and an iron-containing protein named MagR.

In the same issue of Nature Materials, Lohmann described how “maddeningly difficult” it has been to discover the secret shared by such diverse animals as sea turtles, birds, mollusks, and insects. Resolving the debate between the cryptochrome hypothesis and the more orthodox magnetite hypothesis has been difficult because magnetic fields pass right through the body and are not localized to a specific organ.

“Trying to locate a small number of submicroscopic structures of unknown appearance, scattered throughout an animal’s body in unknown places, is an enormous challenge,” he says. The new cryptochrome hypothesis looks promising, but Lohmann offers this caveat:

…the putative magnetoreceptor has been identified largely on the basis of theory, genomics, biochemistry and three-dimensional protein-structure modelling. Pigeon genes for MagR and cryptochrome were expressed in bacteria and the resulting proteins were found to co-purify. This is certainly an important first step, yet whether the complex actually exists in any animal, much less whether it functions as a magnetoreceptor, remains unknown. Also, at least two crucial elements of the current model remain to be elucidated. The first is whether and how cryptochromes interact with MagR to mediate light-dependent effects. The second involves the fundamental question of transduction — how the putative magnetoreceptor converts a stimulus into electrical signals that can be interpreted by nerve cells. [Emphasis added.]

A team of theoretical physicists at Oxford has now added their support to the cryptochrome model. Their March 2016 paper in the Proceedings of the National Academy of Sciences doesn’t, unfortunately, satisfy either of Lohmann’s crucial elements. But if they are right, magnetoreception puts animals on the cutting edge of human understanding of quantum mechanics. The American Physical Society explains:

One explanation [for why radiofrequency interference disrupts a bird’s magnetic sense] is that the electromagnetic noise has quantum-level effects on cryptochrome’s performance. This would suggest that the radical pairs in cryptochrome preserve their quantum coherence for much longer than previously believed possible. Such a finding could have broader implications for physicists hoping to extend coherence for more efficient quantum computing.

The fact that they are considering a biomimetic application implies that the physicists didn’t know quantum coherence could last so long. Birds know more about quantum mechanics than the human experts! The paper says:

…the spike discussed here is undeniably a quantum effect, arising from the mixing of states associated with avoided energy-level crossings, and is not captured by the semiclassical theory. In this sense, radical pair magnetoreception may be more of a quantum phenomenon than hitherto realized.

For reasons unrelated to the actual lab research, the lead physicist, Peter Hore of Oxford, lugged Darwin’s theory into the discussion. “Physicists are excited by the idea that quantum coherence could not just occur in a living cell, but could also have been optimized by evolution,” he said. “There’s a possibility that lessons could be learned about how to preserve coherence for long periods of time.” Here’s how the Oxford team inserted evolutionary speculation into their paper:

  • …the compass could have been optimized by evolution….

  • We conclude that there is ample scope for a cryptochrome-based radical pair compass to have evolved with a heading precision sufficient to explain the navigational behavior of migratory birds both in the laboratory and in the wild.

  • random mutations in the sequence of the protein … could have provided evolution with the scope to optimize the compass precision.

  • …this is another property [spin relaxation time] that could have been optimized by evolution…. Because the spike only emerges when the coherence time exceeds 1 μs, its presence could explain why slow relaxation might have evolved.

All of these comments are superfluous to the research. The authors provide no evidence of specific beneficial mutations that arose and were selected, or how the compass arose in the first place so that it could be “optimized” in some aimless fashion. The comments are all hedged in suppositions: “could have been” and “might have evolved.” Lohmann points out that MagR is “conserved evolutionarily,” allowing “a sort of universal magnetic-sensing structure that can be adapted for different purposes by different animals.” Is it helpful to say that a structure with a purpose “can be adapted” by all kinds of unrelated animals? How many miracles of chance did that take?

Far more productive to the research is the reason they tackled the problem in the first place. Here’s what they describe as significant:

Billions of birds fly thousands of kilometers every year between their breeding and wintering grounds, helped by an extraordinary ability to detect the direction of the Earth’s magnetic field. The biophysical sensory mechanism at the heart of this compass is thought to rely on magnetically sensitive, light-dependent chemical reactions in cryptochrome proteins in the eye. Thus far, no theoretical model has been able to account for the <5° precision with which migratory birds are able to detect the geomagnetic field vector. Here, using computer simulations, we show that genuinely quantum mechanical, long-lived spin coherences in realistic models of cryptochrome can provide the necessary precision. The crucial structural and dynamical molecular properties are identified.

The precision, the mechanism, and the extraordinary ability of birds motivated this research. Design, not evolution, made them excited to understand it.

To migrate successfully over large distances, it is not sufficient simply to distinguish north from south (or poleward from equatorward). A bar-tailed godwit (Limosa lapponica baueri), for example, was tracked by satellite flying from Alaska to New Zealand in a single 11,000-km nonstop flight across the Pacific Ocean. A directional error of more than a few degrees could have been fatal. Because the magnetic compass seems to be the dominant source of directional information, and the only compass available at night under an overcast (but not completely dark) sky, migratory birds must be able to determine their flight direction with high precision using their magnetic compass. Studies have shown that migratory songbirds can detect the axis of the magnetic field lines with an accuracy better than 5°.

The news item, indeed, begins by pointing to one of the champions of Illustra’s documentary Flight: The Genius of Birds.

Each year, the Arctic Tern travels over 40,000 miles, migrating nearly from pole to pole and back again. Other birds make similar (though shorter) journeys in search of warmer climes. How do these birds manage to traverse such great distances when we need a map just to make our way to the next town over?

The next film in Illustra’s Design of Life series, Living Waters, includes a list of two dozen animals, including birds, reptiles, mammals, insects, and fish that are able to navigate by the earth’s magnetic field. How could evolution deal with those empirical observations? As Tim Standish notes, there’s a better explanation.

Darwinian natural selection is blind. It doesn’t know that a solution has been arrived at in some other organism.

It can’t think, “Wow, the salmon have that elegant solution to the problem. Why don’t we evolve towards that in the turtles?” It’s not an explanation that’s possible.

Now conversely it’s not surprising at all to see an intelligent agent know of a solution to a problem and apply that same solution under different circumstances over, and over, and over again. This is what we see intelligence do. And it’s not what we would expect from an unguided process.

As the current work shows, a cause able to provide birds, sea turtles and salmon with quantum-mechanical precision points to intelligence at a very high level.

Image: Arctic tern, by Jos Zwarts [CC BY-SA 4.0], via Wikimedia Commons.