Although – as far as anyone knows – humans lack the cerebral hardware to visually perceive magnetic fields, we do carry some of the necessary sensory equipment. And some recent experiments confirm that, in the right context, that equipment might be secretly operational…like the second version of the Death Star.
The key lies in cryptochromes, proteins that (many scientists suspect) enable certain migratory animals – like sea turtles, honeybees, salmon, and robins – to sense earth’s geomagnetic field, and use this information to orient their bodies along a path of migration. In many animals – including humans – these proteins also seem to be involved in regulating circadian rhythms.
The chemical pathways triggered by cryptochromes aren’t very well understood yet, but the general consensus is that these proteins respond to magnetic energy by triggering chemical changes in the cells that contain them, similar to the way photoreceptor proteins in retinal cells respond to light.
In short, a huge number of animals (humans included) possess cryptochromes in certain cells, and scientists are still working to understand the variety of roles these proteins play.
One hypothesis suggests that cryptochromes may impact visual perception in animals that use them for navigation:
Suppose that the products of a radical pair reaction in the retina of a bird could in some way affect the sensitivity of light receptors in the eye, so that modulation of the reaction products by a magnetic field would lead to modulation of the bird’s visual sense, producing brighter or darker regions in the bird’s field of view. (The last supposition must be understood to be speculative; the particular way in which the radical pair mechanism interfaces with the bird’s perception is not well understood.) When the bird moves its head, changing the angle between its head and the earth’s magnetic field, the pattern of dark spots would move across its field of vision and it could use that pattern to orient itself with respect to the magnetic field.
Humans don’t seem to have any ability like this (though if you do, I’d love to hear about it). Our eyes do, however, contain a protein known as human cryptochrome 2, or hCRY2.
To test whether hCRY2 can actually respond to geomagnetic fields, a team led by University of Massachusetts neuroscientist Steven Reppert replaced cryptochrome found in the ever-popular fruit fly (Drosophila melanogaster) with hCRY2. Reppert’s team discovered something pretty amazing: human cryptochrome allowed the flies to magnetically orient themselves.
[Reppert’s] team engineered flies to be cryptochrome-deficient: They struggled to orient within a magnetically-charged maze. When the researchers spliced human cryptochrome into the flies, they again found their bearings.
One obvious question remains, though: why can’t we humans use our hCRY2 to sense magnetic fields, as many other animals apparently can? Well, aside from the obvious answer that a geomagnetic sense wasn’t particularly helpful for human reproduction and survival, it seems that a geomagnetic compass may also be a biologically expensive tool to maintain. Klaus Schulten, a biophysicist at the University of Illinois at Urbana-Champaign, thinks oxygen anions known as superoxides may play an important part in the geomagnetic navigation pathway:
[Schulten’s] research suggests that the cryptochrome compass needs superoxide, a type of free radical oxygen molecule, to work. Free radicals tend to destroy DNA. That’s fine for a relatively short-lived animal, but not for one that intends to live for decades.
As you can probably tell, this research is still in the early stages, and new data may cast the whole discussion in another light. Still, it’s exciting to think that long before we learned to make maps, we may have carried around our own built-in GPS.