Every animal on Earth may house the molecular machinery to sense magnetic fields, even those organisms that don't navigate or migrate using this mysterious 'sixth sense'.

Scientists working on fruit flies have now identified a ubiquitous molecule in all living cells that can respond to magnetic sensitivity if it is present in high enough amounts or if other molecules assist it.

The new findings suggest that magnetoreception could be much more common in the animal kingdom than we ever knew. If researchers are right, it might be an astonishingly ancient trait shared by virtually all living things, albeit with differing strengths.

That doesn't mean all animals or plants can actively sense and follow magnetic fields, but it does suggest that all living cells might, including ours.

"How we sense the external world, from vision, hearing through to touch, taste, and smell, are well understood," says neuroscientist Richard Baines from the University of Manchester.

"But by contrast, which animals can sense and how they respond to a magnetic field remains unknown. This study has made significant advances in understanding how animals sense and respond to external magnetic fields - a very active and disputed field."

Magnetoreception might sound like magic to us, but plenty of fish, amphibians, reptiles, birds, and other mammals in the wild can sense the tug of Earth's magnetic field and use it to navigate space.

Because this force is essentially invisible to our species, it took a remarkably long time for scientists to notice it.

Only in the 1960s did scientists show that bacteria can sense magnetic fields and orient themselves in relation to those fields; in the 1970s, we found that some birds and fish follow Earth's magnetic field when migrating.

Even to this day, however, it's still unclear how so many animals achieve these incredible feats of navigation.

In the 1970s, scientists suggested that this magnetic-compass sense could involve radical pairs, molecules with unpaired outer shell electrons that form a pair of entangled electrons whose spins are altered by the Earth's magnetic field.

Twenty-two years later, that study's lead author co-authored a new paper proposing a specific molecule in which the radical pairs could be formed.

This molecule – a receptor in the retina of migrating birds called a cryptochrome – can sense light and magnetism, and it seems to work through quantum entanglement.

In basic terms, when a cryptochrome absorbs light, the energy triggers one of its electrons, pushing it to occupy one of two spinning states, each of which is differently influenced by Earth's geomagnetic field.

Cryptochromes have been a leading explanation for how animals sense magnetic fields for two decades, but now researchers at the Universities of Manchester and Leicester have identified another candidate.

Manipulating the genes of fruit flies, the team found that a molecule called Flavin Adenine Dinucleotide (FAD), which usually forms a radical pair with cryptochromes, is actually a magnetoreceptor in and of itself.

This basic molecule is found at differing levels in all cells, and the higher the concentration, the more likely it is to impart magnetic sensitivity, even when cryptochromes are lacking.

In fruit flies, for instance, when FAD is stimulated by light, it generates a radical pair of electrons that are responsive to magnetic fields.

However, when cryptochromes are present alongside FADs, a cell's sensitivity to magnetic fields increases.

The findings suggest that cryptochromes are not as essential as we thought for magnetoreception.

"One of our most striking findings, and one that is at odds with current understanding, is that cells continue to 'sense' magnetic fields when only a very small fragment of cryptochrome is present," explains University of Manchester neuroscientist Adam Bradlaugh.

"That shows cells can, at least in a laboratory, sense magnetic fields through other ways."

The discovery could help explain why human cells show sensitivity to magnetic fields in the lab. The form of cryptochrome present in the cells of our species' retina has proved capable of magnetoreception at a molecular level when expressed in fruit flies.

However, this doesn't mean humans utilize that function, nor is there evidence that cryptochrome guides our cells to line up along magnetic fields in the lab.

Perhaps FAD is the reason why.

Even though human cells show sensitivity to Earth's magnetic field, we don't have a conscious sense of that force. Maybe that's because we don't have any cryptochromes assisting.

"This study may ultimately allow us to better appreciate the effects that magnetic field exposure might potentially have on humans," says genetic biologist Ezio Rosato from the University of Leicester.

"Moreover, because FAD and other components of these molecular machines are found in many cells, this new understanding may open new avenues of research into using magnetic fields to manipulate the activation of target genes. That is considered a holy grail as an experimental tool and possibly eventually for clinical use."

The study was published in Nature.