Why Is There Magnetite in the Human Brain?
Like the Marvel comic book hero, we are all iron men and women. Iron composes the heme of hemoglobin, the red pigment in our blood. It's stored in ferritin, a protein ubiquitous in almost all living organisms. And, most curiously, it's found in mineral form within the human brain.
That iron mineral is magnetite (Fe3O4), and its name is an obvious clue to its most intriguing property: magnetite is the most magnetic of Earth's naturally-occurring minerals.
So what is a magnetic mineral doing inside our brains? Caltech geobiologist Joseph Kirschvink thinks he might have the answer: Magnetite allows us to sense Earth's magnetic field.
This remarkable ability is present all across the animal kingdom, from bacteria and birds to turtles and bats. Sensing the planet's geomagnetism grants these creatures a keen sense of direction and location, known as magnetoreception. Being able to orient on Earth is obviously a huge advantage, so it's no wonder that this ability is so widespread.
However, there wasn't strong evidence that humans could sense magnetic fields until earlier this year, when a new study was published. Kirschvink and a team of researchers sat subjects down in a chamber shielded from electromagnetic interference and altered magnetic fields inside whilst watching participants' brainwaves. They found a "clear cut, quantifiable and reproducible" effect on the amplitude of subjects' alpha waves, a type of electrical brain activity.
In other words, we do seem to have a rudimentary magnetic "sense", although it is undeniably weaker than comparable senses in other animals.
So how does it work?
Kirschvink thinks magnetite is the key. Receptor cells containing crystals of magnetite could register changes in magnetic fields and report this information to the brain. This is almost identical to what magnetotactic bacteria do. They have structures containing nanoscale magnetite crystals called magnetosomes. These essentially act as biological compasses, allowing the bacteria to navigate.
A study published last year found that magnetite is indeed present in its crystalline form throughout the human brain, with particularly high concentrations in the cerebellum and brain stem. Moreover, it's highly likely that this magnetite is produced within our cells, the researchers wrote. Isolating individual cells containing magnetite could provide a "Eureka!" moment, allowing scientists to analyze up close what's going on inside. If the internal dynamics are similar to such cells found in birds and bacteria, it would be a big boost to Kirschvink's theory.
"It is, he explained, the most rational evolutionary pathway for migratory behavior seen in the full range of animals. Natural selection took something that worked marginally – the receptivity of magnetite to the magnetic field – and through mutation and gene replication made it better and better until it produced navigational wizards..."
Are there humans out there who demonstrate this navigational wizardry? O'Connor describes numerous peoples in her book who possess a superhuman sense of direction, including the Inuit and some Aboriginal Australians. Their brains might just be full of magnetite!