b'Biological geomagnetic field sensingFeatureposition, while their cryptochromes provide the directional information (Wiltschko and Wiltschko, 2019)\x08A recent paper by Natan and Vortman (2017) claims to have found a mechanism for the elusive connection between the sensor and the brain by proposing the involvement of the GMF sensing ability of magnetotactic bacteria, which they note would otherwise appear to have no adaptive value\x08 They propose a mutualism or symbiosis between it and the Figure 2.An electron micrograph image of the chains of magnetosomesparticular animal concerned to provide a connection that from a sockeye salmon that are similar to those of magnetotactic bacteria (fromgives the animal a sensor and the bacteria a value, or purpose\x08 Mann et al, 1988). However, a follow-up paper by Natan et al (2020) published in the prestigious Philosophical Transactions of the Royal Society Living cells have the ability to build these nanocrystals ofB, sought to address the criticism and support raised by magnetic material as a form of biogenetic ferromagnetism\x08their hypothesis presented in the previous paper\x08 Finally, they X-ray absorption spectroscopy analyses show it to be mostlysuggest the future research directions required to confirm or iron oxides\x08 The microscopic amounts of material involvedrefute the possibility of symbiotic magnetic sensing and admit are detected by Superconducting Quantum Interferencethat they have not yet found the mechanism by which the host Device (SQUID) magnetometers and observed by electronand the bacteria communicate! It would seem that this idea microscopes3\x08 Is this magnetic material the sensor of the GMF?of symbiosis using magnetotactic bacteria has much scope for Possibly, but it is still not known how these cells produce neuralfurther investigation\x08signals in the organisms brain\x08 In the case of homing pigeons, the magnetic material in the beaks is less than 3 nm in size andThe situation with humansdispersed in a fluid\x08 This fluid sac is elongated in one direction, and it is thought that pressure changes caused by its directionIf a magnetic sense is common in the animal kingdom, then why relative to the GMF are sensed by the pigeons nervous system\x08 not in humans? Did we once have this ability and then gradually In 2015 a team of neuroscientists at the University of Texas,lose it over time? Or, is it still present but suppressed below the Austin reported on their study of the particular nematodecurrent level of detection? To research this question requires Caenorhaditis elegans\x08 This worm is known for its ability to aligntesting the subject when the GMF is varied or non-existent, as is with the GMF and to track it if the direction is altered\x08 Theydone more easily with other animals\x08 Some researchers suggest discovered in the head of the worm an unusual micro-structurethat human health and behaviour are related to the changes in at the tip of a neuron which they believed to be its GMF sensor\x08GMF, possibly without our being conscious of the phenomenon\x08 They noted that when this feature is removed by geneticOne study (Nicholls, 2016) found an increase in suicides during engineering, C\x08 elegans becomes disoriented (Vidal-Gadea, etmagnetic storms\x08 Another found that blood pressure also varies al, 2015 and Bainbridge, et al, 2016)\x08 As some other animals likewith activity in the GMF\x08 Does this activity cause the headaches butterflies and birds have similar brain structures, the researchfelt by some at this time? Finally, some people believe they team suggested it is possible that they may also have thissleep better when aligned to the field\x08feature\x08 Biologically developed magnetic material was first detected The other general theory for the existence of a sensor, a morein humans in 1992; in the brain, heart, spleen and liver\x08 SQUID chemically based theory known as the radical-pair hypothesis,magnetometers have detected particles weighing only a few was first suggested in 1978\x08 It relates to a molecule thatnanograms and less than one ppm in volume5\x08 Cryptochromes, contains two unpaired electrons that are seen to be sensitivethe chemical sensors, are also found in the retinas of humans, to changes in the relatively weak GMF\x08 Rodgers and Horewhich are directly connected to the brain\x08 Thus, both proposals (2009) affirmed that the radical-pair mechanism is currently thefor a sensing mechanism could apply to humans\x08only plausible way in which weak magnetic fields can affect aIn a recent paper by Gilder et al (2018), the magnetic intensity chemical reaction4\x08 Such a molecule sensitive to the GMF is theof brain tissue was closely mapped on seven laboratory cryptochrome protein, which exists in many animals includingspecimens6\x08 The results from over 800 separate brain samples the monarch butterfly, the common fruit fly and the retinas ofwere measured with a magnetometer and shown as contours many migratory birds\x08 The location in the retina gives a directof magnetic intensity (see one result in Figure 3)\x08 The results are connection to the brain through the optic nerve\x08 Specifically, inindependent of the sex and the various ages of the specimens\x08 European robins the molecule is found in the right eye which,Some areas were found to be more magnetic than others\x08 In if covered, causes the bird to become disoriented\x08 Could theparticular, the cerebellum and the brain stem were found to be cryptochrome molecule be a sensor? twice as magnetic as the upper regions of the brain\x08 The authors As many animals and, in particular, birds and monarchthought this could have evolutionary significance\x08butterflies have magnetic material and cryptochromes, bothIn an even more recent paper by Wang et al (2019), changes sensor mechanisms may be employed together\x08 Many workersto human alpha brain waves were observed in response in this field propose that for birds the magnetic material in theto a changing magnetic field\x08 Thirty four participants were birds beak measures the strength of the GMF and hence sensesindividually enclosed in a Faraday cage that enabled the 3There is a branch of this study called biomineralisation and textbooks5A general reference for this topic is found by searching: brain magnetism published on it, include Kirschvink, etal\x08, (1985) and Evans and Hellerwhich yields many references including some mentioned here\x08(2003)\x08 6Stuart Gilder, one of the authors, is a geophysicist who normally studies 4This paper has 109 references\x08 palaeomagnetism of rocks\x0835 PREVIEW FEBRUARY 2021'