The planet’s magnetic field last flipped some 42,000 years ago. Few biological things from that era are still here to be studied, save a few well-preserved, ancient trees. One such tree, an astonishingly intact Waipoua tree from New Zealand, was alive at the time of the last magnetic field flip.
Scientists believe, due to data gathered from the ancient arboreal specimen, that the flipping of the magnetic field was much more devastating to life on earth than was previously thought. Scientists have long known that one of the last major dying-off events among mammals occurred during this period, though this was thought to be coincidental. Now, there is evidence that, maybe, the movement of the Earth’s magnetic field impacted ancient life more than we believed.
The Earth is a giant magnet, thanks to its solid iron core surrounded by a swirling ocean of molten metal. These metals below our very feet generate a powerful magnetic field that envelops our planet, shielding organic life on the surface from harmful cosmic radiation. Even as the sun pours ultraviolet radiation towards the planet, the magnetic field helps to block out some of the most harmful amounts of the radiation.
For reasons that aren’t currently well-understood, the magnetic field around Earth can sometimes become highly unstable. This weakening could, in turn, lead to an increase in the amount of cosmic radiation that buffets the planet, making life much harder for those that live on the surface. And, sometimes, the weakening is so profound that the poles themselves flip, an event that would see compasses claiming north to be south, and vice versa.
The last time this happened has been dubbed the “Laschamp Excursion” by scientists, after the bizarre makeup of lava flows from the era discovered in the Laschamp region of France. The lava from that era has a bizarre inclusion: iron filaments that are pointing in the “wrong” direction, suggesting that magnetic north was situation over what we now call the South Pole when the lava burst from the Earth’s crust.
Thanks to information pulled from well-preserved Waipoua trees, scientists now believe that life on Earth was buffeted by cosmic radiation during this period of magnetic instability. The ozone layer likely was diminished at this point in time as well, leaving much of the planet’s natural atmospheric defenses down. Some have even suggested this could have made the sun’s rays miserable to even be exposed to, which could have led to humans taking to caves in daylight hours.
Of course, it’s far from clear when or if this could happen again. For the time being, we’ll have to simply trust our planet to keep the magnetic field we rely on stable.