evidence: An ancient, well-preserved tree that was alive the last time the Earth's magnetic poles flipped has helped scientists pin down more precise timing of that event, which occurred about 42,000 years ago. This new information has led them to link the flipping of the poles to key moments in the prehistoric record, like the sudden appearance of cave art and the mysterious extinction of large mammals and the Neanderthals. They argue that the weakening of the Earth's magnetic field would have briefly transformed the world by altering its climate and allowing far more ultraviolet light to pour in. [...] Sometimes, for reasons scientists do not fully understand, the magnetic field becomes unstable and its north and south poles can flip. The last major reversal, though it was short-lived, happened around 42,000 years ago.