Magnetic field over the Earth weakened 42,000 years ago, possibly influencing climate, evolution, and the organisms that inhabit the planet.
All around the same time, the Neanderthals and large megafauna died out, cave art started popping up in Asia and Europe, North America saw swaths of new ice sheets, and, if that wasn’t enough, the magnetic poles went for a twist that caused elements like radioactive carbon to be more heavily featured in the atmosphere, and eventually in living animals and plants. And while scientists have hypothesized in the past that these events might somehow be linked, pinning down those exact mechanisms—without a time machine that would have us walking around with woolly mammoths and dire wolves, that is—has been tough.
For more, check out this PopSci / MSN story.