If you like the smell of spring roses, the sounds of summer birdsong, and the colors of fall foliage, you have the stabilization of the ozone layer to thank for it. Located in the stratosphere, where it shields the Earth from harmful ultraviolet radiation, the ozone layer plays a key role in preserving the planet's biodiversity. And now we may have a better idea of why that took so long -- more than 2 billion years -- to happen. According to a new study, Earth's early atmosphere hosted a 'battle royale' between iodine and oxygen -- effectively delaying the creation of a stable ozone layer that would shield complex life from much of the sun's ultraviolet radiation (UVR). The new theory may solve a mystery that has puzzled scientists for hundreds of years.