Ancient mass extinction event tied to breakdown of the ozone layer

A mass extinction 360 million years ago was likely caused by a rapidly-warming climate that triggered a breakdown in the ozone layer. Plant samples from the time period reveal DNA that had been damaged by UV radiation. Scientists warn that we may be in store for another such event if we can't control our own warming of the climate. When the miles-wide asteroid that arrived at Earth some 66 million years ago came crashing down, it wiped out an estimated 75% of all species on the planet. It was an...