250 million years ago, the largest mass extinction in biological history took place. It is what is known as the the Permian-Triassic boundary. In Our Time discusses the details and angleson this extinction.
What do we know of what was life like during the Permian? What was the earth like? And what caused the extinction. Current theory points at vulcanic eruptions from fissures in the large pangaea continent. This and the consecutive greenhouse effect, made life unbearable for more than 90% of the species. In the Triassic, life slowly recovers and a whole new evolutional string is set off.
What helped, specifically, those species that did survive, doesn't seem to to be much of what would generally be evolutionary helpful. In addition, the occurrance of this particular cataclysm, does not have any cyclic quality. Hence it is one unexpected disaster and the survival is by chance. Nobody will call this the Hand of God, but the metaphor jumped to my mind. Yet another podcast from the BBC that had me gripped, even if the subject at firs hand appeared not so promising.