The Black Swan of Cairo, and here, here and here.
Until about 65 million years ago dinosaurs filled every ecological niche imaginable. We poor mammals survived by our wits and tactics we evolved to protect our young for as long as possible. Not only we mammals but a few other species evolved tactics that kept their young hidden - crocodiles, turtles, birds. They were not to know that a Black Swan event would turn these strategies to another advantage. But that's how natural selection works - traits that aid survival for one reason can lead to survival for an altogether different reason.
The dinosaurs happily laid their eggs in the open air and for tens of millions of years the balmy climate incubated them. Temperature volatility was unknown. Mom dinosaur need only be nearby to protect her clutch - the climate would do the rest.
Eggs can incubate in a very narrow temperature range. Vary outside that range and nothing hatches. If by chance you were a species that evolved to hide eggs where how you did so also provided insulation or warming from the ocean or decomposing vegetable matter your eggs would still hatch. If you were carrying your young around inside they too were protected by your high metabolism (developed to better run from hungry dinosaurs).
Came an unforeseen event - a meteor impact that lowered global temperatures by more than eggs could tolerate and lo - there were no longer any dinosaurs (other than the few that survived in some remote isolated warm places that became dwindling islands of last refuge against the explosion of other species that survived - like us).
I sometimes wonder if by blanket inoculation against all disease there will come a time that, because of collapse of our infrastructure, vaccines will no longer be available. Nor insulin, nor plentiful clean water.
Of course, Murphy always being ready to do his best (worst), such a collapse will come when there are no books left anywhere - we all depend on google for everything and with it gone not only will we not have access to vaccines and modern medicine but we'd not have access to information with which we might recover. We could go from 9 billion to 90 in a matter of decades. Whatever species had been suppressed by us would then be let loose - from viruses and bacteria to everything you can imagine - rats, cockroaches.....
Safety is a two edged sword - one day we too might find ourselves like the dinosaurs - totally dependent on a very narrow set of conditions.
But then, all species have their life cycle.
What do other's have to say?
One-way Evolution (New Scientist)