Life has existed on the Earth for a pretty long time, perhaps longer than you might imagine. Biology seems pretty resilient, though, there have
been five major mass extinctions (the last of the five killed off the dinosaurs) -- and at least 20 total mass extinction events over the last half billion years or so. Maybe we're working on the sixth major extinction event by messing around with nuclear weapons or the Large Hadron Collider. Or perhaps we'll bring back something from the past that we'll regret. Here are a few of examples of ancient organisms that we might not want to revive.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post