I was contributing to another topic when this question occured to me.

I have always believed that Humans possibly came here from elsewhere in the universe. Could it be possible, that homosapien created a nuclear winter on earth wiping out all of the dangerous predators that roamed the planet? I mean, if we were searching the galaxy and came across one that had an abundance of life and everything needed for us to survive, would we destroy the dangerous predators before colonising it?

A handful of humans could then have been placed on Earth to breed and insure the future survival of our species.

The major problem with biblical, or otherwise supernatural, cataclysm hypotheses is that there are many animals that were alive, but now are not, that would most likely survive if somehow brought back; conversely, many modern animals are similar enough to ancient animals that they would not have survived a catastrophe formidable enough to have taken out the ancient animals.

Take Trilobites, for example. They are thought to have evolved in the early Cambrian, 500 million years ago. They survived two extinctions before finally succumbing to the Permian extinction event. However, if they were to be brought back nowadays, they could very possibly inhabit an ecological role of scavengers. Their heavy armour, while unlikely to be capable of resisting the strongest bites, would offer them some protection from smaller predators.

For a more familiar example, take Pliosaurs, Ichthyosaurs, or Nothosaurs. There is no reason they couldn't have survived the Flood, and they would probably be capable of competing with modern large marine predators. Some of the larger pliosaurs would probably be capable of hunting whales. Dinosaurs could similarly survive quite easily in the tropics and temperate regions, either as scavengers or as hunters. Entelodonts, a less known relative of modern pigs, would probably have no trouble whatsoever competing with modern animals; they could probably even become a top predator.

Finally, man knows much more dangerous foes than the dinosaurs. It goes without a doubt that more humans have been killed by viruses and bacteria than by large predators. To make the world "safe", why only remove the superficially dangerous foes?