Hunting Predator

Predators tend to select young, weak, and ill individuals.
Evolution of predation
Predation appears to have become a major selection pressure shortly before the Cambrian period—around 550 million years ago—as evidenced by the almost simultaneous development of calcification in animals and algae, and predation-avoiding burrowing. However, predators had been grazing on micro-organisms since at least 1,000 million years ago.