"For the last 2.6 million years, the planet has experienced severe climatic fluctuations, during which the Earth waxed and waned between frigid ice ages and warm interglacial periods. Until approximately 60,000 years ago, there were few apparent extinctions. Suddenly, hundreds of species of mammals, flightless birds, and reptiles went extinct largely over the course of 50,000 years. These extinctions marked the beginning of a tidal wave of disappearing species that is cresting today. Because large body size is correlated with slow breeding, the largest animals are more susceptible to extinction."