Review of the Cretaceous-Paleogene Extinction Event More than 98% of all organisms that ever lived on Earth are now extinct. A mass extinction event occurs when a large number of species become extinct in a short period of time (relative to the age of the Earth). Mass extinctions are extensively studied for both their causes and their effects, as there is usually room for debate regarding the catalysts that precede extinction and the massive influx of new biological species that follows. There were five major mass extinctions, nicknamed the "Big Five", which wiped out at least 50% of the species living at that time. The best-known mass extinction of the Big Five, with the decimation of all non-avian dinosaur species, is the Cretaceous-Paleogene (K-Pg) extinction. The K-Pg extinction occurred as continuous natural disasters followed by an asteroid collision with Earth, which affected plant life, animal life, and hydrology (Cobb, n.d.). Some paleontologists believe that the global ecosystem was already in decline before the final blow, an asteroid, which destroyed much of life on Earth. Continental drift and seafloor spreading may have altered the circular configuration of the oceans, causing dramatic changes in sea levels and Earth's climate..