An animal or plant becomes extinct when the last of its kind dies. Since the beginning of life on Earth, countless plants, animals, and other organisms have become extinct. In fact, most species that have lived on Earth are now gone. Extinction occurs in the blink of an eye, but factors leading up to it can build up over thousands of years.

Image credits: main image, courtesy of Discovery Channel Online; quote, Mike Novacek courtesy of Discovery Channel Online.