There have been at least five mass extinctions in the history of the Earth. One memorable one was 65 million years ago, when dinosaurs became extinct. Mammals actually co-existed during part of the time that dinosaurs dominated the planet. However, they were small and insignificant. After the extinction of dinosaurs occurred, this opened up new opportunities and niches for mammals to exploit and adapt to. So, without competition and predation by dinosaurs, mammals were able to evolve into all the many mammal species we currently have today, as well as others which have themselves, become extinct. Mammals have a variety of forms and are adapted to land, sea and even the air. Extinction which seems to be a cruel event can be seen as a catalyst for the evolution of other species which survive.