Though Earth is densely packed with living beings, the greater part of the species that ever existed on it has become extinct. Yet before humans started spilling oil into the sea and depleting ozone layer with greenhouse gasses, species died out for certain logical natural reasons. After all, overpopulation is a threat not only to humans, and animals have their own mechanisms to regulate populations.
Climate change is one of the most trustworthy explanations for the extinction of species. About 11,000 years ago, megafauna mammals failed to adapt to a sudden climate change. It is also likely that instead of food, dinosaurs faced early humans that presented more of a threat than food to gigantic reptiles. Anyway, megafauna species by no means could survive in this new environment that formed on Earth, and their extinction made way for present-day reptiles and amphibians.
Several more reasons of natural extinction can be derived from climate change. These are a lack of food, lack of genetic diversity, predation of better-adapted species, unbearable weather conditions, and an invasion of disease. The evidence of meteor strikes is another popular theory to explain the extinction of pre-historic species.
With such a broad evidence that nature regulates populations itself, we cannot deny the impact of humans on the degradation of species. Construction and industrial activity deprive animals of their natural habitat, and the lucky ones have to migrate. Some species that have no alternative habitat (such as polar bears) can simply die out as glaciers melt. A huge human responsibility lies on pollution. If we cannot take away resources from marine species, we can still pollute them. And that is not the activity to be proud of.