Extinction

The Great Ice Age that occurred during the Pleistocene era (which began about 2 million years ago and ended 10,000 years ago) also caused the extinction of many plants and animal species. This period is of particular interest because it coincides with the evolution of the human species. This was a time of diverse animal life, and mammals were the dominant large surface forms (i.e., in contrast to the reptiles of previous periods; as always, insects and bacteria dominated the animal world in absolute numbers, numbers of species, and total biomass). In the late Pleistocene (50,000–10,000 years ago), several other extinctions occurred. These wiped out several species known collectively as the megafauna ("big animals"), including mammoths, mastodons, ground sloths, and giant beavers. The late Pleistocene extinctions of megafauna did not occur all at once, nor were they of equal magnitude throughout the world. However, the continents of Africa, Australia, Asia, and North America were all affected. Recent work has shown that these extinctions did not, as long thought, single out megafauna as such, but rather all vertebrate species with slow reproduction rates (e.g., one offspring per female per year or less)—megafauna merely happen to be members of this larger class. It has been speculated that human beings caused these late-Pleistocene extinctions, hunting slow-reproducing species to extinction over centuries. Paleontologists continue to debate the pros and cons of different versions of this "overkill" theory.

The causes of the extinction events of the late Pleistocene are still debated, as are those of the larger mass extinctions that have punctuated earlier geological history; most likely several factors were involved, of which the global spread of human beings seems to have been one. Indeed, the past 10,000 years have seen dramatic changes in the biosphere caused by human beings. The invention of agriculture and animal husbandry and the eventual spread of these practices throughout the world has allowed humans to utilize a large portion of the available resources of Earth—often in an essentially destructive and nonsustainable way. For example, in pursuit of lumber, farmland, and living room, human beings have reduced Earth's forest area from over one half of total land area less than one third.