Introduction
Extinction is nothing new, and (a different thing) it is no longer surprising or controversial, at least to most people. (At an earlier period the possibility of extinction attracted the same kind of resistance that the possibility of evolution still does). The disappearance of the dinosaurs has been mainstreamed, even if in some accounts, such as that offered by the Creation Museum in Kentucky, it appears to have been surprisingly recent, and even the much earlier disappearance of the trilobites is not especially esoteric.
Of course these extinctions happened without human assistance; both were at least expedited by climate change associated with massive geological disruptions due to volcanism and extraterrestrial impact.
Humans are not the only animals to bear more direct responsibility for the disappearance of their fellow species. For example, when North America and South America were joined by the closing of the isthmus of Panama (scientists disagree about the timing of this conjunction, but at least three to four million years ago) two very different mammalian communities came into contact. South America was home to a varied group of marsupials, reflecting its earlier connection with Australia, while the inhabitants of North America more closely resembled today's mammalian fauna. Animals migrated in both directions, but ultimate result of this contact was the replacement by placental mammals of all the South American marsupials except the opossums and shrew opossums.
Many organisms, whether as large as elephants or as small as locusts or fungi, can have devastating impacts on particular ecosystems. And even our impressive capacity for environmental transformation may not set us so very far apart from other life forms. The evolution of the earth's atmosphere in its current form, containing plenty of oxygen for animals like us to breathe, is normally recounted as a story of progress, with a happy ending: the earliest atmosphere was inhospitable to such life, until the emergence of blue-green algae or cyanobacteria that, through photosynthesis, slowly produced the air that now sustains us. It can also, however, be cast as tragedy, since it resulted in the extinction of most of the anaerobic life that had flourished previously, including most of the cyanobacteria themselves. This was the first of the mass extinctions that our planet has witnessed (although it does not bear that label--rather, it is usually called the Great Oxygenation Event) , and perhaps the only one before the present that was caused when the dominant organisms transformed their environment so radically that they could no longer thrive in it.
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