As death inevitably follows birth, extinction is the predictable consequence of evolution on our planet. The fossil record chronicles the comings and goings of species through geologic history, but its most prominent feature may be surprising. Extinctions are not distributed evenly through time: Rather, at five moments over the past 500 million years, most of Earth’s animal species disappeared over a short time scale, permanently altering ecosystems on land and in the sea.
The best known of these mass extinctions wiped out the dinosaurs and many less evocative creatures 66 million years ago. A large body of evidence supports the hypothesis that this event reflects the impact of a massive asteroid, underscoring that life can be influenced by factors beyond our planet.
Another, more destructive mass extinction may be more instructive to 21st century citizens. Two hundred and fifty million years ago, volcanic activity a million times greater than anything ever witnessed by humans spewed massive amounts of carbon dioxide and other gases into the atmosphere, resulting in the extinction of an estimated 90% of marine animal species. Known as the “Great Dying,” the extinctions were selective, with some species more vulnerable than others, and the observed pattern of extinction and survival supports the view that volcanically induced global warming, ocean acidification, and oxygen loss from subsurface seawater drove the extinctions.
Andrew H. Knoll, Fisher research professor of natural history and professor of earth and planetary sciences at Harvard University, examines how these three threats are accelerating today due to human activities, not volcanism. He discusses how observations and experiments made to better understand 21st-century global change resonate strongly with the Great Dying 252 million years ago, suggesting Earth’s future if we choose to do nothing.
But, says Knoll, the distant mirror of past mass extinction should be seen as something like Charles Dickens’ ghost of Christmas yet to come: a warning of what might transpire if we choose to do nothing. He reminds us however, that like Scrooge, we also have the capacity to change our ways and so preserve Earth’s precious biodiversity for future generations.
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