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Cosmic bombs and the evolution of life on Earth |
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Geologist Robert S. Dietz used to say that when it came to life on Earth, evolution served as the game plan and catastrophic meteorite impacts played the role of wild card. Nature dealt now-extinct creatures such as the pteranodon and the dinosaurs just such a card approximately 65 million years ago. |
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Seascape of the Devonian Period, 360 million years ago, before life emerged onto land. One of the greatest mass extinctions in Earth's history occured 365 million years ago, when seven of every 10 marine creatures died off. Castastrophic meteorite impact is one suspected but unverified cause. |
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Life may have developed more than once early in Earth's 4.5-billion-year history, only to be repeatedly snuffed out by a giant comet or asteroid. Life had become well-established, though, by 550 million years ago, as shown in the seascape of the Cambrian Period. |
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Another seascape of the Cambrian Period. A trilobite is prominent in this image and the one above. Nine of every 10 marine species became extinct, including the trilobites, at the end of the Paleozoic Era 250 million years ago. The cause remains a mystery. |
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