As children, we learn that volcanoes, plate tectonics, and erosion are natural processes that can have very dramatic effects on Earth, or any planet for that matter. What we may not have learned is that the geological sculpting of Earth, the Moon, and other planets is influenced not only by internal forces but by external forces as well. By influencing the geological conditions of a planet, these forces are also shaping the evolution of environmental and biological conditions on that planet. Our external cosmic environment has undoubtedly shaped the history of life on Earth.
Evidence from the geologic record indicates that Earth has experienced its fair share of impacts. Detailed studies lead scientists to believe that some of the smaller impact craters on the surface of Earth coincide with the dates of more modest species extinction earlier in Earth's history. For instance, one study found evidence of an impact coinciding with the end of the Devonian Period, 340 million years ago. The fossil record during this same time period suggests that 70% of species became extinct in less than a few million years. The most recent extinction was a short 11 million years ago, when it is estimated that 30% of species disappeared. Up to a dozen mass extinctions have been identified in the fossil record. However, an impact crater of the correct age to correlate with a mass extinction has only been identified in a couple of cases.
Evidence of mass extinctions has led to a radical adjustment of our original theories of evolution. The work of Darwin and his successors, starting in the 1850s, showed that evolution results from natural selection, driven by competition among species, along with mutations in the genetic structure of organisms and the exploitation of new environmental niches. The result is the continual emergence of genetic variability and new species. In classic Darwinian theory, natural selection is motivated by events within the Earth's biosphere. However, the discovery of the mass extinction event corresponding to the beginning of the end for the dinosaurs 65 million years ago furthered Darwin's original conception. Major events such as these indicate that drastic changes in the environment, and hence in the course of evolution, can also come from random external astronomical phenomena, such as the fall of an asteroid. Random processes can range from the decay of an atom to the impact of an asteroid. Random means the exact timing is not predictable but the rate of events averaged over time is a well-determined number.
Asteroid or comet impacts are just one example of external influences on terrestrial life. Beyond the Solar System there are other events that could disrupt the smooth flow of evolution. Stars in the neighborhood of the Sun have random motions that occasionally take them close to the Solar System. Direct collisions between stars or between stars and planets are exceptionally rare. But when a star gets close enough it can disrupt the Oort comet cloud and send a cascade of comets into the inner Solar System, greatly increasing the probability of an impact on the Earth. Gravity is a long range force so even a "brush" with a nearby star can have consequences. Such a close encounter may happen every few million years. If a supernova goes off within 30 light years, or a hyper nova goes off within 1000 light years, it could blast the Earth with damaging radiation, causing a spike in the mutation rate and killing some species. In its 250 million year orbit of the center of the galaxy, the Solar System may pass close to star clusters or near molecular clouds and suffer the same disruption as when a star passes nearby. All of these event have most likely occurred multiple times in the long history of life on Earth.
This realization has dramatically transformed our notion of our role in the universe. Medieval scholars believed that Earth was stable and unchanging. As natural science developed from the Renaissance to the time of Darwin, scientists assumed that all changes on Earth are gradual and that all the important influences on Earth's environment and biology (not counting heat and light from the Sun) come from within the Earth's system itself. Now, we realize capricious events such as asteroid or comet impacts can radically alter the course of evolution. One reminder, which you can personally visit, is the famous impact crater in Arizona. Nearly a mile wide, this crater formed only 20,000 to 50,000 years ago. Rest assured, the chance of a catastrophic impact within a human lifetime is slight. As humans, we rarely see the direct effect of such large cosmic forces on our planet. Nevertheless, over the long term of geological history, these forces have shaped the world in which we live.