The Earth does not operate as a closed system. Basic planetary and stellar processes may be involved in encouraging or hindering biological evolution. Starting at the geological level, we note that convection within a planet can not only cause plate tectonics, but also change sea levels, ocean currents, wind patterns, and seasonal extremes. This process has been responsible for isolating landmasses such as Australia and allowing different species to evolve there. Similarly, volcanic eruptions may have spewed enough dust into the upper atmosphere to reduce the sunlight reaching the surface. For example, widespread reductions in sunlight of up to 25% occurred after the 1883 Krakatoa (Indonesia), 1912 Katmai (Alaska), and 1982 El Chichon (Mexico) eruptions. Rarer, larger events could change temperatures enough to cause the decline of some species and the ascendancy of new ones. Current discoveries of dust layers in active ice packs are beginning to allow dating and reconstruction of prehistoric volcanic cataclysms.
Cosmic intruders have been decisive in the history of life. The first wave of impacts early in the Earth's history was beneficial to life, bringing in substantial amounts of life's essential gases and organic materials. Since then, the much rarer large impacts have been catastrophic. The impact of an asteroid or comet could have damaged the ozone (O3) layer, exposing organisms to enhanced radiation. The Tunguska explosion of 1908 generated as much as 30 million tons of nitrogen oxide (NO) in the upper atmosphere, reacting to deplete up to 45% of the ozone in the Northern Hemisphere, consistent with Smithsonian measurements made from 1909 to 1911. An asteroid impact occurred 65 million years ago, probably causing the dramatic break in the fossil record between the Cretaceous and the Tertiary periods. The ejected dust and soot dwarfed that produced by volcanism and probably caused massive climate change. A burst of volcanism around the same time may have dealt many lifeforms a lethal one-two punch.
Subtle changes in the Earth-Sun system can also influence life. During the Earth's history, the Sun has brightened by 30%, and even smaller changes in the Sun's radiation since then may have caused climate change. Astronomers believe that slight changes in the Earth's orbit and the tilt of the planetary axis to the plane of the ecliptic due to gravitational forces caused major climate changes such as the ice ages. Recent marine studies of traces from the past 700,000 years have shown decisively that cycles of climatic change, including several pulses of continental glaciation, can be tied to the so-called Milankovich cycles of orbital change in the Earth-Sun-Moon system.
During life's tenure, the Solar System has probably been influenced by the gravity of nearby passing field stars and by blasts of radiation and high-energy particles from nearby supernovae. On a larger scale, life's history has encompassed 18 galactic "years" &mdash complete 250-million-year orbits of the Milky Way disk — and the Earth has passed through denser material in the plane of the Milky Way galaxy perhaps 200 times. We are only just beginning to discover the signatures of these cosmic events in our rock and fossil records and the effects they have had on the evolution of advanced life.