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According to abundant geological evidence, an asteroid roughly 10 km (6 miles) across hit Earth about 65 million years ago. This impact made a huge explosion and a crater about 180 km (roughly 110 miles) across. Debris from the explosion was thrown into the atmosphere, severely altering the climate, and leading to the extinction of roughly 3/4 of species that existed at that time, including the dinosaurs. Many asteroids of this type are now known; their orbits pass through the inner solar system and cross Earth's orbit. Some of these could potentially hit Earth in the future. Most, but not all are smaller than the one that hit us 65 million years ago.
Fossils found in soil layers of different ages show a record of slow, gradual changes in species, with simple organisms gradually being replaced by more complex organisms, apparently by evolutionary processes driven by natural selection. For example, 1000 million years (abbreviate My) ago, the oceans held only simple organisms like algae, while the land was relatively lifeless. Fish fossils appear in strata after about 500-600 My ago; dinosaurs and giant reptiles were on the land by 200 My ago. Mammals were not common until after 65 My ago, and humanlike creatures appeared only in the last 4 My.
[Side comment: The above description has been challenged for two centuries by religious fundamentalists. Fundamentalists are defined as people who believe that the primary way of learning about nature should not be the scientific method, or compilation of evidence tested in different labs in different countries, but rather interpretation of ancient manuscripts, such as the Koran, the Old Testament of the Bible, the New Testament, or other ancient writings. The scientific method was hammered out mainly in the 1600s, when naturalists of that period agreed that information about nature could best be determined by direct observations of nature, and experiments, which would be published openly, in international literature. The key to the method is that assertions published by one scientist can be tested by new, independent observations and measures made by other scientists. Daniel Boorstein's book, "The Discovers," gives an excellent overview of how the scientific method was established. This method of learning about nature continues to be under challenge in America from fundamentalist groups, for example in school boards and state legislatures. In 1998, the Arizona state school board recommended that the term "evolution" be dropped from public school curriculum in the state, only to reverse themselves some months later after a public outcry. In 1999 the Kansas sate school board dropped evolution and the Big Being theory of the universe's origin from the subjects tested in their school system. Several other state school boards subsequently followed in their footsteps. This battle is not new; it is four centuries old!]
Ever since the fossil sequence began being mapped around 1800, geologists noticed that striking "breaks" occurred in the sequence, when one group of fossilized species gave way to other groups during short intervals. Indeed, these breaks were the basis for dividing geologic time into different eras with different names. For example, the Paleozoic era gave way to the Mesozoic era of middle life forms, which in turn gave way to the present Cenozoic era of recent life. Prior to the 1980s, the causes of the breaks were unknown...
In the late 1970s, a team of geochemists headed by Lewis Alvarez of Berkeley was studying chemical changes in soil layers corresponding to breaks in the fossil record. In the soil layer that separates the Mesozoic Era from the Cenozoic Era, dating 65 My ago, they found an excess of the element iridium, which is common in meteorites. Meteorites are believed to be fragments of asteroids. Therefore, the Alvarez team theorized that an asteroid had hit Earth at this time, and that the debris ejected from the explosion were spread in the soil layer.
For about ten years, this theory was extremely controversial. However, compelling evidence has accumulated to support the theory.
There are now many lines of evidence to prove that a relatively large impact happened 65 My ago.
Asteroids hit Earth typically at high speeds of 10 to 20 km/sec (16-32 miles/sec). During the impact, the kinetic energy in the asteroid (or energy of motion) is converted to explosive energy, blowing debris of dust, soil, and rocks not only into the atmosphere, but out into space, where it fell back into the top of the atmosphere. Early calculations in the 1980s (using in part ideas worked out by Carl Sagan and his colleagues) showed that so much dust entered the high atmosphere that the Earth was shrouded in a dust layer that blocked sunlight for several weeks or months. This would have killed some plants, disrupting the food chain.
Later calculations (especially by Jay Melosh at the University of Arizona) indicated that for the first few hours after the impact, rocky debris would have fallen back into the high atmosphere, creating a storm of glowing fireballs in the sky. The radiant energy from these would have heated the surface to boiling temperatures for some minutes, and would have been enough to kill many animals and plants on the surface. However, in regions of heavy rainstorms or snowstorms, these organisms would have survived the first few hours. Sea creatures would have been buffered from effects in the first hours, but plankton on the surface might have died out over the weeks of darkness, decreasing the food supply for small fish, which affected the bigger fish, and so on.
These examples show how hard it is to predict the exact effects of the impact. Many species who lived on the surface (such as dinosaurs) might have been decimated in hour or weeks. Species who lived in burrows, or hibernated (like some mammals) might have survived. This may explain why mammals replaced giant reptiles after the impact. Tiny primitive mammals may have emerged from their dens, to find that their giant reptile competitors were mostly gone.
Probably not. The biggest known break is the break between the Paleozoic Eara and the Mesozoic Era, about 230 to 250 My ago, when something like 90% of then-existing species died out. This has been called the "Great Dying," but has not been explained. So far there is no evidence of an asteroid impact at that time. The second greatest break is the one that we have discussed, 65 My ago, caused by an asteroid impact.
Geologists have divided the eras into shorter intervals called Periods, such as the Jurassic Period, noted for its large dinosaurs. These Periods are also defined by breaks in the fossil record, smaller than the breaks between eras. Many species went extinct during these breaks, but not as many as in the breaks between the Eras. Evidence for impacts, smaller than the one 65 My ago, has been found at some of the breaks, but not at others.