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Evolution and Extinction
Dinosaur origins are uncertain. During the 140-million-year reign of the dinosaurs, many new breeds of their kind evolved and many primitive predecessors died out. However, not all kinds disappeared at once. The last of the dinosaurs disappeared at the end of the Cretaceous Period, which marked a major change in the history of life on Earth. Their extinction has been a major cause of controversy. Some of the common reasons included were increases in cosmic radiation to changes in climate, incresed volcanic activity, and to continental drift. In fact, evidence indicates that sea levels did fall and climatic temperatures did drop at the end of the Mesozoic Era.
A controversial, however popular extinction theory was proposed in the 1980s by a physicist, Luis Alvarez. They found a greater than usual amount of iridium in samples of sedimentary layers between rocks of Late Cretaceous Period and Early Tertiary Period time and concluded that the iridium came from outer space, most likely from an asteroid that struck the Earth at that time. They theorized that the impact caused an enormous cloud of dust created by the collision to circle the Earth, enshrouding it from sunlight and striking at the food chain by killing most plants. Research on this theory continues, and a possible location for the crater caused by the asteroid has been found on the Yucatan Peninsula of Mexico. Many geologists have accepting the iridium evidence and that theory, and many have suggested that the iridium may have resulted from massive volcanic eruptions over a longer period of time, resulting in worldwide climatic changes.
Whatever the cause, the dinosaurs are now gone. As suggested above, however, in a way they may remain. Many paleontologists consider birds almost certainly to have evolved from small bipedal dinosaurs during the Jurassic. If so, the children of the dinosaurs are still here today.
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