Why did dinosaurs become extinct?

There are many theories as to why dinosaurs disappeared from Earth about 65 million years ago. Scientists debate whether dinosaurs became extinct gradually or all at once. The gradualists believe that the dinosaur population steadily declined at the end of the Cretaceous period. Numerous reasons have been proposed for this. Some claim the dinosaurs' extinction was caused by biological changes that made them less competitive with other organisms, especially the mammals that were just beginning to appear. Overpopulation has been argued, as has the theory that mammals ate so many dinosaur eggs that dinosaur reproduction was irrevocably harmed. Others believe that disease-everything from rickets to constipation-wiped the dinosaurs out. Changes in climate, continental drift, volcanic eruptions, and shifts in Earth's axis, orbit, and/or magnetic field have also been held responsible.The catastrophists argue that a single disastrous event caused the extinction not only of the dinosaurs but also of a large number ofother species that coexisted with them. In 1980 the American physicist Luis Alvarez (1911-1988) and his geologist son Walter Alvarez (1940-) proposed that a large comet or meteoroid struck Earth 65 million years ago. They pointed out that there is a high concentration of the element iridium in the sediments at the boundary between the Cretaceous and Tertiary periods. Iridium is rare on Earth, so the only source ofsuch a large amount of it had to be outer space. This iridium anomaly has since been discovered at over fifty sites around the world. In 1990 tiny glass fragments, which could have been caused by the extreme heat of an impact, were identified in Haiti. A 1l0-mi (l77·km) wide crater in the Yucatan Peninsula, long covered by sediments, has been dated to 64.98 million years ago, making it a leading candidate for the site of this impact.
A hit by a large extraterrestrial object, perhaps as much as 6 mi (9.3 km) wide, would have had a catastrophic effect upon the world's climate. Huge amounts ofdust and debris would have been thrown into the atmosphere, reducing the amount of sunlight reaching the surface. Heat from the blast may also have caused large forest fires, which would have added smoke and ash to the air. Lack of sunlightwould kill off plants and have a dominolike effect on other organisms in the food chain, including the dinosaurs.
It is possible that the reason for the dinosaurs' extinction may have been a combination of both theories. The dinosaurs may have been gradually declining, for whatever reason. The impact ofa large object from space merely delivered the coup de grace.
The fact that dinosaurs became extinct has been cited as proof of their inferiority and that they were evolutionary failures. However, these animals flourished for 150 million years. By comparison, the earliest ancestors of humanity appeared only about three million years ago. Humans have a long way to go before they can claim the same sort ofsuccess as the dinosaurs.