It is very hard to know why were dinosaurs alive. The exact origin and timing of the evolution of dinosaurs is still under research. Scientists are still working on that.

The question of why the dinosaurs died or became extinct has puzzled scientists for a long time. We can never know for sure what killed the dinosaurs because we cannot go back in time to 65 million years ago when they disappeared.

In 1980 a famous scientist named Luis Alvarez discovered that when the dinosaurs first began to disappear, rocks and fossils from all around the planet had a thick layer of iridum dust. Iridum is a metal, like iron or aluminum, and is present in large amounts on asteroids. From this finding, Alvarez guessed that a huge asteroid hit the earth 65 million years ago and caused the dinosaurs to go extinct.

In the 1990's, paleontologists (scientists who study events that happened hundreds or millions of years ago) found the crater from this asteroid on the Yucatan peninsula in Mexico, and found rock fragments from this impact which had been blown as far away as Nebraska!

Judging from the amount of iridum dust and the size of the crater, Alvarez and other scientists guessed that huge amounts of dust and ash from the explosion were thrown into the sky and calculated that this blanket of soot would have blocked out the sun for many years If the sun disappeared for even a year there would be no plant growth, and eventually there would be nothing to eat. Alvarez also guessed that without the sun, the earth would have become very cold, and as most people believe that dinosaurs are like reptiles and cannot regulate their temperature, many of them would have died from the cold.

Lastly, the asteroid crater was found in a certain kind of rock, which when vaporized would have caused acid rain to fall for years. This acid would have poisoned drinking water and killed the dinosaur's food, if not the dinosaurs themselves.

Many scientists disagree with Alvarez. The biggest problem with his theory is that dinosaurs had been going extinct long before the asteroid impact occurred. What caused the earlier extinctions? Also, why did some types of animals (especially mammals) survive the extinction when so many others did not? Many paleontologists argue that the dinosaurs went extinct gradually, over a long period of time, for a lot of different reasons, while Alvarez and his supporters believe they went extinct suddenly, after the asteroid impact.