BEIJING (Xinhuanet) -- Dinosaurs didn't know -- they were too busy evolving -- but some scientists now say the reason they went extinct is because two big rocks ran into each other out in space about 160 million years ago.

The 10km-wide (6.2 miles) asteroid that crashed into the Yucatan peninsula of Mexico 65 million years ago, causing the end of the dinosaurs, was almost certainly created by a head-on collision between two even-larger space rocks about 160 million years ago, scientists have discovered.
An asteroid 170km (105 miles) in diameter was ripped into thousands of smaller fragments, known as the Baptistina family, when it was struck by another, 60km wide, at a speed of 3km per second.
It peppered the Earth, the moon, Venus and Mars with debris that has scarred their surfaces to this day, a study led by Dr. William Bottke, of the Southwest Research Institute in Boulder, Colorado, has found.
One of the larger pieces wandered through space for 95 million years before hitting Earth 65 million years ago, forming the Chicxulub crater and triggering the Cretaceous/tertiary mass extinction event in which the dinosaurs died out.
The study, which is published in the journal Nature, sought to explain why the cratering rate on the Earth and the moon appears to have doubled over the past 100 million to 200 million years.
Bottke's team traced the increase to an event 160 million years ago, in which the asteroid family was born. It takes its name from the largest member that still exists today, a 40km asteroid known as Baptistina.
Bottke said chemical analysis showed the asteroid shower was the most likely source, a 90 percent probability, of the impact that produced the mass extinction of dinosaurs 65 million years ago. The other possibilities -- that it was a comet from the edges of the solar system, or a preexisting asteroid -- had only a 10 per cent chance of being correct