Paris: The extinction of the dinosaurs 65 million years ago can be traced to a collision between two monster rocks in the asteroid belt nearly 100 million years earlier, scientists report today.

The smash drove a giant sliver of rock into Earth's path, eventually causing the climate-changing impact that ended the reign of the dinosaurs and enabled the rise of mammals -- including, eventually, us. Other asteroid fragments smashed into the Moon, Venus and Mars, pocking their faces with mighty craters, the US and Czech researchers believe.

Mixing skills in time travel, jigsaw-making and carbon chemistry, the trio carried out a computer simulation of the jostling among orbital rubble left from the building of the Solar System. The sleuths were guided by an intriguing clue -- a large asteroid called (298) Baptistina, which shares the same orbital track as a group of smaller rocks.

Turning the clock back, the simulation found that the Baptistina bits not only fitted together, they were also remnants of a giant parent asteroid, around 170 kilometers across, that once cruised the innermost region of the asteroid belt. Around 160 million years ago -- the best bet in a range of 140-190 million years -- this behemoth was whacked by another giant some 60 kms across.