The rock that did for dinosaurs (Image: Joe Tucciarone/SPL)

Read more: Cosmic accidents: 10 lucky breaks for humanity

A 10-kilometre-wide rock did for the dinosaurs, but smashed open a window of opportunity for unimpressive little animals called mammals

The upping of atmospheric oxygen was followed by a frenzy of evolutionary innovation, during which most of the animal groups known today popped up. By 350 million years ago, the coal beds laid down in the carboniferous period speak of a world covered in lush greenery, too. Soon this verdant Earth became home to animals of a size never seen before: the dinosaurs. The age of the reptiles lasted for more than 160 million years. It took an extraterrestrial intervention to clear the way for a new world order.

Nothing like the late heavy bombardment has hit Earth in recent geological time, but every 100 million years or so something big wallops the planet. If it happened now we would be wiped out. Yet curiously, we probably owe our existence to the last such impact.

Around 65.5 million years ago an asteroid some 10 kilometres across slammed into the Yucatan peninsula in present-day Mexico. The release of carbon and sulphur-rich gases from the blasted rock layers precipitated a global catastrophe in which fires raged, the sky darkened, Earth cooled and acid rain showered down. Within months the dinosaurs were dead. So too were almost all other reptiles of sea and air, along with ammonites and most birds and land plants (Science, vol 327, p 1241).

For mammals, it was a different story. They didn’t exactly sail through – around half of …