Cosmic accidents: Killer asteroid with a silver lining – life – 01 October 2010 – New Scientist

via Cosmic accidents: Killer asteroid with a silver lining – life – 01 October 2010 – New Scientist.

Cosmic accidents: Killer asteroid with a silver lining

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 the species went extinct – but those that survived were small, fast-breeding and versatile, and could scavenge the abundant detritus created by the impact. They were able to burrow or hide to escape the fires and acid rain. They often lived in or around freshwater ecosystems, which are fed largely by dead organic matter and so were more resilient in the face of catastrophe than oceans and dry land.

These survivors went on to inherit the Earth. As the biosphere gradually recovered, mammals filled the niches left vacant by the dinosaurs, and eventually those of the marine reptiles too. The fossil record suggests this happened in a burst of evolutionary creativity between 65 and 55 million years ago. Some “molecular clock” studies, which compare the genomes of related living species to reconstruct their evolutionary tree, paint a slightly different picture, implying that mammalian evolution didn’t gear up until more than 10 million years after the impact.

Either way, one lineage that makes its debut is ours, the primates. Reason enough to say that if that asteroid had not been there and then, we would not be here and now.