James Crampton, CC BY-SA Scientists are increasingly worried we may be witnessing the start of the “sixth mass extinction” – the first to be caused by human activities. Five earlier mass extinctions, caused by dramatic but natural events, have devastated life on Earth during the past half billion years. The most famous, though not the largest, was triggered by an asteroid impact 66 million years ago and caused the demise of the dinosaurs – with the exception of a feathered, meat-eating group t