Overview: Chris Packham tells the miraculous story of how plant life turned Earth from a barren rock into a vibrant green world - a four-billion year saga of extraordinary highs and lows that almost wiped out all life on the planet. Beginning as a water world without land masses, a giant asteroid bombardment triggered Earth's plate tectonics, leading to huge fungi, soil and giant swamp forests. But the early plants locked so much carbon dioxide that global temperatures suddenly plunged.