2.7 billion years ago, oxygen loving cells emerge! The first global environmental crisis is averted by the creativity of these tiny cellular creatures who invent a use for oxygen as they breathe it in. By doubling their outer walls cellular defences against oxygen were strengthened. By replacing the magnesium atom at the core of chlorophyll with an atom of iron, the haem molecule emerges which will one day become the haemoglobin protein that transports oxygen through the bloodstream in all animal bodies. Oxygen levels continue to rise until they reach near present-day levels.