Survivors: The Animals and Plants that Time has Left Behind. Richard Fortey

      6. Endosymbiotic/endosymbiont theory: a complex cell ‘in the making’ engulfs a formerly free-living prokaryote, which is then retained as a symbiont rather than being digested. This process happened several times, thereby introducing many more possibilities for life.

      I should mention that none of these distinguished scientists conforms to the common preconceptions of the ‘geeky’ white-coated specialist. Bill Schopf is a bon viveur and raconteur, with a very persuasive laugh, and a relentless drive for discovery. Andrew Knoll is the kind of American who makes most of us poor Europeans feel as if we were only given half a ration of energy at birth. He has projects spanning most of the world, and a stable of exceptional students to take the work forward. He manages everything with a kind of urbane good humour and insouciance that defuses any possible resentment at his omniscience. Lynn Margulis is unique. I know of no other professor who would, or indeed could, quote the poet Emily Dickinson at length in a supermarket. As a long-term maverick she is always on the look out for ideas that will provoke and encourage new ideas (and completely undeterred that some of them may well prove to be wrong). Her dress style is equally distinctive, featuring embroidered waistcoats and pleated skirts, as if she were about to take part in a folk festival. She has an intuitive grasp of the important collaborations that makes the world work; not just the ubiquitous and versatile bacteria, but also chemistry, and geology, and politics.

      This account of the geological importance of stromatolites and their discoverers has entailed a distraction from the little bubbles that provide the nub of this chapter. Recall the tiny gas beads rising from the top of the stromatolite in its tank, arising from a living biofilm breathing out oxygen. Now combine that scene with the picture I have attempted to paint of the fledgling earth in the Archaean and Proterozoic, where shallow seas and lagoons were covered with stromatolites, some of them gargantuan by recent standards. Imagine thousands and thousands of dimpled miles, exhaling oxygen by day in a thousand billion tiny bubbles, stimulated by the primeval sunshine. Even the slimy surface of the threads had a part to play in mitigating the effects of harmful ultra-violet radiation; we should all be grateful to slime. Now imagine this process continuing for billions of years, six times longer than the history of the velvet worm. The effect was to change the atmosphere, bubble by bubble. The early earth had little or no free oxygen; the ‘blue greens’ changed the air itself. Animals need to breathe oxygen to power their life functions. They could not exist before the slow, relentless preparation of the atmosphere effected by lowlier organisms on the tree of life: no gills, no lungs, no blood blue or red. If some malevolent God were instantly to reverse the work of the stromatolites we should all be gasping on the ground like beached trout within minutes. So we are, in a sense, the children of the sticky mounds.