A short history of nearly everything - Bill Bryson [147]
Anniversaries were few and far between in the Archaean world. For two billion years bacterial organisms were the only forms of life. They lived, they reproduced, they swarmed, but they didn't show any particular inclination to move on to another, more challenging level of existence. At some point in the first billion years of life, cyanobacteria, or blue-green algae, learned to tap into a freely available resource—the hydrogen that exists in spectacular abundance in water. They absorbed water molecules, supped on the hydrogen, and released the oxygen as waste, and in so doing invented photosynthesis. As Margulis and Sagan note, photosynthesis is “undoubtedly the most important single metabolic innovation in the history of life on the planet”—and it was invented not by plants but by bacteria.
As cyanobacteria proliferated the world began to fill with O2 to the consternation of those organisms that found it poisonous—which in those days was all of them. In an anaerobic (or a non-oxygen-using) world, oxygen is extremely poisonous. Our white cells actually use oxygen to kill invading bacteria. That oxygen is fundamentally toxic often comes as a surprise to those of us who find it so convivial to our well-being, but that is only because we have evolved to exploit it. To other things it is a terror. It is what turns butter rancid and makes iron rust. Even we can tolerate it only up to a point. The oxygen level in our cells is only about a tenth the level found in the atmosphere.
The new oxygen-using organisms had two advantages. Oxygen was a more efficient way to produce energy, and it vanquished competitor organisms. Some retreated into the oozy, anaerobic world of bogs and lake bottoms. Others did likewise but then later (much later) migrated to the digestive tracts of beings like you and me. Quite a number of these primeval entities are alive inside your body right now, helping to digest your food, but abhorring even the tiniest hint of O2. Untold numbers of others failed to adapt and died.
The cyanobacteria were a runaway success. At first, the extra oxygen they produced didn't accumulate in the atmosphere, but combined with iron to form ferric oxides, which sank to the bottom of primitive seas. For millions of years, the world literally rusted—a phenomenon vividly recorded in the banded iron deposits that provide so much of the world's iron ore today. For many tens of millions of years not a great deal more than this happened. If you went back to that early Proterozoic world you wouldn't find many signs of promise for Earth's future life. Perhaps here and there in sheltered pools you'd encounter a film of living scum or a coating of glossy greens and browns on shoreline rocks, but otherwise life remained invisible.
But about 3.5 billion years ago something more emphatic became apparent. Wherever the seas were shallow, visible structures began to appear. As they went through their chemical routines, the cyanobacteria became very slightly tacky, and that tackiness trapped microparticles of dust and sand, which became bound together to form slightly weird but solid structures—the stromatolites that were featured in the shallows of the poster on Victoria Bennett's office wall. Stromatolites came in various shapes and sizes. Sometimes they looked like enormous cauliflowers, sometimes like fluffy mattresses (stromatolite comes from the Greek for “mattress”), sometimes they came in the form of columns, rising tens of meters above the surface of the water—sometimes as high as a hundred meters. In all their manifestations, they were a kind of living rock, and they represented the world's first cooperative venture, with some varieties of primitive organism living just at the surface and others living just underneath, each taking advantage of conditions created by the other. The world had its first ecosystem.
For many years, scientists knew about stromatolites from fossil formations, but in 1961 they got a real surprise with the discovery of a community of living stromatolites at Shark Bay on the remote northwest coast of Australia.