Bill Bryson - A short history of nearly everything

The Earth Sciences compound was an odd combination of things-part offices, part labs, part machine shed. “We used to build everything here,” Bennett said. “We even had our own glassblower, but he’s retired. But we still have two full-time rock crushers.” She caught my look of mild surprise. “We get through a lot of rocks. And they have to be very carefully prepared. You have to make sure there is no contamination from previous samples-no dust or anything. It’s quite a meticulous process.” She showed me the rock-crushing machines, which were indeed pristine, though the rock crushers had apparently gone for coffee. Beside the machines were large boxes containing rocks of all shapes and sizes. They do indeed get through a lot of rocks at the ANU.

Back in Bennett’s office after our tour, I noticed hanging on her wall a poster giving an artist’s colorfully imaginative interpretation of Earth as it might have looked 3.5 billion years ago, just when life was getting going, in the ancient period known to earth science as the Archaean. The poster showed an alien landscape of huge, very active volcanoes, and a steamy, copper-colored sea beneath a harsh red sky. Stromatolites, a kind of bacterial rock, filled the shallows in the foreground. It didn’t look like a very promising place to create and nurture life. I asked her if the painting was accurate.

“Well, one school of thought says it was actually cool then because the sun was much weaker.” (I later learned that biologists, when they are feeling jocose, refer to this as the “Chinese restaurant problem”-because we had a dim sun.) “Without an atmosphere ultraviolet rays from the sun, even from a weak sun, would have tended to break apart any incipient bonds made by molecules. And yet right there”-she tapped the stromatolites-“you have organisms almost at the surface. It’s a puzzle.”

“So we don’t know what the world was like back then?”

“Mmmm,” she agreed thoughtfully.

“Either way it doesn’t seem very conducive to life.”

She nodded amiably. “But there must have been something that suited life. Otherwise we wouldn’t be here.”

It certainly wouldn’t have suited us. If you were to step from a time machine into that ancient Archaean world, you would very swiftly scamper back inside, for there was no more oxygen to breathe on Earth back then than there is on Mars today. It was also full of noxious vapors from hydrochloric and sulfuric acids powerful enough to eat through clothing and blister skin. Nor would it have provided the clean and glowing vistas depicted in the poster in Victoria Bennett’s office. The chemical stew that was the atmosphere then would have allowed little sunlight to reach the Earth’s surface. What little you could see would be illumined only briefly by bright and frequent lightning flashes. In short, it was Earth, but an Earth we wouldn’t recognize as our own.

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 O 2to 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 O 2. 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. This was most unexpected-so unexpected, in fact, that it was some years before scientists realized quite what they had found. Today, however, Shark Bay is a tourist attraction-or at least as much of a tourist attraction as a place hundreds of miles from anywhere much and dozens of miles from anywhere at all can ever be. Boardwalks have been built out into the bay so that visitors can stroll over the water to get a good look at the stromatolites, quietly respiring just beneath the surface. They are lusterless and gray and look, as I recorded in an earlier book, like very large cow-pats. But it is a curiously giddying moment to find yourself staring at living remnants of Earth as it was 3.5 billion years ago. As Richard Fortey has put it: “This is truly time traveling, and if the world were attuned to its real wonders this sight would be as well-known as the pyramids of Giza.” Although you’d never guess it, these dull rocks swarm with life, with an estimated (well, obviously estimated) three billion individual organisms on every square yard of rock. Sometimes when you look carefully you can see tiny strings of bubbles rising to the surface as they give up their oxygen. In two billion years such tiny exertions raised the level of oxygen in Earth’s atmosphere to 20 percent, preparing the way for the next, more complex chapter in life’s history.