If you are into very old rocks, and Bennett indubitably is, the ANU has long been a prime place to be. This is largely thanks to the ingenuity of a man named Bill Compston, who is now retired but in the 1970s built the world’s first Sensitive High Resolution Ion Micro Probe—or SHRIMP, as it is more affectionately known from its initial letters. This is a machine that measures the decay rate of uranium in tiny minerals called zircons. Zircons appear in most rocks apart from basalts and are extremely durable, surviving every natural process but subduction. Most of the Earth’s crust has been slipped back into the oven at some point, but just occasionally—in Western Australia and Greenland, for example—geologists have found outcrops of rocks that have remained always at the surface. Compston’s machine allowed such rocks to be dated with unparalleled precision. The prototype SHRIMP was built and machined in the Earth Science department’s own workshops, and looked like something that had been built from spare parts on a budget, but it worked great. On its first formal test, in 1982, it dated the oldest thing ever found—a 4.3-billion-year-old rock from Western Australia.

“It caused quite a stir at the time,” Bennett told me, “to find something so important so quickly with brand-new technology.”

She took me down the hall to see the current model, SHRIMP II. It was a big heavy piece of stainless-steel apparatus, perhaps twelve feet long and five feet high, and as solidly built as a deep-sea probe. At a console in front of it, keeping an eye on ever-changing strings of figures on a screen, was a man named Bob from Canterbury University in New Zealand. He had been there since 4A.M., he told me. SHRIMP II runs twenty-four hours a day; there’s that many rocks to date. It was just after 9A.M.and Bob had the machine till noon. Ask a pair of geochemists how something like this works, and they will start talking about isotopic abundances and ionization levels with an enthusiasm that is more endearing than fathomable. The upshot of it, however, was that the machine, by bombarding a sample of rock with streams of charged atoms, is able to detect subtle differences in the amounts of lead and uranium in the zircon samples, by which means the age of rocks can be accurately adduced. Bob told me that it takes about seventeen minutes to read one zircon and it is necessary to read dozens from each rock to make the data reliable. In practice, the process seemed to involve about the same level of scattered activity, and about as much stimulation, as a trip to a laundromat. Bob seemed very happy, however; but then people from New Zealand very generally do.

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 alot 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 O2to 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 (stromatolitecomes 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.

It has been suggested that the cyanobacteria at Shark Bay are perhaps the slowest-evolving organisms on Earth, and certainly now they are among the rarest. Having prepared the way for more complex life forms, they were then grazed out of existence nearly everywhere by the very organisms whose existence they had made possible. (They exist at Shark Bay because the waters are too saline for the creatures that would normally feast on them.)

One reason life took so long to grow complex was that the world had to wait until the simpler organisms had oxygenated the atmosphere sufficiently. “Animals could not summon up the energy to work,” as Fortey has put it. It took about two billion years, roughly 40 percent of Earth’s history, for oxygen levels to reach more or less modern levels of concentration in the atmosphere. But once the stage was set, and apparently quite suddenly, an entirely new type of cell arose—one with a nucleus and other little bodies collectively calledorganelles (from a Greek word meaning “little tools”). The process is thought to have started when some blundering or adventuresome bacterium either invaded or was captured by some other bacterium and it turned out that this suited them both. The captive bacterium became, it is thought, a mitochondrion. This mitochondrial invasion (or endosymbiotic event, as biologists like to term it) made complex life possible. (In plants a similar invasion produced chloroplasts, which enable plants to photosynthesize.)

Mitochondria manipulate oxygen in a way that liberates energy from foodstuffs. Without this niftily facilitating trick, life on Earth today would be nothing more than a sludge of simple microbes. Mitochondria are very tiny—you could pack a billion into the space occupied by a grain of sand—but also very hungry. Almost every nutriment you absorb goes to feeding them.

We couldn’t live for two minutes without them, yet even after a billion years mitochondria behave as if they think things might not work out between us. They maintain their own DNA. They reproduce at a different time from their host cell. They look like bacteria, divide like bacteria, and sometimes respond to antibiotics in the way bacteria do. In short, they keep their bags packed. They don’t even speak the same genetic language as the cell in which they live. It is like having a stranger in your house, but one who has been there for a billion years.

The new type of cell is known as a eukaryote (meaning “truly nucleated”), as contrasted with the old type, which is known as a prokaryote (“prenucleated”), and it seems to have arrived suddenly in the fossil record. The oldest eukaryotes yet known, called Grypania, were discovered in iron sediments in Michigan in 1992. Such fossils have been found just once, and then no more are known for 500 million years.

Compared with the new eukaryotes the old prokaryotes were little more than “bags of chemicals,” in the words of the geologist Stephen Drury. Eukaryotes were bigger—eventually as much as ten thousand times bigger—than their simpler cousins, and carried as much as a thousand times more DNA. Gradually a system evolved in which life was dominated by two types of form—organisms that expel oxygen (like plants) and those that take it in (you and me).

Single-celled eukaryotes were once calledprotozoa (“pre-animals”), but that term is increasingly disdained. Today the common term for them isprotists . Compared with the bacteria that had gone before, these new protists were wonders of design and sophistication. The simple amoeba, just one cell big and without any ambitions but to exist, contains 400 million bits of genetic information in its DNA—enough, as Carl Sagan noted, to fill eighty books of five hundred pages.

Eventually the eukaryotes learned an even more singular trick. It took a long time—a billion years or so—but it was a good one when they mastered it. They learned to form together into complex multicellular beings. Thanks to this innovation, big, complicated, visible entities like us were possible. Planet Earth was ready to move on to its next ambitious phase.

But before we get too excited about that, it is worth remembering that the world, as we are about to see, still belongs to the very small.