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had the machine till noon。 ask a pair ofgeochemists how something like this works； and they will start talking about isotopicabundances 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 withstreams of charged atoms； is able to detect subtle differences in the amounts of lead anduranium 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 toread dozens from each rock to make the data reliable。 in practice； the process seemed toinvolve about the same level of scattered activity； and about as much stimulation； as a trip to alaundromat。 bob seemed very happy； however； but then people from new zealand verygenerally do。
the earth sciences pound was an odd bination of things—part offices； part labs；part machine shed。 “we used to build everything here；” bennett said。 “we even had our ownglassblower； but he’s retired。 but we still have two full…time rock crushers。” she caught mylook of mild surprise。 “we get through a lot of rocks。 and they have to be very carefullyprepared。 you have to make sure there is no contamination from previous samples—no dustor 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。 besidethe machines were large boxes containing rocks of all shapes and sizes。 they do indeed getthrough a lot of rocks at the anu。
back in bennett’s office after our tour； i noticed hanging on her wall a poster giving anartist’s colorfully imaginative interpretation of earth as it might have looked 3。5 billion yearsago； just when life was getting going； in the ancient period known to earth science as thearchaean。 the poster showed an alien landscape of huge； very active volcanoes； and asteamy； 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 andnurture life。 i asked her if the painting was accurate。
“well； one school of thought says it was actually cool then because the sun was muchweaker。” （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 atmosphereultraviolet rays from the sun； even from a weak sun； would have tended to break apart anyincipient bonds made by molecules。 and yet right there”—she tapped the stromatolites—“youhave 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 wewouldn’t be here。”
it certainly wouldn’t have suited us。 if you were to step from a time machine into thatancient archaean world； you would very swiftly scamper back inside； for there was no moreoxygen to breathe on earth back then than there is on mars today。 it was also full of noxiousvapors from hydrochloric and sulfuric acids powerful enough to eat through clothing andblister skin。 nor would it have provided the clean and glowing vistas depicted in the poster invictoria bennett’s office。 the chemical stew that was the atmosphere then would haveallowed little sunlight to reach the earth’s surface。 what little you could see would beillumined only briefly by bright and frequent lightning flashes。 in short； it was earth； but anearth we wouldn’t recognize as our own。
anniversaries were few and far between in the archaean world。 for two billion yearsbacterial 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 levelof 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 spectacularabundance in water。 they absorbed water molecules； supped on the hydrogen； and releasedthe 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 historyof 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 thoseorganisms that found it poisonous—which in those days was all of them。 in an anaerobic （or anon…oxygen…using） world； oxygen is extremely poisonous。 our white cells actually useoxygen to kill invading bacteria。 that oxygen is fundamentally toxic often es as a surpriseto those of us who find it so convivial to our well…being； but that is only because we haveevolved to exploit it。 to other things it is a terror。 it is what turns butter rancid and makes ironrust。 even we can tolerate it only up to a point。 the oxygen level in our cells is only about atenth the level found in the atmosphere。
the new oxygen…using organisms had two advantages。 oxygen was a more efficient way toproduce energy； and it vanquished petitor 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 primevalentities are alive inside your body right now； helping to digest your food； but abhorring eventhe 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’taccumulate in the atmosphere； but bined with iron to form ferric oxides； which sank to thebottom of primitive seas。 for millions of years； the world literally rusted—a phenomenonvividly recorded in the banded iron deposits that provide so much of the world’s iron oretoday。 for many tens of millions of years not a great deal more than this happened。 if youwent back to that early pr