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uge； 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鈥檛 look like a very promising place to create andnurture life。 i asked her if the painting was accurate。
鈥渨ell； one school of thought says it was actually cool then because the sun was muchweaker。鈥潯。╥ later learned that biologists； when they are feeling jocose； refer to this as the鈥渃hinese restaurant problem鈥濃�攂ecause we had a dim sun。） 鈥渨ithout 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鈥濃�攕he tapped the stromatolites鈥斺�測ouhave organisms almost at the surface。 it鈥檚 a puzzle。鈥�
鈥渟o we don鈥檛 know what the world was like back then？鈥�
鈥渕mmm；鈥潯he agreed thoughtfully。
鈥渆ither way it doesn鈥檛 seem very conducive to life。鈥�
she nodded amiably。 鈥渂ut there must have been something that suited life。 otherwise wewouldn鈥檛 be here。鈥�
it certainly wouldn鈥檛 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鈥檚 office。 the chemical stew that was the atmosphere then would haveallowed little sunlight to reach the earth鈥檚 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鈥檛 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鈥檛 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鈥攖he 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 鈥渦ndoubtedly the most important single metabolic innovation in the historyof life on the planet鈥濃�攁nd 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鈥攚hich 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鈥檛accumulate 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鈥攁 phenomenonvividly recorded in the banded iron deposits that provide so much of the world鈥檚 iron oretoday。 for many tens of millions of years not a great deal more than this happened。 if youwent back to that early proterozoic world you wouldn鈥檛 find many signs of promise for earth鈥檚 future life。 perhaps here and there in sheltered pools you鈥檇 encounter a film of livingscum or a coating of glossy greens and browns on shoreline rocks； but otherwise life remainedinvisible。
but about 3。5 billion years ago something more emphatic became apparent。 wherever theseas were shallow； visible structures began to appear。 as they went through their chemicalroutines； the cyanobacteria became very slightly tacky； and that tackiness trappedmicroparticles of dust and sand； which became bound together to form slightly weird but solidstructures鈥攖he stromatolites that were featured in the shallows of the poster on victoriabennett鈥檚 office wall。 stromatolites came in various shapes and sizes。 sometimes they lookedlike enormous cauliflowers； sometimes like fluffy mattresses （stromatolite es from thegreek for 鈥渕attress鈥潱弧ometimes they came in the form of columns； rising tens of metersabove the surface of the water鈥攕ometimes as high as a hundred meters。 in all theirmanifestations； they were a kind of living rock； and they represented the world鈥檚 firstcooperative venture； with some varieties of primitive organism living just at the surface andothers living just underneath； each taking advantage of conditions created by the other。 theworld had its first ecosystem。
for many years； scientists knew about stromatolites from fossil formations； but in 1961they got a real surprise with the discovery of a munity of living stromatolites at sharkbay on the remote northwest coast of australia。 this was most unexpected鈥攕o unexpected；in fact； that it was some years before scientists realized quite what they had found。 today；however； shark bay is a tourist attraction鈥攐r at least as much of a tourist attraction as a placehundreds of miles from anywhere much and dozens of miles from anywhere at all