icular 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 can ever be。
boardwalks have been built out into the bay so that visitors can stroll over the water to get agood look at the stromatolites； quietly respiring just beneath the surface。 they are lusterlessand gray and look； as i recorded in an earlier book； like very large cow…pats。 but it is acuriously giddying moment to find yourself staring at living remnants of earth as it was 3。5billion years ago。 as richard fortey has put it： 鈥渢his is truly time traveling； and if the worldwere attuned to its real wonders this sight would be as well…known as the pyramids of giza。鈥�
although you鈥檇 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 asthey give up their oxygen。 in two billion years such tiny exertions raised the level of oxygenin earth鈥檚 atmosphere to 20 percent； preparing the way for the next； more plex chapter inlife鈥檚 history。
it has been suggested that the cyanobacteria at shark bay are perhaps the slowest…evolvingorganisms on earth； and certainly now they are among the rarest。 having prepared the way formore plex life forms； they were then grazed out of existence nearly everywhere by thevery organisms whose existence they had made possible。 （they exist at shark bay becausethe waters are too saline for the creatures that would normally feast on them。）one reason life took so long to grow plex was that the world had to wait until thesimpler organisms had oxygenated the atmosphere sufficiently。 鈥渁nimals could not summonup the energy to work；鈥潯s fortey has put it。 it took about two billion years； roughly 40percent of earth鈥檚 history； for oxygen levels to reach more or less modern levels ofconcentration in the atmosphere。 but once the stage was set； and apparently quite suddenly； anentirely new type of cell arose鈥攐ne with a nucleus and other little bodies collectively calledorganelles （from a greek word meaning 鈥渓ittle tools鈥潱！he process is thought to have startedwhen 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； itis thought； a mitochondrion。 this mitochondrial invasion （or endosymbiotic event； asbiologists like to term it） made plex life possible。 （in plants a similar invasion producedchloroplasts； which enable plants to photosynthesize。）mitochondria manipulate oxygen in a way that liberates energy from foodstuffs。 withoutthis niftily facilitating trick； life on earth today would be nothing more than a sludge ofsimple microbes。 mitochondria are very tiny鈥攜ou could pack a billion into the spaceoccupied by a grain of sand鈥攂ut also very hungry。 almost every nutriment you absorb goesto feeding them。
we couldn鈥檛 live for two minutes without them； yet even after a billion years mitochondriabehave 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 likebacteria； and sometimes respond to antibiotics in the way bacteria do。 in short； they keep theirbags packed。 they don鈥檛 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 鈥渢ruly nucleated鈥潱弧s contrastedwith the old type； which is known as a prokaryote （鈥減renucleated鈥潱弧nd it seems to havearrived suddenly in the fossil record。 the oldest eukaryotes yet known； called grypania； werediscovered in iron sediments in michigan in 1992。 such fossils have been found just once； andthen no more are known for 500 million years。
pared with the new eukaryotes the old prokaryotes were little more than 鈥渂ags ofchemicals；鈥潯n the words of the geologist stephen drury。 eukaryotes were bigger鈥攅ventuallyas much as ten thous