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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。 “animals could not summonup the energy to work；” as fortey has put it。 it took about two billion years； roughly 40percent of earth’s 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—one with a nucleus and other little bodies collectively calledorganelles （from a greek word meaning “little tools”）。 the 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—you could pack a billion into the spaceoccupied by a grain of sand—but also very hungry。 almost every nutriment you absorb goesto feeding them。
we couldn’t 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’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 contrastedwith the old type； which is known as a prokaryote （“prenucleated”）； and 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 “bags ofchemicals；” in the words of the geologist stephen drury。 eukaryotes were bigger—eventuallyas much as ten thousand times bigger—than their simpler cousins； and carried as much as athousand times more dna。 gradually a system evolved in which life was dominated by twotypes of form—organisms that expel oxygen （like plants） and those that take it in （you andme）。
single…celled eukaryotes were once called protozoa （“pre…animals”）； but that term isincreasingly disdained。 today the mon term for them is protists 。 pared with thebacteria 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 400million bits of genetic information in its dna—enough； as carl sagan noted； to fill eightybooks of five hundred pages。
eventually the eukaryotes learned an even more singular trick。 it took a long time—abillion years or so—but it was a good one when they mastered it。 they learned to formtogether into plex multicellular beings。 thanks to this innovation； big； plicated；visible entities like us were possible。 planet earth was ready to move on to its next ambitiousphase。
but before we get too excited about that； it is worth remembering that the world； as we areabout to see； still belongs to the very small。
……
20SMALL WORLD
it’s probably not a good idea to take too personal an interest in your microbes。 louispasteur； the great french chemist and bacteriologist； became so preoccupied with them that hetook to peering critically at every dish placed before him with a magnifying glass； a habit thatpresumably did not win him many repeat invitations to dinner。
in fact； there is no point in trying to hide from your bacteria； for they are on and around youalways； in numbers you can’t conceive。 if you are in good health and averagely diligent abouthygiene； you will have a herd of about one trillion bacteria grazing on your fleshy plains—about a hundred thousand of them on every square centimeter of skin。 they are there to dineoff the ten billion or so flakes of skin you shed every day； plus all the tasty oils and fortifyingminerals that seep out from every pore and fissure。 you are for them the ultimate food court；with the convenience of warmth and constant mobility thrown in。 by way of thanks； they giveyou b。o。
and those are just the bacteria that inhabit your skin。 there are trillions more tucked awayin your gut and nasal passages； clinging to your hair and eyelashes； swimming over thesurface of your eyes； drilling through the enamel of your teeth。 your digestive system alone ishost to more than a hundred trillion microbes； of at least four hundred types。 some deal withsugars； some with starches； some attack other bacteria。 a surprising number； like theubiquitous intestinal spirochetes； have no detectable function at all。 they just seem to like tobe with you。 every human body consists of about 10 quadrillion cells； but about 100quadrillion bacterial cells。 they are； in short； a big part of us。 from the bacteria’s point ofview； of course； we are a rather small part of them。
because we humans are big and clever enough to produce and utilize antibiotics anddisinfectants； it is easy to convince ourselves that we have banished bacteria to the fringes ofexistence。 don’t you believe it。 bacteria may not build cities or have interesting social lives；but they will be here when the sun explodes。 this is their planet； and we are on it onlybecause they allow us to be。
bacteria； never forget； got along for billions of years without us。 we couldn’t survive a daywithout them。 they process our wastes and make them usable again； without their diligentmunching nothing would rot。 they purify our water and keep our soils productive。 bacteriasynthesize vitamins in our gut； convert the things we eat into useful sugars andpolysaccharides； and go to war on alien microbes that slip down our gullet。
we depend totally on bacteria to pluck nitrogen from the air and convert it into usefulnucleotides and amino acids for us。 it is a prodigious and gratifying feat。 as margulis andsagan note； to do the same thing industrially （as when making fertilizers） manufacturers mustheat the source materials to 500 degrees centigrade and squeeze them to three hundred timesnormal pressures