按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
tens of millions of years not a great deal more than this happened。 if youwent 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 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—the stromatolites that were featured in the shallows of the poster on victoriabennett’s office wall。 stromatolites came in various shapes and sizes。 sometimes they lookedlike enormous cauliflowers； sometimes like fluffy mattresses （stromatolite es from thegreek for “mattress”）； sometimes they came in the form of columns； rising tens of metersabove the surface of the water—sometimes as high as a hundred meters。 in all theirmanifestations； they were a kind of living rock； and they represented the world’s 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—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 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： “this 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。”
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 asthey give up their oxygen。 in two billion years such tiny exertions raised the level of oxygenin earth’s atmosphere to 20 percent； preparing the way for the next； more plex chapter inlife’s 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。 “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—orga