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and aspirations and desires。 we want to take constantadvantage of all the intoxicating existence we’ve been endowed with。 but what’s life to alichen？ yet its impulse to exist； to be； is every bit as strong as ours—arguably even stronger。
if i were told that i had to spend decades being a furry growth on a rock in the woods； ibelieve i would lose the will to go on。 lichens don’t。 like virtually all living things； they willsuffer any hardship； endure any insult； for a moment’s additional existence。 life； in short； justwants to be。 but—and here’s an interesting point—for the most part it doesn’t want to bemuch。
this is perhaps a little odd because life has had plenty of time to develop ambitions。 if youimagine the 4；500…billion…odd years of earth’s history pressed into a normal earthly day；then life begins very early； about 4a。m。； with the rise of the first simple； single…celled organisms； but then advances no further for the next sixteen hours。 not until almost 8：30 inthe evening； with the day five…sixths over； has earth anything to show the universe but arestless skin of microbes。 then； finally； the first sea plants appear； followed twenty minuteslater by the first jellyfish and the enigmatic ediacaran fauna first seen by reginald sprigg inaustralia。 at 9：04p。m。 trilobites swim onto the scene； followed more or less immediately bythe shapely creatures of the burgess shale。 just before 10p。m。 plants begin to pop up on theland。 soon after； with less than two hours left in the day； the first land creatures follow。
thanks to ten minutes or so of balmy weather； by 10：24 the earth is covered in the greatcarboniferous forests whose residues give us all our coal； and the first winged insects areevident。 dinosaurs plod onto the scene just before 11p。m。 and hold sway for about three…quarters of an hour。 at twenty…one minutes to midnight they vanish and the age of mammalsbegins。 humans emerge one minute and seventeen seconds before midnight。 the whole of ourrecorded history； on this scale； would be no more than a few seconds； a single human lifetimebarely an instant。 throughout this greatly speeded…up day continents slide about and bangtogether at a clip that seems positively reckless。 mountains rise and melt away； ocean basinse and go； ice sheets advance and withdraw。 and throughout the whole； about three timesevery minute； somewhere on the planet there is a flashbulb pop of light marking the impact ofa manson…sized meteor or one even larger。 it’s a wonder that anything at all can survive insuch a pummeled and unsettled environment。 in fact； not many things do for long。
perhaps an even more effective way of grasping our extreme recentness as a part of this4。5…billion…year…old picture is to stretch your arms to their fullest extent and imagine thatwidth as the entire history of the earth。 on this scale； according to john mcphee in basin andrange； the distance from the fingertips of one hand to the wrist of the other is precambrian。
all of plex life is in one hand； “and in a single stroke with a medium…grained nail file youcould eradicate human history。”
fortunately； that moment hasn’t happened； but the chances are good that it will。 i don’twish to interject a note of gloom just at this point； but the fact is that there is one otherextremely pertinent quality about life on earth： it goes extinct。 quite regularly。 for all thetrouble they take to assemble and preserve themselves； species crumple and die remarkablyroutinely。 and the more plex they get； the more quickly they appear to go extinct。 whichis perhaps one reason why so much of life isn’t terribly ambitious。
so anytime life does something bold it is quite an event； and few occasions were moreeventful than when life moved on to the next stage in our narrative and came out of the sea。
land was a formidable environment： hot； dry； bathed in intense ultraviolet radiation；lacking the buoyancy that makes movement in water paratively effortless。 to live onland； creatures had to undergo wholesale revisions of their anatomies。 hold a fish at each endand it sags in the middle； its backbone too weak to support it。 to survive out of water； marinecreatures needed to e up with new load…bearing internal architecture—not the sort ofadjustment that happens overnight。 above all and most obviously； any land creature wouldhave to develop a way to take its oxygen directly from the air rather than filter it from water。
these were not trivial challenges to overe。 on the other hand； there was a powerfulincentive to leave the water： it was getting dangerous down there。 the slow fusion of thecontinents into a single landmass； pangaea； meant there was much； much less coastline thanformerly and thus much less coastal habitat。 so petition was fierce。 there was also an omnivorous and unsettling new type of predator on the scene； one so perfectly designed forattack that it has scarcely changed in all the long eons since its emergence： the shark。 neverwould there be a more propitious time to find an alternative environment to water。
plants began the process of land colonization about 450 million years ago； acpanied ofnecessity by tiny mites and other organisms that they needed to break down and recycle deadorganic matter on their behalf。 larger animals took a little longer to emerge； but by about 400million years ago they were venturing out of the water； too。 popular illustrations haveencouraged us to envision the first venturesome land dwellers as a kind of ambitious fish—something like the modern mudskipper； which can hop from puddle to puddle duringdroughts—or even as a fully formed amphibian。 in fact； the first visible mobile residents ondry land were probably much more like modern wood lice； sometimes also known as pillbugsor sow bugs。 these are the little bugs （crustaceans； in fact） that are monly thrown intoconfusion when you upturn a rock or log。
for those that learned to breathe oxygen from the air； times were good。 oxygen levels inthe devonian and carboniferous periods； when terrestrial life first bloomed； were as high as35 percent （as opposed to nearer 20 percent now）。 this allowed animals to grow remarkablylarge remarkably quickly。
and how； you may reasonably wonder； can scientists know what oxygen levels were likehundreds of millions of years ago？ the answer lies in a slightly obscure but ingenious fieldknown as isotope geochemistry。 the long…ago seas of the carboniferous and devonianswarmed with tiny plankton that wrapped themselves inside tiny protective shells。 then； asnow； the plankton created their shells by drawing oxygen from the atmosphere and biningit with other elements （carbon especially） to form durable pounds such as calciumcarbonate。 it’s the same chemical trick that goes on in （and is discussed elsewhere in relationto） the long…term carbon cycle—a process that doesn’t make for terribly exciting narrative butis vital for creating a livable planet。
eventually in this process all the tiny organisms die and drift to the bottom of the sea；where they are slowly pressed into limestone。 among the tiny atomic structures theplankton take to the grave with them are two very stable isotopes—oxygen…16 and oxygen…18。
（if you have forgotten wh