"Life and the Evolution of Earth's Atmosphere." Earth

Life and the Evolution ofEarth’s AtmosphereStephen J. MojzsisWhat’s Life to a Geochemist?Thinking about it, there’s a subtle kind of poetry involved withits scientific definition. Life can be said to be a self-replicating,encapsulated, chemical system that undergoes Darwinianevolution. In other words, groups of related organisms haveevolved in response to changes in the environment throughnatural selection. This process has been in effect ever sincelife began from simple organic molecules in water, over fourStephen J. Mojzsis is an Assistant Professor in the Department of Geological Sciences at the University ofColorado, Boulder.

Stromatolites in Western Australia, today. Did ancientEarth look like this?billion years ago. So, the origin of life isprobably the origin of evolution. Life isresourceful and entrepreneurial. It takesadvantage and it changes the chemistry of itssurroundings. Life is a fantastically complexsystem, the emergence of which remains thegreatest mystery in science.Long-term changes in the composition of theatmosphere and oceans are intimately linked toboth the geophysical changes in the solid Earthitself and with the ongoing evolution of life. Theatmosphere and oceans first appeared about4.5 billion years ago, soon after the Earth andMoon completed their formational phase. Thiswas the time when gases escaping throughvolcanoes made an envelope of atmospherearound the young Earth, and a primitive crustsolidified and cooled to the point where liquidwater could condense. Water began poolinginto the first lakes, seas, and oceans. Theinteraction of water, heat, and rock set thestage for the origin of life.From the beginning, a number of factors haveaffected the makeup of the atmosphere tochange it from its initial state to what we havetoday. Several of these factors, such as platetectonics, weathering (which recycle rocks,water, and gases), and chemical changesinduced by the byproducts of life itself, areinternal to the planet. However, external factorssuch as the slowly but ever-increasingluminosity of the Sun over billions of years,gradual changes in the Earth’s orbit over manytens of thousands of years, and the rare butcatastrophic impacts of giant meteorites andcomets, have also played an important role.The atmosphere of our planet did not originallycontain all the free, breathable oxygen it doesnow. The first permanent atmosphere arosewhen gases that had been dissolved in themolten planet during its assembly from smallerbodies, called “planetesimals,” were released tothe surface by volcanism. That first, primitiveatmosphere was probably several times denserthan what we have now, and was dominatednot by oxygen, but by carbon dioxide—amajor greenhouse gas. Other gases, such asmolecular nitrogen, water vapor, and smallamounts of carbon monoxide, sulfur gases, andtrace quantities of methane, and hydrogen werealso present.Astrophysical computer models based on thestudy of young stars and of star-forming regionsin the galaxy strongly suggest that the Sun wasmuch dimmer when the first life emerged onEarth, over 4 billion years ago. A dimmer Sunwould have supplied less solar radiation towarm the early Earth. To keep the Earth fromstarting out as a frozen wasteland, with no hopefor beginning life to take hold, an atmospheric“greenhouse” must have kept the surface zonewarm enough to maintain water in liquid form.Liquid water is the prerequisite for life.Greenhouse gases in much higher abundancethan today, primarily water vapor, carbondioxide, and methane, would have formed athermal blanket over the surface of the earlyEarth that strongly absorbed outgoing thermalradiation. This leads to a significantly enhanced,warming “greenhouse effect” that offset thedimmer Sun. Without this very different,greenhouse gas-rich and oxygen-poor earlyatmosphere to begin with, life would havegotten a frozen start. (See the essay by CharlesF. Keller in Section Five, which discussesgreenhouse gases and global warming.)The composition of this early atmosphere, sodifferent from the one we have now, would bedeadly for most life that is not a primitivebacterium. Soon after the emergence of thefirst life more than 4 billion years ago, theactivities of organisms began to influence thecomposition of the atmosphere. As Earth’sbiosphere and atmosphere co-evolved over

illions of years, the product of photosynthesis,mainly, free oxygen, has come to dominate thechemistry of the atmosphere. Without thebyproducts of crucial biological processes suchas photosynthesis, there would be little or nofree oxygen for animals to breathe, nor enoughto form a protective ozone layer in the upperatmosphere. Ozone (O3) is formed by therecombination of oxygen (O2) by solar radiationin the upper atmosphere into the new moleculewith three oxygen atoms. This molecule screensharmful ultraviolet sunlight and prevents most ofit from reaching the surface. Without aneffective ozone layer in the upper atmosphere,more advanced life would not have been able toemerge from the seas to colonize the land. Wecan safely say that the vast amount of ozoneneeded to permit the colonization of landbecame possible only after significant freeoxygen accumulated from photosynthesisbeginning early in Earth’s history.Since little evidence from the Earth’s formationaltime has survived for scientists to study, it ispossible to only make a few basic conclusionsabout the nature of the first atmosphere and theconditions in which life originated. Most of whatis known about early planetary history comesfrom three sources: the sparse geologic recordremaining from more than 3.5 billion years ago,computer models of atmospheres changing withtime, and comparing Earth to its planetaryneighbors, Mars and Venus, which evolvedalong very different paths. Scientists have alsorecently started to study the sophisticatedbiochemical machinery of primitivephotosynthetic bacteria to learn more abouttheir early evolution. These primitive bacteriaare descended from the pioneeringmicroorganisms that forever changed thechemistry of the atmosphere from a primitiveunbreathable mix to oxygen-rich air.How Earth’s Atmosphere OriginatedThe elements and compounds that make up theatmosphere and oceans evaporate readily undernormal surface conditions; that’s the definitionof “volatile.” Elsewhere in the universe, thesevolatiles would assume very different physicalstates. In the space between planets and stars,where it is very cold, they would be presentmostly as solid ices; however, near starsor in regions where new stars are beingformed, where it is hot, they would be presentas plasma.Plasma is a fourth state of matter after solid,liquid, and gas. It exists only at extremely hightemperatures when electrons are stripped offatoms to form charged particles. Plasma is infact the most common state of matter in theuniverse, yet this state is useless for life; onlysolid bodies like planets can support matter inthe usable solid-liquid-gas states. Life as weknow it can occur only where a volatile such aswater is stable in the liquid state, and whereenergy resources like sunlight or chemicalenergy are available for exploitation for makingor supplying food. Such special conditions forlife seem to be satisfied only on planetaryTable 1: Comparison of atmospheric compositions of Venus, Mars, Earthpressure (bars)* CO 2 (%) N 2 (%) Argon-36 (%) H 2 0 (%) 0 2 (%)Venus 92 96.5 3.5 0.00007

LIFE AND THE EVOLUTION OF EARTH’S ATMOSPHERECO 2partial pressure ( bar )Partial Pressure of COPartial Pressure of CO 2in the Atmospheres Overtime2 in the Atmosphere Over Time10 4Dense degassed CO atmosphere ? ( Marty & Mojzsis 1999 model )210 3Ocean covered Earth ( Kasting 1993 model )10 2?10 1Huronian glaciationlate Precambrian( 5 to 20 o C )glaciation( 5 to 20 o C )10 0ARCHEAN10 -1PROTEROZOIC10 -2C310 -3PAL10 -4LUNAR FORMATION4500 4000 3500 3000 2500 2000 1500 1000 500 0Time before present ( Ma )surfaces. On Earth, volatiles that constitute theatmosphere are principally represented by thegases nitrogen, which makes up seventy-eightpercent, oxygen (twenty percent), water vapor(percentage varies according to weather andgeography), argon gas (about one percent), andcarbon dioxide, or CO2 (about 0.03 percent),and trace amounts of other gases. This mixtureis vastly different from the lifeless atmospheresof nearby Venus and Mars, both of which haveno free oxygen and are completely dominatedby carbon dioxide (Table 1). This comparison ofthe atmospheres of our planetary neighborhoodillustrates the role of life in maintaining surfaceconditions amenable for habitation.The air and oceans probably represent most ofthe surface reservoir of the volatile compoundsNitrogen (N2) and water (H20) on Earth. Oxygenis dominantly locked in minerals in the crust andthe Earth’s interior, and would remain so entirelyif not for the actions of photosynthesizers.As for carbon, the amount that is present asCO2 in the atmosphere is small compared tothe majority of the Earth’s carbon which issequestered in carbonate rocks such asFigure 1: Changing concentrations of atmosphericcarbon dioxide with time on Earth in response to the steadyincrease in solar luminosity (after Kasting 1993). Theamount of past CO2 that has been calculated in this figure(using the model of J.F. Kasting, 1993) is the concentrationrequired to keep the surface warm enough for liquid waterto exist even with past lower solar output. PAL = presentatmospheric level of CO2. The Moon formed between 4.5and 4.45 billion years ago (dark blue field). The Earth couldhave started with an atmosphere extremely enriched in carbondioxide (gray field). The rise in land plants after 500million years ago ("C3" on the figure) defines a minimumlimit for CO2 in the air after that time.limestone and marble, organic matter insediments, fossil fuels, and biomass on or nearthe surface. If all of this carbon were oxidizedinto CO2 and put into the atmosphere, it wouldoverwhelm all other gaseous components. Theamount of CO2 released in such a case wouldgive our planet a surface pressure of aboutsixty bars, and result in an atmosphericcomposition very similar to that of Venus. Therewould also be a massive greenhouse effect, likethe one that keeps the surface of Venus so hot(greater than 460°C). This massive greenhouseeffect would result in temperature increases

that were more than enough to vaporize theoceans. If the enormous volume of water inthe ocean were vaporized, a heavy (250 bar)and deadly greenhouse blanket would pushtemperatures up beyond even the meltingpoint of rocks. Therefore it is easy tounderstand how a delicate balance is necessaryamong the biosphere, atmosphere, crust,and the oceans to keep the Earth habitableover long time scales.Over the past few hundred million years, theamount of CO2 in the atmosphere hasdecreased from high levels to stabilize near itspresent relatively low level (Figure 1). Thisappears to be a response of the environment tothe steady increase in solar luminosity, whichhas kept the surface within an equitabletemperature range for life to flourish over time.Photosynthesis Produces the Oxygen inthe AtmospherePhotosynthesis is the process through whichcertain bacteria and plants use carbon dioxide,water, and light energy to make food andoxygen. The first life on Earth was probably notphotosynthetic, as conditions on the surfacewere harsh because of multiple meteoriteimpacts early on, and a deadly bath of ultravioletradiation before the establishment of anozone screen. Without an ozone layer, theremight still be a biosphere, but it would be tinyand struggling to eke out an existence deep inthe crust away from the surface. The bottom ofthe sea was protected from the realities of adangerous surface zone on the young Earth, solife might have originated and subsequentlyevolved there, only to later migrate upwards. Inthe darkness of the ocean depths, it would havehad to rely on chemical rather than light energy.An environment where this is possible—thedeep-sea hydrothermal vent environments—hasexisted throughout much of Earth’s history alongthe mid-ocean ridges (to learn more about thisenvironment, read Deborah S. Kelley’s essay inSection Six). We don’t know how long it took forphotosynthetic life to evolve and begin theproduction of oxygen, but we do know thatphotosynthesis is the dominant metabolicprocess that sustains the biosphere today.Photosynthesis, although a cornerstone of thepresent biosphere, is a complex process thatas yet remains poorly understood. In order toconvert light energy into chemical energy,photosynthetic organisms use molecularpigments that absorb sunlight of differentwavelengths and reflect others. Differentcolored molecules in plants absorb light energyfrom different parts of the solar spectrum. Theprincipal light-collecting pigment in plants ischlorophyll, which gives them their green color.Interestingly, the Sun’s output is dominated byblue-yellow light, making this the most intensepart of the visible solar spectrum. So,chlorophyll is green because it absorbs all ofthe intense, visible, blue-yellow light and reflectsthe green. Plants, like all forms of life, areefficient, so they make special use of theirpigments to reflect the least intense parts,including the green light of the solar spectrum.The intense light energy is used by thechlorophyll in plants to make carbohydrates orfood, with oxygen as a waste product.How the Biosphere Affects the AtmosphereOrganisms affect the composition of theatmosphere in several significant ways. Most ofus are aware that animals inhale oxygen andexhale carbon dioxide when they breathe. Thesegases are exchanged between the biosphere,the atmosphere, the hydrosphere (oceans) andthe lithosphere (crust) in enormous quantities.Only a tiny proportion of the oxygen in theatmosphere is made when ultraviolet light splitswater molecules high in the atmosphere in aprocess known as photolysis. The vast bulk ofoxygen is continuously replenished via plant

LIFE AND THE EVOLUTION OF EARTH’S ATMOSPHEREphotosynthesis. The high concentration ofoxygen needed to keep the contemporarybiosphere going, including all of the humansbreathing it, is maintained despite its continualremoval through oxidative processes likeweathering of rocks (which tend to give them areddish color as in the Grand Canyon), rusting,burning, and so on. In fact, if all photosynthesiswere to cease, which would be a catastrophicevent indeed, the oxygen in the atmosphere ofthe Earth would be mostly gone in less than amillion years, along with the protective ozonelayer. In the absence of photosynthesis, theoxygen present in the atmosphere wouldcombine with carbon in organic matter at thesurface to make carbon dioxide, or react withiron in minerals, to soon be removed from theatmosphere altogether. In 1979, in his bookGaia: A New Look at Life on Earth, JamesLovelock emphasized that the atmosphericcomposition of the Earth is far from achievingchemical equilibrium with the surface and mustbe sustained by biological processes. Accordingto Lovelock’s Gaia Hypothesis, life isresponsible for making the unique conditions forhabitability on Earth; life regulates the globalenvironment. The other planets near the Earthprovide a useful measure of how different ourplanet is from places that are in equilibrium.Again, take a look at the lifeless planets of Marsand Venus in Table 1. They have negligibleamounts of oxygen in their atmospheres, andare dominated by carbon dioxide. The samewould be true for the Earth if life had neverappeared and changed the atmosphere to keepthe planet habitable.The Build-up of OxygenWhen oxygen began to first accumulate onEarth billions of years ago, it caused a crisis forprimitive anaerobic bacterial life, which isactually poisoned by excess oxygen in theenvironment. Such life still exists today inorganic-rich soils, swamps, deep in sediments,and lake bottom muds where levels of oxygenare low. This early oxygen crisis, however,created an opportunity and led to importantnew evolutionary changes as life responded tothis challenge with metabolic innovations thatmade use of the newly-available oxygen.Organisms evolved which used oxygen in theirmetabolic cycles to get more energy out oftheir food—twenty times more than anaerobicorganisms get.A coupled biological and geological mechanismtermed the “carbon cycle” began to manifestitself early in the history of life on Earth, as lifetook over the production of organic matter andoxygen and began to regulate the balance ofcarbon between the atmosphere and theoceans. This cycle has helped to regulate theplanet’s surface temperature by balancing CO2output from volcanoes with weathering, burial oforganic matter in sediments, and other meansof removing CO2 from the atmosphere. (Tolearn more details about how this works, readRachel Oxburgh’s essay on the carbon cycle inSection Six). In a sense, industrialization andthe burning of fossil fuels, which have releasedvast amounts of carbon dioxide into theatmosphere, represent a small reversal of thegeneral trend toward higher oxygenconcentrations and lower carbon dioxideconcentrations with geologic time. Humanactivity, the long-term actions of plate tectonics,and variations in Earth’s orbit will inevitablycause large fluctuations in the amount of carbondioxide in the atmosphere again, and perhapschanges in oxygen concentration as well.When Did These Changes Occur?Life originated more than 4 billion years ago.How soon after the origin of life did oxygenlevels begin to rise? This is a question thatscientists have been exploring for some time(Figure 2). Data gleaned from ancient soils

(paleosols) and a variety of other geologicalevidence indicates that starting about 2.5 billionyears ago, sedimentary rocks began to becomeincreasingly affected by higher amounts ofoxygen in the atmosphere, and appear red, asthough “rusted.” Scientists believe that oxygenconcentration began to rapidly increase at thattime because more oxygen began to beproduced by photosynthesis than could bewhisked away by reactions with theatmosphere, oceans, and rocks. A thresholdhad been reached, and the rise of oxygen hadbegun. At around that same time, morecomplex bacterial life evolved in response to theoxygen-rich conditions, including organisms witha true cell nucleus (Eukaryotes) from which allplants, animals, and fungi are descended.The final significant section in the emergence ofan oxygen-rich atmosphere on Earth beganabout 700 million years ago, when the fossilrecord documents the emergence ofmulticellular organisms. When oxygen levelsbecame high enough, a little more than onepercent of the present level, a powerful ozonescreen started to form in the upper atmospherethat protected the first land-dwelling creaturesfrom the Sun’s harmful ultraviolet radiation.Abundant oxygen in the atmosphere wasessential to the rise of animal and plant lifeon land.A Hot FutureKnowing what we do about the past trends inatmospheric evolution of the Earth, what can wesay about the distant future for life on thisplanet? The Sun will continue to grow brighterover the 5 billion years or so that remain in itsstellar life span. Its luminosity has beenincreasing on a near-linear path for more than4.5 billion years because hydrogen iscontinually converted into the heavier element,helium, by nuclear fusion. This conversionreleases tremendous amounts of energy everysecond and is responsible for sunlight. Ashelium accumulates, the average density of theSun increases over time, which in turn causespressures and temperatures in the Sun’s interiorto increase. These higher pressures andtemperatures increase the rate of fusion, andthe extra energy that is generated leads toincreased luminosity. As this positive feedbackcontinues, the Sun will continue to increase inintensity by about six percent or more in thenext billion years.One of two things needs to happen to keep theEarth from slowly going the way of deadly hotVenus. Either almost all of the greenhousegases have to be removed from theatmosphere, or the planet’s albedo—theamount of sunlight the surface can reflect backinto space—must increase. Unless new plantsevolve a type of photosynthesis that requiresvery little carbon dioxide, or some future culturesomehow protects the Earth from a brighterSun, life on land will have to be abandoned. Inthe end, some 5 billion years from now, muchof the Sun’s fuel will be exhausted. The Sun willbegin to grow as it approaches the Red Giantphase of its stellar life cycle. By then, theoceans on Earth will have long since boiledaway. The last life, primitive bacteria much likethe first that appeared, residing in deep crustalrocks and sediments, will have perished. As thesurface temperature rises, all of the volatilegases stored over many eons in the Earth’scrust will be released to form a thick, hot,blanketing atmosphere. Finally, there will betwin Venuses from that time until the end of thesolar system.SummaryA great deal can be learned by approachingthe history of life on Earth from the standpointof geology. The rocks that make up thegeologic record preserve changes in theplanet’s crust, oceans, and atmosphere thathave accompanied the history of theevolution of life. Studies of the geologic record

LIFE AND THE EVOLUTION OF EARTH’S ATMOSPHEREhelp us fine-tune theoretical models that attemptto shed light on conditions at the Earth’ssurface when life emerged. Studyingneighboring worlds is also relevant because itallows us to compare the very different pathstaken by these planets in the absence of life. Itis also necessary to keep in mind theimmense expanse of time involved. Geologistsapproach the geologic record much like adetective tackles problems in a detective story;the plot thickens as new evidence isuncovered, but it often yields more questionsthan answers.In science, theories evolve as evidenceaccumulates. But theories must be grounded inthe solid principles of physics, mathematics,and chemistry that describe the behavior ofmatter in the universe. We are now on thethreshold of being able to evaluate the placeof Earth in the universe; we may discoverthat we are not all that unique in having awater-rich planet with abundant life and oxygen.The search for other worlds that contain, orcontained, life stands as the grand quest forthe next millennium.Partial Pressure Pressure of O 2 of in Othe 2 in Atmospheres the OvertimeTimeO 2partial pressure ( bar )10 010 -210 -310 -410 -510 -610 -710 -8LUNAR FORMATION?10 -910 -10?10 -1110 -1210 -13?? ?paleosolsfossil evidencefor Eukaryotes( Grypania )BIFland plantsPAL10 -1 4500 4000 3500 3000 2500 2000 1500 1000 500 0Time before present ( Ma )Figure 2: This plot (after Kasting, 1993) shows estimates ofchanging concentrations of free oxygen in the atmosphereover geologic time. The area (in light blue) represents therange in possible concentrations of oxygen based on modelcalculations, and the study of ancient soils (paleosols),fossil organisms, and marine sediments that only form inthe absence of oxygen. These sediments are preserved asBanded Iron Formations (BIF) and only appear in the geologicrecord up to about 1.8 billion years ago. Although it is notknown when photosynthesis began, it is clear thatphotosynthesis only became an important producer ofoxygen in the atmosphere late in Earth’s history. PAL =present atmospheric level of O2.