Theoldestfossilslookrathersimilartosomeoftoday’sbacteria.Thisindicatesthatlifemayalready havebeenquitewellestablishedbythatpoint,withmembrane-enclosedcells,ahereditarysystembased onDNA,andametabolismbasedonproteins.
But which came first? Replicating DNA-based genes, protein-based metabolism, or enclosing membranes?Intoday’slivingorganismsthesesystemsformamutuallyinterdependentsystemthatonly worksproperlyasawhole.DNA-basedgenescanonlyreplicatethemselveswiththeassistanceofprotein enzymes.ButproteinenzymescanonlybebuiltfromtheinstructionsheldintheDNA.Howcanyouhave one without the other? Then there’s the fact that both genes and metabolism rely on the cell’s outer membrane to concentrate the necessary chemicals, capture energy and protect them from the environment. But we know that cells alive today use genes and enzymes to build their sophisticated membranes.It’shardtoimaginehowoneofthiscrucialtrinityofgenes,proteinsandmembranescould havecomeaboutonitsown:ifyoutakeoneelementaway,thewholesystemrapidlycomesapart.
Theformationofmembranesmightbetheeasiestparttoaccountfor.Weknowthatthekindoflipid moleculesthatmakeupmembranescanformviaspontaneouslyoccurringchemicalreactionsthatinvolve substances and conditions thought to have been present on the young Earth. And when scientists put these lipids into water, they do something unexpected: they assemble themselves spontaneously into hollow,membrane-boundedspheresthatareaboutthesamesizeandshapeassomebacterialcells.
With a plausible mechanism for forming membrane-bounded entities, that leaves the question of whetherDNAgenesorproteinscamefirst.Thebestsolutionscientistshaveyetfoundforthisparticular chicken and egg-type problem is to say that neither of them did! Instead, it may have been DNA’s chemicalcousin,RNA,thatcamefirst.
LikeDNA,RNAmoleculescanstoreinformation.Theycanalsobecopied,witherrorsinthatcopying processintroducingvariability.ThatmeansRNAcanactasahereditarymoleculethatcanevolve.That’s whatRNA-basedvirusesstilldotoday.TheothercrucialpropertyofRNAmoleculesisthattheycanfold up to form more complicated three-dimensional structures that can function as enzymes. RNA-based enzymes are not nearly as complex or versatile as protein enzymes, but they can catalyze certain chemical reactions. Several of the enzymes crucial to the function of today’s ribosomes are made from
RNA,forexample.IfthesetwopropertiesofRNAwerecombined,theymayhavebeenabletoproduce RNAmoleculesthatworkasbothgeneandenzyme:ahereditarysystemandaprimitivemetabolismin thesamepackage.Whatthiswouldamounttoisaself-sustaining,RNA-basedlivingmachine.
Some researchers think these RNA machines might have first formed within the rocks that surround deep-ocean hydrothermal vents. Tiny pores in the rock may have provided a protected environment, whilstthevolcanicactivityboilingoutoftheEarth’scrustwouldhaveofferedasteadyflowofenergyand chemical raw materials. In these circumstances, it’s possible that the nucleotides needed to make RNA polymerscouldbemadefromscratch,byassemblingthemfromsimplermolecules.Atfirst,metalatoms embeddedintherockmayhaveactedaschemicalcatalysts,allowingreactionstoproceedwithouttheaid ofbiologicalenzymes.Eventually,aftermillenniaoftrialanderror,thiscouldhaveledtotheformationof machinesmadefromRNAthatwerealive,self-maintainingandself-replicatingandthat,sometimelater, couldhavebeenincorporatedintomembrane-boundedentities.Thatwouldhavebeenalandmarkevent intheemergenceoflife:theappearanceofthefirsttruecells.
ThestoryIhavejusttoldyouisplausible,butpleaserememberitisalsohighlyspeculative.Thefirst lifeformsleftnotrace,soitisverydifficulttoknowwhatwashappeningatthedawnoflifeorevenwhat precisestatetheEarthitselfwasinmorethan3.5billionyearsago.
Oncethefirstcellshadsuccessfullyformed,however,itiseasiertoimaginewhathappenednext.First, thesingle-celledmicrobeswouldhavespreadthroughtheworld,gradually,colonizingsea,landandair.
Then,2billionyearsorsolater,thelargerandmorecomplex–butforaverylongtimestillsingle-celled–
eukaryotes joined them. True multicellular eukaryotic organisms came much later, after another billion yearsorsohadelapsed.Thatmeansthatmulticellularlifehasbeenhereforabout600millionyears,just onesixthoflife’stotalhistory.Butinthattimethey’vegivenrisetoallthelargestandmostvisibleliving forms that surround us, including towering forests, swarming colonies of ants, huge networks of underground fungi, herds of mammals on the African savannah, and very much more recently, modern humans.
Allofthishashappenedthroughtheblindandunguided,buthighlycreative,processofevolutionby natural selection. But when considering life’s successes, we should remember that evolutionary change can only happen efficiently when some members of a population fail to survive and reproduce. So althoughlifeasawholehasproveditselftobetenacious,long-lastingandhighlyadaptable,individuallife formstendtohavealimitedlifespanandarestrictedabilitytoadaptwhentheirenvironmentchanges.
Whichiswherenaturalselectioncomesintoplay,killingofftheoldorderand,ifmoresuitablevariants existinapopulation,makingwayforthenew.Itseemsthatitisonlythroughdeaththattherecanbelife.
Theruthlesswinnowingprocessofnaturalselectionhascreatedmanyunexpectedthings.Oneofthe mostextraordinaryoftheseisthehumanbrain.Sofarasweknow,nootherlivingthingshareswithus quitethesameawarenessofitsownexistence.Ourself-consciousmindsmusthaveevolved,atleastin part, to give us more leeway to adjust our behaviour when our worlds change. Unlike butterflies, and perhaps all other known organisms, we can deliberately choose and reflect upon the purposes that motivateus.
Thebrainisbasedonthesamechemistryandphysicsasallotherlivingsystems.Yetsomehow,from thesamerelativelysimplemoleculesandwell-understoodforces,springourabilitiestothink,todebate, toimagine,tocreateandtosuffer.Howallthisemergesfromthewetchemistryofourbrainsprovidesus withanextraordinarilychallengingsetofquestions.
We know that our nervous systems are based on immensely complex interactions between billions of nerve cells (neurons) that make trillions of connections between themselves, called synapses. Together, these unfathomably elaborate and constantly changing networks of interconnected neurons establish signallingpathwaysthattransmitandprocessrichstreamsofelectricalinformation.
Asissooftenthecaseinbiology,weknowmostofthisfromstudyingsimpler‘model’organisms,such asworms,fliesandmice.Weknowquitealotaboutthewaysthesenervoussystemsgatherinformation fromtheirenvironmentsthroughtheirsenseorgans.Researchershavedoneathoroughjoboftracking the movement of visual, sound, touch, smell and taste signals through the nervous system, as well as mapping some of the neuronal connections that form memories, generate emotional responses, and createoutputbehaviours,suchasflexingmuscles.
This is all important work, but it is only a beginning. We have barely scratched the surface of understanding how the interactions between billions of individual neurons can combine to generate abstract thought, self-consciousness, and our apparent free will. Finding satisfactory answers to these questionswillprobablyoccupythetwenty-firstcenturyandlikelybeyond.AndIdonotthinkwecanrely only on the tools of the traditional natural sciences to get there. We will have to additionally embrace insightsfrompsychology,philosophyandthehumanitiesmoregenerally.Computersciencecanhelptoo.
Today’s most powerful �
�AI’ computer programs are built to mimic, in a highly simplified form, the way life’sneuralnetworkshandleinformation.
These computer systems perform increasingly impressive data-crunching feats, but display nothing that even vaguely resembles abstract or imaginative thought, self-awareness, or consciousness. Even definingwhatwemeanbythesementalqualitiesisverydifficult.Here,anovelist,apoetoranartistcan help,bycontributingtothebasisofcreativethoughts,bymoreclearlydescribingemotionalstates,orby interrogating what it really means to be. If we have more of a common language, or at least greater intellectualconnection,betweenthehumanitiesandthesciencestodiscussthesephenomena,wemaybe better placed to understand how and why evolution has allowed us to develop as chemical and informational systems that have somehow become aware of their own existence. It will take all our
imaginationandcreativitytounderstandhowimaginationandcreativitycancomeabout.
Theuniverseisunimaginablyvast.Bythelawsofprobability,itseemsveryunlikelythatacrossallthat timeandspacelife–letalonesentientlife–hasonlyeverblossomedonce,righthereonEarth.Whether ornotwewillevermeetalienlifeformsisadifferentissue.Butifweeverdo,Iamconfidentthey,likeus, willbeself-sustainingchemicalandphysicalmachines,builtaroundinformation-encodingpolymersthat havebeenproducedthroughevolutionbynaturalselection.
Ourplanetistheonlycorneroftheuniversewhereweknowforcertainlifeexists.Thelifethatweare partofhereonEarthisextraordinary.Itconstantlysurprisesusbut,inspiteofitsbewilderingdiversity, scientistsaremakingsenseofit,andthatunderstandingmakesafundamentalcontributiontoourculture andourcivilization.Ourgrowingunderstandingofwhatlifeishasgreatpotentialtoimprovethelotof humankind.Butthisknowledgegoesevenfurther.Biologyshowsusthatallthelivingorganismsweknow of are related and closely interacting. We are bound by a deep connectedness to all other life: to the crawling beetles, infecting bacteria, fermenting yeast, inquisitive mountain gorillas and flitting yellow butterflies that have accompanied us during our journey through this book, as well as to every other memberofthebiosphere.Together,allthesespeciesarelife’sgreatsurvivors,thelatestdescendantsofa single,immeasurablyvastfamilylineagethatstretchesbackthroughanunbrokenchainofcelldivisions intothefarreachesofdeeptime.
Asfarasweknow,wehumansaretheonlylifeformswhocanseethisdeepconnectivityandreflecton whatitmightallmean.Thatgivesusaspecialresponsibilityforlifeonthisplanet,madeupasitisbyour relatives,someclose,somemoredistant.Weneedtocareaboutit,weneedtocareforit.Andtodothat weneedtounderstandit.
ACKNOWLEDGEMENTS
DavidandRosieFickling,foralltheireffortstomakethisbookaccessible;andtofriendsandcolleagues inmylabandbeyondovertheyears,fordiscussionsanddisagreementsaboutthenatureoflife.Finally, toBenMartynoga,forhelpingmegreatlyandmakingthisbookenjoyabletowrite.
AbouttheAuthor
Paul Nurse is a geneticist and cell biologist who has worked on how the reproduction of cells is controlled.Thisprocessisthebasisofgrowthanddevelopmentinalllivingorganisms.HeisDirectorof the Francis Crick Institute in London and has served as Chief Executive of Cancer Research UK, PresidentofRockefellerUniversityandPresidentoftheRoyalSociety.Hesharedthe2001NobelPrizein Physiology or Medicine and has received the Albert Lasker Award and the Royal Society’s Royal and CopleyMedals.
Hewasknightedin1999andreceivedtheLégiond’honneurfromFrancein2003andtheOrderofthe Rising Sun from Japan in 2018. He served for fifteen years on the Council of Science and Technology, advising the Prime Minister and Cabinet, and is presently a Chief Scientific Advisor for the European CommissionandatrusteeoftheBritishMuseum.
Paulfliesglidersandvintageaeroplanesandhasbeenaqualifiedbushpilot.Healsolikesthetheatre, classicalmusic,hill-walking,goingtomuseumsandartgalleries,andrunningveryslowly.
WhatisLife? ishisfirstbook.
Copyright
WhatisLife?–UnderstandBiologyinFiveSteps
Firstpublishedin2020
byDavidFicklingBooks,31BeaumontStreet,Oxford,OX12NP
Thisebookeditionfirstpublishedin2020
Allrightsreserved
Text©PAULNURSE,2020
EditedbyBenMartynoga
CoverdesignbyPaulDuffield
TherightofPAULNURSEtobeidentifiedasauthorofthisworkhasbeenassertedinaccordancewithSection77ofthe Copyright,DesignsandPatentsAct1988.
Thisebookiscopyrightmaterialandmustnotbecopied,reproduced,transferred,distributed,leased,licensedorpublicly performedorusedinanywayexceptasspecificallypermittedinwritingbythepublishers,asallowedunderthetermsand conditionsunderwhichitwaspurchasedorasstrictlypermittedbyapplicablecopyrightlaw.Anyunauthoriseddistributionor useofthistextmaybeadirectinfringementoftheauthor’sandpublisher’srights,andthoseresponsiblemaybeliableinlaw accordingly.
Extractfrom TheTempleofNaturebyErasmusDarwinpublishedbypermissionofTimaiosPress.
ISBN978–1–78845–141–3
Paul Nurse - What Is Life Page 13