Why Life Does Not Really Exist

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Permanent Address: http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-does-not-really-exist/ADVERTISEMENT A native bee in my backyard (Credit: Ferris Jabr)A K'Nex contraption (Credit: Druyts.t viaWikimedia Commons)Why Life Does Not Really ExistBy Ferris Jabr | December 2, 2013I have been fascinated withliving things sincechildhood. Growing up innorthern California, I spenta lot of time playingoutdoors among plants andanimals. Some of myfriends and I would sneakup on bees as theypollinated flowers and trapthem in Ziploc bags so wecould get a close look attheir obsidian eyes and golden hairs before returning the insects to theirdaily routines. Sometimes I would make crude bows and arrows from bushes in my backyard, using stripped bark for string and leavesfor fletchings. On family trips to the beach I learned how to quickly dig crustaceans and arthropods out of their hiding spots bywatching for bubbles in the sand as the most recent wave retreated. And I vividly recall an elementary school field trip to a grove ofeucalyptus trees in Santa Cruz, where thousands of migrating monarch butterflies had stopped to rest. They clung to branches in greatbrown globs, resembling dead leavesuntil one stirred and revealed the fiery orange inside of its wings.Moments like thatalong with a number of David Attenborough television specialsintensified my enthrallment with the planetscreatures. Whereas my younger brother was obsessed with his KNex setmeticulously building elaborate roller coastersI wanted tounderstand how our cat, well, worked. How did she see the world? Why did she purr? What were fur and claws and whiskers made of?One Christmas I asked for an encyclopedia of animals. After ripping the wrapping paper off a massive book that probably weighed halfas much as I did, I sat near the tree reading for hours. Not too surprising, then, that I ended up writing about nature and science for aliving.Recently, however, I had an epiphany that has forced me to rethink why I love living things somuch and reexamine what life is, really. For as long as people have studied life they have struggledto define it. Even today, scientists have no satisfactory or universally accepted definition of life.While pondering this problem, I remembered my brothers devotion to KNex roller coasters andmy curiosity about the family cat. Why do we think of the former as inanimate and the latter asalive? In the end, arent they both machines? Granted, a cat is an incredibly complex machinecapable of amazing behaviors that a KNex set could probably never mimic. But on the mostfundamental level, what is the difference between an inanimate machine and a living one? Dopeople, cats, plants and other creatures belong in one category and KNex, computers, stars androcks in another? My conclusion: No. In fact, I decided, life does not actually exist.Allow me to elaborate.Formal attempts to precisely define life date to at least the time of ancient Greek philosophers.Aristotle believed that, unlike the inanimate, all living things have one of three kinds of souls:vegetative souls, animal souls and rational souls, the last of which belonged exclusively to humans.Greek anatomist Galen proposed a similar, organ-based system of vital spirits in the lungs, blood and nervous system. In the 17thcentury, German chemist George Erns Stahl and other researchers began to describe a doctrine that would eventually become known asvitalism. Vitalists maintained that living organisms are fundamentally different from non-living entities because they contain somenon-physical element or are governed by different principles than are inanimate things and that organic matter (molecules thatcontained carbon and hydrogen and were produced by living things) could not arise from inorganic matter (molecules lacking carbonthat resulted primarily from geological processes). Subsequent experiments revealed vitalism to be completely untruethe inorganicWhy Life Does Not Really Exist | Brainwaves, Scientific American Blo... http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-d...1 de 5 02/03/2014 20:27A tardigrade can survive without food or water in adehyrated state for more than 10 years (Credit: Goldtseinlab via Wikimedia Commons via Flickr)can be converted into the organic both inside and outside the lab.Instead of imbuing organisms with some non-physical element, other scientists attempted to identify a specific set of physicalproperties that differentiated the living from the nonliving. Today, in lieu of a succinct definition of life, Campbell and many otherwidely used biology textbooks include a rather bloated list of such distinguishing characteristics, for instance: order (the fact that manyorganisms are made from either a single cell with different compartments and organelles or highly structured groups of cells); growthand development (changing size and shape in a predictable manner); homeostasis (maintaining an internal environment that differsfrom an external one, such as the way cells regulate their pH levels and salt concentrations); metabolism (expending energy to growand to delay decay); reacting to stimuli (changing behavior in response to light, temperature, chemicals or other aspects of theenvironment); reproduction (cloning or mating to produce new organisms and transfer genetic information from one generation to thenext); and evolution (the change in the genetic makeup of a population over time).Its almost too easy to shred the logic of such lists. No one has ever managed to compile aset of physical properties that unites all living things and excludes everything we labelinanimate. There are always exceptions. Most people do not consider crystals to be alive,for example, yet they are highly organized and they grow. Fire, too, consumes energy andgets bigger. In contrast, bacteria, tardigrades and even some crustaceans can enter longperiods of dormancy during which they are not growing, metabolizing or changing at all,yet are not technically dead. How do we categorize a single leaf that has fallen from atree? Most people would agree that, when attached to a tree, a leaf is alive: its many cellswork tirelessly to turn sunlight, carbon dioxide and water into food, among other duties.When a leaf detaches from a tree, its cells do not instantly cease their activities. Does itdie on the way to the ground; or when it hits the ground; or when all its individual cellsfinally expire? If you pluck a leaf from a plant and keep its cells nourished and happyinside a lab, is that life?Such dilemmas plague just about every proposed feature of life. Responding to the environment is not a talent limited to livingorganismswe have designed countless machines that do just that. Even reproduction does not define a living thing. Many anindividual animal cannot reproduce on its own. So are two cats alive because they can create new cats together, but a single cat is notalive because it cannot propagate its genes by itself? Consider, also, the unusual case of turritopsis nutricula, the immortal jellyfish,which can indefinitely alternate between its adult form and its juvenile stage. A jelly vacillating in this way is not producing offspring,cloning itself or even aging in the typical fashionyet most people would concede it remains alive.But what about evolution? The ability to store information in molecules like DNA and RNA, to pass on this information to onesoffspring and to adapt to a changing environment by altering genetic informationsurely these talents are unique to living things. Manybiologists have focused on evolution as lifes key distinguishing feature. In the early 1990s, Gerald Joyce of the Scripps ResearchInstitute was a member of an advisory panel to John Rummel, manager of NASAs exobiology program at the time. During discussionsabout how best to find life on other worlds, Joyce and his fellow panelists came up with a widely cited working definition of life: aself-sustaining system capable of Darwinian evolution. Its lucid, concise and comprehensive. But does it work?Lets examine how this definition handles viruses, which have complicated the quest to define life more than any other entity. Virusesare essentially strands of DNA or RNA packaged inside a protein shell; they do not have cells or a metabolism, but they do have genesand they can evolve. Joyce explains, however, that in order to be a self-sustaining system, an organism must contain all theinformation necessary to reproduce and to undergo Darwinian evolution. Because of this constraint, he argues that viruses do notsatisfy the working definition. After all, a virus must invade and hijack a cell in order to make copies of itself. The viral genome onlyevolves in the context of the host cell, Joyce said in a recent interview.When you really think about it, though, NASAs working definition of life is not able toaccommodate the ambiguity of viruses better than any other proposed definition. A parasiticworm living inside a persons intestineswidely regarded as a detestable but very real form oflifehas all the genetic information it needs to reproduce, but it would never be able to do sowithout cells and molecules in the human gut from which it steals the energy it needs tosurvive. Likewise, a virus has all the genetic information required to replicate itself, but doesnot have all the requisite cellular machinery. Claiming that the worms situation is categoricallydifferent from that of the virus is a tenuous argument. Both the worm and virus reproduce andevolve only in the context of their hosts. In fact, the virus is a much more efficient reproducerthan the worm. Whereas the virus gets right down to business and needs only a few proteinsinside a cells nucleus to initiate replication on a massive scale, the parasitic wormsreproduction requires use of an entire organ in another animal and will be successful only ifWhy Life Does Not Really Exist | Brainwaves, Scientific American Blo... http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-d...2 de 5 02/03/2014 20:27A cluster of bacteriophages, viruses that evolvedto infect bacteria (Credit: Dr Graham Beards viaWikimedia Commons)A geothermal pool in Wyoming. Nearly four billion yearsago, what we call life may have first evolved in similar"warm little ponds," as Darwin put it. (Credit: CalebDorfman, via Flickr)the worm survives long enough to feed, grow and lay eggs. So if we use NASAs workingdefinition to banish viruses from the realm of life, we must further exclude all manner of muchlarger parasites including worms, fungi and plants.Defining life as a self-sustaining system capable of Darwinian evolution also forces us to admitthat certain computer programs are alive. Genetic algorithms, for instance, imitate natural selection to arrive at the optimal solution toa problem: they are bit arrays that code traits, evolve, compete with one another to reproduce and even exchange information.Similarly, software platforms like Avida create digital organisms that are made up of digital bits that can mutate in much the sameway DNA mutates. In other words they, too, evolve. Avida is not a simulation of evolution; it is an instance of it, Robert Pennock ofMichigan State University told Carl Zimmer in Discover. All the core parts of the Darwinian process are there. These things replicate,they mutate, they are competing with one another. The very process of natural selection is happening there. If thats central to thedefinition of life, then these things count.I would argue that Joyces own lab delivered another devastating blow to NASAs working definition of life. He and many otherscientists favor an origin of life story known as the RNA world hypothesis. All life on our planet depends on DNA and RNA. In modernliving organisms, DNA stores the information necessary to build the proteins and molecular machines that together form a bustling cell.At first, scientists thought only proteins known as enzymes could catalyze the chemical reactions necessary to construct this cellularmachinery. In the 1980s, however, Thomas Cech and Sidney Altman discovered that, in collaboration with various protein enzymes,many different kinds of RNA enzymesor ribozymesread the information coded in DNA and build the different parts of a cell pieceby piece. The RNA world hypothesis posits that the earliest organisms on the planet relied solely on RNA to perform all these taskstoboth store and use genetic informationwithout the help of DNA or an entourage of protein enzymes.Heres how it might have happened: Nearly four billion years ago, in Earths primordialsoup, free-floating nucleotidesthe building blocks of RNA and DNAlinked into longerand longer chains, eventually producing ribozymes that were big enough and complexenough to make new copies of themselves and thus had a much greater chance ofsurviving than RNAs that could not reproduce. Simple self-assembling membranesenveloped these early ribozymes, forming the first cells. In addition to making moreRNA, ribozymes may have joined nucleotides into chains of DNA; nucleotides may havespontaneously formed DNA as well. Either way, DNA replaced RNA as the maininformation-storing molecule because it was more stable. And proteins took on manycatalytic roles because they were so versatile and diverse. But the cells of modernorganisms still contain what are likely remnants of the original RNA world. Theribosome, for examplea bundle of RNA and proteins that builds proteins one aminoacid at a timeis a ribozyme. Theres also a group of viruses that use RNA as theirprimary genetic materialTo test the RNA world hypothesis, Joyce and other researchers have tried to create the types of self-replicating ribozymes that mayhave once existed in the planets primordial soup. In the mid-2000s, Joyce and Tracey Lincoln constructed trillions of randomfree-floating RNA sequences in the lab, similar to the early RNAs that may have competed with one another billions of years ago, andisolated sequences that, by chance, were capable of bonding two other pieces of RNA. By pitting these sequences against one another,the pair eventually produced two ribozymes that could replicate one another ad infinitum as long as they were supplied with sufficientnucleotides. Not only can these naked RNA molecules reproduce, they can also mutate and evolve. The ribozymes have altered smallsegments of their genetic code to adapt to fluctuating environmental conditions, for example.They meet the working definition of life, Joyce says. Its self-sustaining Darwinian evolution. But he hesitates to say that theribozymes are truly alive. Before he goes all Dr. Frankenstein, he wants to see his creation innovate a completely new behavior, not justmodify something it can already do. I think whats missing is that it needs to be inventive, needs to come up with new solutions, hesays.But I dont think Joyce is giving the ribozymes enough credit. Evolution is a change in genes over time; one does not need to witnesspigs sprouting wings or RNAs assembling into the letters of the alphabet to see evolution at work. The advent of blue eye color between6,000 and 10,000 years agosimply another variation of iris pigmentsis just as legitimate an example of evolution as the firstfeathered dinosaurs. If we define life as a self-sustaining system capable of Darwinian evolution, I cannot see any legitimate reason todeny self-replicating ribozymes or viruses the moniker of life. But I do see a reason to ditch this working definition and all otherdefinitions of life altogether.Why is defining life so frustratingly difficult? Why have scientists and philosophers failed for centuries to find a specific physicalWhy Life Does Not Really Exist | Brainwaves, Scientific American Blo... http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-d...3 de 5 02/03/2014 20:27A photo taken with an electron scanning microscope of theALH 84001 meteorite, which supposedly formed on Mars 4billion years ago before eventually reaching Earth. Ahandful of scientists think the chain-like structures in thephoto are fossilized Martian nanobacteria, but mostresearchers are skeptical (Credit: NASA, via WikimediaCommons)property or set of properties that clearly separates the living from the inanimate? Because such a property does not exist. Life is aconcept that we invented. On the most fundamental level, all matter that exists is an arrangement of atoms and their constituentparticles. These arrangements fall onto an immense spectrum of complexity, from a single hydrogen atom to something as intricate as abrain. In trying to define life, we have drawn a line at an arbitrary level of complexity and declared that everything above that border isalive and everything below it is not. In truth, this division does not exist outside the mind. There is no threshold at which a collection ofatoms suddenly becomes alive, no categorical distinction between the living and inanimate, no Frankensteinian spark. We have failed todefine life because there was never anything to define in the first place.I nervously explained these ideas to Joyce on the phone, anticipating that he would laugh and tell me they were absurd. After all, this issomeone who helped NASA define life. But Joyce said the argument that life is a concept is perfect. He agrees that the mission todefine life is, in some ways, futile. The working definition was really just a linguistic convenience. We were trying to help NASA findextraterrestrial life, he says. We couldnt use the word life in every paragraph and not define it.Carol Cleland, a philosopher at the University of Colorado Boulder who has spent years researching attempts to deliniate life, alsothinks that the instinct to precisely define life is misguidedbut she is not yet ready to deny lifes physical reality. Its just as prematureto reach the conclusion that there is no intrinsic nature to life as it is to define life, she says. I think the best attitude is to treat whatare normally taken as the definitive criteria of life as tentative criteria.What we really need, Cleland has written, is a well-confirmed, adequately generaltheory of life. She draws an analogy to chemists in the sixteenth century. Beforescientists understood that air, dirt, acids and all chemical substances were made ofmolecules, they struggled to define water. They could list its propertiesit was wet,transparent, tasteless, freezable and it could dissolve many other substancesbut theycould not precisely characterize it until researchers discovered that water is twohydrogen atoms bonded to an oxygen atom. Whether salty, muddy, dyed, liquid orfrozen, water is always H20; it may have other elements mixed in, but the tripartitemolecules that make what we call water water are always there. Nitric acid may resemblewater, but it is not water because the two substances have different molecular structures.Creating the equivalent of molecular theory for life, Cleland says, will require a largersample size. She argues that, so far, we have only one example of what life isthe DNAand RNA-based life on Earth. Imagine trying to create a theory about mammals byobserving only zebras. Thats the situation we find ourselves in when trying to identifywhat makes life life, Cleland concludes.I disagree. Discovering examples of alien life on other planets would undoubtedly expand our understanding of how the things we callliving organisms work and how they evolved in the first place, but such discoveries would probably not help us formulate arevolutionary new theory of life. Sixteenth century chemists could not pinpoint what distinguished water from other substancesbecause they did not understand its fundamental nature: they did not know that every substance was made of a specific arrangement ofmolecules. In contrast, modern scientists know exactly what the creatures on our planet are made ofcells, proteins, DNA and RNA.What differentiates molecules of water, rocks, and silverware from cats, people and other living things is not life, but complexity.Scientists already have sufficient knowledge to explain why what we have dubbed organisms can in general do things that most of whatwe call inanimate cannotto explain how bacteria make new copies of themselves and quickly adapt to their environment, and whyrocks do notwithout proclaiming that life is this and non-life that and never the twain shall meet.Recognizing life as a concept in no way robs what we call life of its splendor. Its not that theres no material difference between livingthings and the inanimate; rather, we will never find some clean dividing line between the two because the notion of life and non-life asdistinct categories is just thata notion, not a reality. Everything about living creatures that fascinated me as a boy are equallywondrous to me now, even with my new understanding of life. I think what truly unites the things we say are alive is not any propertyintrinsic to those things themselves; rather, it is our perception of them, our love of them andfranklyour hubris and narcissism.First, we announced that everything on Earth could be separated into two groupsthe animate and inanimateand it is no secretwhich one we think is superior. Then, not only did we place ourselves in the first group, we further insisted on measuring all other lifeforms on the planet against ourselves. The more similar something is to usthe more it appears to move, talk, feel, thinkthe morealive it is to us, even though the particular set of attributes that makes a human a human is clearly not the only way (or, in evolutionaryterms, even the most successful way) to go about being a living thing.Truthfully, that which we call life is impossible without and inseparable from what weregard as inanimate. If we could somehow see the underlying reality of our planettoWhy Life Does Not Really Exist | Brainwaves, Scientific American Blo... http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-d...4 de 5 02/03/2014 20:27Scientific American is a trademark of Scientific American, Inc., used with permission 2013 Scientific American, a Division of Nature America, Inc.All Rights Reserved.Our late family cat, Jasmine (Credit: Jabr family)comprehend its structure on every scale simultaneously, from the microscopic to themacroscopicwe would see the world in innumerable grains of sand, a giant quiveringsphere of atoms. Just as one can mold thousands of practically identical grains of sandon a beach into castles, mermaids or whatever one can imagine, the innumerable atomsthat make up everything on the planet continually congregate and disassemblethemselves, creating a ceaselessly shifting kaleidoscope of matter. Some of those flocks ofparticles would be what we have named mountains, oceans and clouds; others trees, fishand birds. Some would be relatively inert; others would be changing at inconceivablespeed in bafflingly complex ways. Some would be roller coasters and others cats.About the Author: Ferris Jabr is an associate editor focusing on neuroscience and psychology. Follow on Twitter @ferrisjabr.More The views expressed are those of the author and are not necessarily those of Scientific American.Why Life Does Not Really Exist | Brainwaves, Scientific American Blo... http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-d...5 de 5 02/03/2014 20:27