why life does not really exist

Download Why Life Does Not Really Exist

Post on 17-May-2017




0 download

Embed Size (px)


  • Permanent Address: http://blogs.scientificamerican.com/brainwaves/2013/12/02/why-life-does-not-really-exist/


    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, 2013

    I have been fascinated with

    living things since

    childhood. Growing up in

    northern California, I spent

    a lot of time playing

    outdoors among plants and

    animals. Some of my

    friends and I would sneak

    up on bees as they

    pollinated flowers and trap

    them in Ziploc bags so we

    could get a close look at

    their obsidian eyes and golden hairs before returning the insects to their

    daily routines. Sometimes I would make crude bows and arrows from bushes in my backyard, using stripped bark for string and leaves

    for fletchings. On family trips to the beach I learned how to quickly dig crustaceans and arthropods out of their hiding spots by

    watching for bubbles in the sand as the most recent wave retreated. And I vividly recall an elementary school field trip to a grove of

    eucalyptus trees in Santa Cruz, where thousands of migrating monarch butterflies had stopped to rest. They clung to branches in great

    brown 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 planets

    creatures. Whereas my younger brother was obsessed with his KNex setmeticulously building elaborate roller coastersI wanted to

    understand 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 half

    as 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 a


    Recently, however, I had an epiphany that has forced me to rethink why I love living things so

    much and reexamine what life is, really. For as long as people have studied life they have struggled

    to 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 and

    my curiosity about the family cat. Why do we think of the former as inanimate and the latter as

    alive? In the end, arent they both machines? Granted, a cat is an incredibly complex machine

    capable of amazing behaviors that a KNex set could probably never mimic. But on the most

    fundamental level, what is the difference between an inanimate machine and a living one? Do

    people, cats, plants and other creatures belong in one category and KNex, computers, stars and

    rocks 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 17th

    century, German chemist George Erns Stahl and other researchers began to describe a doctrine that would eventually become known as

    vitalism. Vitalists maintained that living organisms are fundamentally different from non-living entities because they contain some

    non-physical element or are governed by different principles than are inanimate things and that organic matter (molecules that

    contained carbon and hydrogen and were produced by living things) could not arise from inorganic matter (molecules lacking carbon

    that resulted primarily from geological processes). Subsequent experiments revealed vitalism to be completely untruethe inorganic

    Why 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 20:27

  • A 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 physical

    properties that differentiated the living from the nonliving. Today, in lieu of a succinct definition of life, Campbell and many other

    widely used biology textbooks include a rather bloated list of such distinguishing characteristics, for instance: order (the fact that many

    organisms are made from either a single cell with different compartments and organelles or highly structured groups of cells); growth

    and development (changing size and shape in a predictable manner); homeostasis (maintaining an internal environment that differs

    from an external one, such as the way cells regulate their pH levels and salt concentrations); metabolism (expending energy to grow

    and to delay decay); reacting to stimuli (changing behavior in response to light, temperature, chemicals or other aspects of the

    environment); reproduction (cloning or mating to produce new organisms and transfer genetic information from one generation to the

    next); 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 a

    set of physical properties that unites all living things and excludes everything we label

    inanimate. 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 and

    gets bigger. In contrast, bacteria, tardigrades and even some crustaceans can enter long

    periods 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 a

    tree? Most people would agree that, when attached to a tree, a leaf is alive: its many cells

    work 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 it

    die on the way to the ground; or when it hits the ground; or when all its individual cells

    finally expire? If you pluck a leaf from a plant and keep its cells nourished and happy

    inside 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 living

    organismswe have designed countless machines that do just that. Even reproduction does not define a living thing. Many an

    individual animal cannot reproduce on its own. So are two cats alive because they can create new cats together, but a single cat is not

    alive 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 ones

    offspring and to adapt to a changing environment by altering genetic informationsurely these talents are unique to living things. Many

    biologists have focused on evolution as lifes key distinguishing feature. In the early 1990s, Gerald Joyce of the Scripps Research

    Institute was a member of an advisory panel to John Rummel, manager of NASAs exobiology program at the time. During discussions

    about how best to find life on other worlds, Joyce and his fellow panelists came up with a widely cited working definition of life: a

    self-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. Viruses

    are essentially strands of DNA or RNA packaged inside a protein shell; they do not have cells or a metabolism, but they do have genes

    and they can evolve. Joyce explains, however, that in order to be a self-sustaining system, an organism must contain all the

    information necessary to reproduce and to undergo Darwinian evolution. Because of this constraint, he argues that viruses do not

    satisfy the working definition. After all, a virus must invade and hijack a cell in order to make copies of itself. The viral genome only

    evolves 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 to

    accommodate the ambiguity of viruses better than any other proposed definition. A parasitic

    worm living inside a persons intestineswidely regarded as a detestable but very real form of

    lifehas all the genetic information it needs to reproduce, but it would never be able to do so

    without cells and molecules in the human gut from which it steals the energy it needs to

    survive. Likewise, a virus has all the g