"The New York Times" December 18, 2001
Defining the Undefinable: The Living Cell
By NATALIE ANGIER
Much as people may wish these days for the comforts of a few black and white
distinctions between good and evil, democracy and tyranny, friends and fiends our
restless color wheel of a world refuses to comply. And that law of equivocation and qualification,
scientists say, extends down to the most basic distinction of all: whether something is alive or
not.
Obvious though the differences may seem between a sleeping cat and its pillow, or a patch of
lichen and its rock, scientists have had a devilishly difficult time specifying, delimiting and
agreeing on the characteristics that define life, and that exclude the apparently nonliving.
The question is not one of mere semantics or taxonomics. As the nation once again takes up the
angst-riven issue of extracting stem cells from human embryos for possible use in the treatment
of a raft of diseases, researchers must weigh not only the question of when human life begins, but
what being alive really means.
"People have long sought to define and understand it," said Dr. Mark A. Bedau, a
philosopher of biology at Reed College in Oregon. "The question is especially interesting now that
science and technology are continually pushing the boundaries of what living systems or lifelike
systems exist, and what they may look like in the future."
As an example of the trickiness that technology can bring to the definition of life, scientists cite
new experiments showing that monkey eggs can be chemically treated and modified to the point
where they begin behaving enough like embryos to generate stem cells all without the addition
of sperm normally required for embryogenesis, and without any capacity to grow into a baby
monkey.
Assuming that human eggs could be similarly stripped down and par thenogenetically
reinvented, how "alive" would these wholly unnatural and almost entirely fabricated
stem cell factories be? How much more alive must they be adjudged,
scientists ask,
than are the chemical reactions in a test tube that yield synthetic versions of vital
hormones, or that attempt to mimic the conditions early in the earth's history, when life
began?
"We've gotten to the point where we have RNA molecules that replicate in a test
tube," just as RNA is thought to have done at the dawn of life four billion years ago,
said Dr. Jeffrey L. Bada, a professor of marine chemistry at the University of California
at San Diego, who studies the origin of life.
"But no one has dared to call that living yet," Dr. Bada said. "We
have to incorporate
all kinds of chemicals to make that replication happen." Keeping embryos alive in the
laboratory, he said, or manipulating eggs for research, "is not too much different than
working with evolving RNA molecules in a test tube."
As Dr. Lawrence M. Krauss, the chairman of the physics department at Case Western
Reserve University, sees it, an embryo floating in a petri dish is
little more than a
chemical factory, not so much alive as prelude to life, and no worthier of being
considered a mini-human being than a block of uncarved marble is worthy of being
called Michelangelo's Pietą.
"The distinction between organic material that has potential one day to do something,
and something already functioning as human being, is very great,"
said Dr. Krauss,
who often writes about the effects of science on society. "I'm
amazed when people
view the potential as if it's real."
A number of scientists argue that humans are too mired in
anthropocentric, or at least
biocentric thinking, and thus are likely to award membership in Club Life solely to
organic life forms, while withholding it from worthy entities or systems that display
many properties of life without being carbon-based.
Some say that the economy and the Internet, for example, are alive
in the same sense
that an ant colony is alive; all are super organisms, with the individual functions
widely distributed yet intimately connected, mutually responsive, dynamic and ever-
changing.
Those in the field of so-called artificial life believe that many properties of life can be
captured in digital format, and that by viewing the performance and behavior of these
computerized species, researchers may better understand what life outside the terminal
is all about.
A-lifers, as they are sometimes known, write computer programs that begin with
relatively simple codes, or instructions, which, once liberated to
run free on anything
from desktop P.C.'s to university supercomputers, follow their own
paths and
aggregate into spectacularly diverse digital "ecosystems."
One of the most famous such programs is Tierra, created in the early 1990's by Dr.
Thomas S. Ray, a professor of zoology at the University of Oklahoma, and since
disseminated to programmers around the world. Tierra began with a bit of computer
code 85 bytes long called the Ancestor, which spawned daughter
codes, which in turn
generated other daughter programs, some of them 10 or 100 times as
long as the
original Ancestor.
The replication parameters were less than perfect, just as DNA replication in the
carbon-based world is less than perfect, resulting in minor
changes, or mutations, from
one generation of Tierrans to the next. The organisms assembled themselves into
habitats, he said. They competed, they cooperated, they cheated, they parasitized,
they parasitized the parasites. They reproduced with varying
success, and, as living
things are wont to do, they died.
Most vividly of all, said Dr. Ray, Tierra abided by Darwinian law:
it evolved. "For me,
the central, defining property of life is evolution by natural
selection," he said. "And
that's a property I've captured in my work."
So while he won't flatly say that Tierra life is alive, he won't dismiss it as a mere model
of life either. "We're creating things in a twilight zone between life and nonlife," he
said. "It leads to the recognition that life isn't cut-and-dried,
yes-or-no. There are
shades of life and aliveness." Yet when asked if he felt sorrow at
the "death" of any of
his Tierrans, he said, "No, no more than I would over the death of
a virus or
bacterium."
Other scientists are more skeptical about the possibility of machine life approaching
real life, or of a computer or robot being so nimbly programmed that it attains some
form of awareness, a conviction that it is alive and that, like HAL of "2001: A Space
Odyssey," it does not want to die.
"There are lots of variations on the idea that computers, if not already alive, are on their way to
becoming alive, and that for better or worse they will be a successor species to the human race,"
said Jaron Lanier, a computer scientist and pioneer in virtual reality. "I've always objected to
that, which has made me the black sheep at a lot of computer conferences."
But Mr. Lanier argues that computer scientists who think of computers as alive, or practically
sentient, write bad computer programs that make everybody
stupider. He cites the classic Turing
test of a computer's sophistication, devised by the great British mathematician Alan Turing.
According to Turing, if a judge cannot distinguish between the answers given by a computer and
those from a person, then the computer must be considered essentially conscious, and should be
given equal rights.
Mr. Lanier proposes another interpretation of such a result. "Turing assumed that the computer
in this case has become smarter or more humanlike," he said. "But the equally likely
conclusion is that the person has become dumber and more computerlike."
"Consider all the ways that we bend to a mechanized process by doing stupid or irrational
things," he added, as when college students are advised to begin building up a good credit rating
by borrowing money and paying interest to feed data into the credit rating system. "So students
are bending over backward and doing economically foolish things to make the credit
algorithm look smart," he said.
Ultimately, in Mr. Lanier's view, the experience of being alive is a subjective one that may not be
amenable to definition or algorithmic recapitulation. "Science studies repeatable
phenomena and empirical things that can be tested," he said. "But the experience of being alive is not
detectable by instruments, and at some point we may have to admit it's a big question mark
that we're never going to be able to answer."
But like a beckoning finger, a question mark can prove
irresistible, and professional
thinkers have long mulled over the definition of life.
Aristotle constructed a stepladder of life "essences."
"Aristotle talked of the
vegetative soul, which is basically equivalent to life at the cellular
level," said Dr. Lee
M. Silver, a professor of molecular biology and public affairs at
Princeton. "Then there
was his sensitive soul, equivalent to organisms with a functioning
nervous system,
and finally his rational soul, the human being."
As Aristotle saw it, then, not only were human beings generally superior to all other
species; they were, at their core, more alive.
At the other end of the scala natura is the virus, a source of many lively scientific
debates. To live means to use energy, some have argued, to
metabolize, to eat. Viruses
do none of these things. "They show no properties of life," said
Dr. Lynn Margulis, a
professor of biology at the University of Massachusetts. "No
metabolism, no self-
maintenance. They're about as alive as is sugar or salt."
Yet some virologists defend the animate nature of their subjects by pointing out that
viruses secondarily metabolize, by forcing their host cells to devote most of their
metabolic energy to the production of new viruses.
The pathogens are all too clever at getting around, and they even
evolve. "Viruses are
smart enough to direct their own reproduction," said Dr. Arnold
Levine, a virologist
and the president of Rockefeller University in New York. "I've
always thought of them
as alive, although in a dormant state."
An even thornier case is the prion, a nasty infectious protein that lacks even the
virus's nucleic blueprint to make more of itself, yet somehow manages to make more of
itself in the brains of its unfortunate hosts, leading to devastating illnesses like mad
cow disease and kuru. Can something that possesses a core trait of
life the impulse
to go forth and multiply be blithely dismissed as lifeless?
And then there is bronze disease, a k a rust. "Rust can be
contagious," said Dr. J.
Doyne Farmer, a professor at the Santa Fe Institute in New Mexico,
who has worked
on problems modeling biological systems. "The more rust you get on
a car's body, the
more the rust propagates. Nobody would say that bronze disease is
alive, but already,
even at the elementary level, there's some notion of contagion, and something that
seems alive."
In this spirit of inclusiveness, many life theorists emphasize systems rather than
individuals as a way of grasping the meaning of life. "To focus on
the superficial
characteristics of what makes a particular fly or a particular human alive is to miss the
underlying unified explanation for those hallmarks," said Dr. Bedau of Reed College.
In his view, the key to life is something he calls "supple
adaptation," the capacity of
populations or groups to respond to changing circumstances by continually creating
new adaptations. Thus, the dinosaurs died out not because they expired one by one,
but because their population somehow lost all capacity to respond to shifting
conditions. Their suppleness fossilized, and they soon followed suit.
In this vein, some experts say, the current stem cell debate, by focusing on human
embryos afloat in the laboratory and divorced from their ordinary biological and
evolutionary context, ignores the connectivity of the human race, the social, emotional
and ultimately physical link among people that is epitomized in the relationship
between the mother and her gestating fetus.
"Until very recently in human history, the moment when somebody was said to be
there was with the quickening," said Dr. Barbara Katz Rothman, a bioethicist and
professor of sociology at the City University of New York. "The
moment a woman felt
movement, when the baby communicated itself to the woman and she communicated
the quickening to others, was the moment that the baby entered the
social world. It
was an inherently social act: thud! I'm here."
A days-old embryo in a petri dish, or frozen away in a fertility
clinic, she said, makes no
thud and has no umbilicus to the world around it. It must of necessity await definition
by the lives that flicker around it, who are themselves caught up in the uncertainties
and disputes that are life's most assured gifts.
Oryginal:
http://www.nytimes.com/2001/12/18/health/genetics/18LIFE.html?ex=1009899992
http://www.nytimes.com/2001/12/18/health/genetics/18LIFE.html?pagewanted=2&ex=1009899992
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