My son had many things to play with during his childhood, but one was special.
Her name was Frizzle. She was a ball of inquisitive fur several inches long
that the pet store classified as a gerbil. Frizzle spent much of her time
in the brief few years of her life in exploring the multichambered residence
that we had prepared for her, and then in trying to escape from it. When
she died, we missed her.
For various reasons, I had no
animal pet when
I was young, but for a time, I attempted to nourish a small cactus
plant. Its activities were less interesting than those of a gerbil; it
grew slightly, but made no effort to escape. Yet I was saddened when it turned
gray and drooped, and I realized that it had lost the struggle for
survival.
We all learn at an early age how profoundly living things change when they
die. We also recognize that the living things we know are a small part of
a larger surrounding universe of things like water,
rocks, and the moon that are not, and have never been, alive. This wisdom
belongs to our own time, however. For centuries, many observers, including
skilled scientists, did not recognize that dead things do not become alive.
They felt, for example, that river mud could give rise to serpents, and raw
meat could give birth to worms in a process called spontaneous generation.
Only through many carefully controlled experiments, culminating in a brilliant
series carried out by Louis Pasteur in the nineteenth century, was
this theory disproven. We now recognize that life comes only from previously
existing life, like a flame that can be divided, and spread, but once
extinguished, can never be rekindled.
How then did life first come into existence, on this planet or anywhere else
that life may exist in the universe?
Many religions and some philosophies avoid
the problem by presuming that life, in the form of a deity or other immortal
being, has existed eternally. An alternative view can be found in science,
where we look for natural answers in preference to
supernatural ones and turn to another possibility:
that life arose from nonlife at least once, sometime after the creation of
the universe.
To learn about life's origins, we obviously cannot
rely upon human records or memories, and must turn
instead to the evidence stored in the earth itself. Those data are left as
fossils in sediments whose age can be deduced from the amount of radioactivity
remaining in surrounding rocks. For example, an unstable isotope of potassium
is sealed into a volcanic rock when it solidifies from lava. Half of this
isotope decays every 1.3 billion years, with part of it converted to the
stable gas argon, which remains trapped in the rocks. By measuring the amounts
of the remaining potassium isotope and the trapped argon, and performing
a simple calculation, we can determine the age of the rock.
The fossil record tells a marvellous tale of the rise of life. It starts
three and a half billion years ago with single-celled forms that resemble
bacteria and algae, and leads up to the overwhelming variety of living creatures,
including ourselves, that are present today. Earth itself is only one billion
years older than the earliest record of life discovered thus far. If we were
to compare Earth's age to that of a sixty-year-old man, then the fraction
of Earth's existence needed for life to appear could be compared to the time
needed for the man to reach puberty. The geological time occupied by all
recorded human history would be equivalent to the last half- hour or so of
the man's existence. Unfortunately, the record of geology peters out beyond
three and a half billion years ago. No rocks remain that tell us anything
about the way in which the first bacteria-like creatures came into existence.
This process remains a profound puzzle.
A human being is so much more than a bacterium, though. If we can understand
the evolutionary process that produced ourselves
from one-celled creatures, and the developmental changes that convert a
one-celled fertilized egg into a multicelled adult, then why is it difficult
to understand how a bacteria could form from nonliving matter?
To appreciate this problem, we must explore the structure of life as it exists
today. Obviously, this topic can fill many volumes, but here I wish to draw
out a single thread that serves to distinguish life from nonlife: Living
things are highly organized. I have chosen a word that is familiar; some
scientists prefer terms like negative entropy, but basically, the
idea is the same. By using organized, I mean to describe the quality
that distinguishes the complete works of William Shakespeare from
a series of letters struck at random on a
typewriter, or a symphony from the sound one gets by dropping dishes
on the floor. Clearly, things produced by the activities of life, for example,
Shakespeare's works, can also be organized, but things unconnected with life,
like a rock on the surface of the moon, are much less so. A bacterium, when
compared with nonliving matter, is still very organized. The comparison between
a dust particle and a bacterium would then be similar to the match of a random
string of letters to a Shakespeare play.
Some scientists who have thought about life's origins have believed that
this gap in organization between living and nonliving matter could be closed
by random chance alone, if enough tries were made. They have drawn great
encouragement from a famous experiment devised by Stanley Miller and Harold
Urey. Miller and Urey showed in 1953 that certain amino acids
could be formed very easily when electrical energy is passed through a simple
mixture of gases. These amino acids are building blocks of
proteins, one of life's key components.
If key chemicals can arise so readily, can the rest of life be far behind?
| To support the theory that pre-biotic methane may have played an important role in the development of life during the Hadean, the Miller-Urey experiments were conducted in the 1950s. These experiments tried to replicate the environmental conditions of the primitive Earth to see if organic compounds would form. When methane was included, the experiments showed that complex reduced carbon compounds, such as amino acids, were formed by non-biological processes. This seemed to prove that complex organic molecules can build up from very simple organic molecules, such as methane. The precise gas mixture used in the Miller-Urey experiments has since fallen out of favor with scientists as the most likely pre-biotic atmosphere. Still, methane could have been a component of the Earth's early atmosphere. |
Unfortunately, life is vastly more organized than the "prebiotic" chemical
mixtures formed in Miller-Urey-type experiments. Imagine that by random strokes
on the typewriter you type the words to be. You might then be reminded
of the famous line: "To be, or not to be: that is the question." A further
jump of imagination might lead you to the idea that the remainder of Hamlet
could then emerge from the random strokes. But any sober calculation of the
odds reveals that the chances of producing a play or even a sonnet in this
way are hopeless, even if every atom of material on Earth were a typewriter
that had been turning out text without interruption for the past four and
a half billion years.
Other thinkers, including a number of religious people,
have argued that the formation of life from nonliving matter by natural means
is impossible. They cite the Second Law of
Thermodynamics, and claim that the formation of organized matter from
disorganized matter is forbidden by it. But the Second Law applies only
to sealed-in (closed) systems. It does
not forbid nonliving chemicals on Earth to absorb energy from an outside
source such as the sun and becoming more organized. The gain in organization
here would be balanced by a greater loss of organization in the sun, and
the Second Law would thus be satisfied.
When chemical systems absorb energy, however, they usually use it to heat
themselves up, or to form new bonds in ways that do not lead to any gain
in organization. We don't know the key recipe-the set of special ingredients
and forms of energy that could lead chemical systems up the ladder of
organization on the first steps to life. These circumstances may be quite
rare and difficult or, once you have grasped the trick, as simple as brewing
beer or fermenting wine.
How can we find out more? One way would be to run more prebiotic experiments.
Many have been run, of course, but they usually have searched for the chemicals
present in life today, rather than seeking to identify the process of
self-organization. It isn't likely that the highly evolved biochemicals of
today-proteins, nucleic acids, and other complexities-were present during
the first faltering steps to life. What we need is more understanding of
how simple chemical substances such as minerals, soaps, and components of
the air will behave when exposed to an abundant and continuing supply of
energy, such as ultraviolet light. Would the material simply turn into a
tar or dissipate the energy as heat? Most of the time this would happen,
and little of importance would have been learned. But perhaps, if the right
mixture were chosen, complex chemical cycles would establish themselves and
continue to evolve. If so, we would have gained an important clue about the
beginning of life. Some experiments of this type could be carried out in
undergraduate and even high-school laboratories, as they would not require
complex and costly equipment. This area remains one where amateurs could
make a significant contribution to fundamental science.
Another scientific approach to seeking our origins is very expensive, but
it would provide great excitement and inspiration along the way. In the past
generation, we have developed the ability to explore our
solar system, but not good enough reasons to
ensure that this is done. By that I mean objectives that would capture the
attention of the public and motivate them to support the costs, rather than
ones of interest only to scientists deeply immersed in their specialities.
The solar system offers a dazzling array of worlds, each containing different
chemical systems that have been exposed to energy for billions of years.
Some of them may have developed in the direction of organization. By discovering
a system that has started on such a path, even a different path from the
one followed on our own planet, we may get vital clues about the principles
involved in self-organization and the nature of our own first steps. A treasure
hunt of this type among the worlds surrounding our sun might or might not
discover early forms of life, but would certainly put some life back into
our space program.
"Where do we come from?" In my title, I put the origin of life in terms of
a question of location, as a child might do. Some scientists have argued
that life started elsewhere, then migrated to planet Earth. Even if this
were so, that fact would still not solve the central question, which is one
of mechanism: "How did we come to be?" Location may be critical, though,
in another way: To learn how we started, even if it happened here, we may
have to venture out into the greater universe that awaits us.
ROBERT SHAPIRO is professor of chemistry at New York University. He is author
or coauthor of over ninety articles, primarily in the area of
DNA chemistry. In particular, he and his coworkers
have studied the ways in which environmental chemicals can damage our hereditary
material, causing changes that can lead to mutations and cancer. His research
has been supported by numerous grants from the National Institutes of Health;
the Department of Energy; the National Science Foundation; and other
organizations.
In addition to his research, Professor Shapiro has written three books for
the general public. The topics have included the extent of life in the universe
(Life Beyond Earth, with Gerald Feinberg); the origin of life on earth
(Origins: A Skeptic's Guide to the Creation of Life on Earth); and
the current effort to read the human genetic message (The Human
Blueprint).
Further Reading
How Things Are: A Science Toolkit for the Mind
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