Will We Figure Out How Life Began?
We may determine what started it all—but that might not tell us
whether life was inevitable or just a lucky accident.
by Stephen Jay Gould
deep
problem—philosophical rather than factual—stymies all our attempts to
define the nature of life. Scientific generalizations require replication, the
demonstration that a given set of forces and substances will yield the same
result when brought together under the same conditions. Ideally, we test for
replication with time-honored procedures that scientists call controlled
experiments—artificially simplified situations manipulated by human
observers to guarantee (within the best of our ability) an exact repetition of
all timings, forces and substances. If we achieve the same result in each of
several replications, we then gain confidence that we may be witnessing a
predictable generality based upon a law of nature. This search for replicates
underlies the efforts—and partial successes—of scientists to
synthesize living matter from the presumed chemical constituents in the
"primordial soup" of the earth's original oceans. Can we create some
rudimentary forms of life by exposing these constituents to known sources of
energy (lightning from electrical storms, heat from oceanic vents, for
example) under the presumed conditions of the earth's early atmosphere and
surface?
In this context of accepted scientific procedures, single
occurrences present a knotty problem. Their "truth" cannot be denied, but how
can we use their existence to assert any generality rather than an explanation
for a singular circumstance? For specific events of history—the rise,
domination and extinction of dinosaurs, for example—we seek no such
generality, and specific narrations for bounded events supply the explanations
we seek. Thus a particular asteroid, striking the earth 65 million years ago
and leaving evidence of its impact off the Yucatan Peninsula, probably
triggered a global extinction that sealed the fate of dinosaurs and many other
creatures. In developing such evidence, we have explained a unique historical
event, but we have not discovered a general law of nature.
But when we ask questions about the nature of life—when
we wonder, for example, how common life may be in the universe or inquire
whether any potential life on other worlds will look like the life we already
know on Earth—then we are seeking to understand general principles about
the essential character of the natural universe and not simply to explicate a
particular set of historical events. To formulate such general principles, as
I argued above, we need replicates, either made in our laboratories or found
elsewhere in the universe.
The life that we know, however wondrous in extent and
variety, all proceeds—or so our best inferences tell us—from one
single experiment. The biochemical features underlying this amazing variety
and the coherent fossil record of 3.5 billion years (implying a single branching
tree of earthly life with a common trunk) indicate that every living thing on
Earth, from the tiniest bacterium on the ocean floor to the highest albatross
that ever flew in the sky, arose as the magnificently diversified evolutionary
outcome of one single experiment performed by nature, one origin of life in the
early history of one particular planet.
Thus we can define the life we know by specifying its
common features and properties. But if we wish to move from this knowledge to
statements about the general nature of any potential life in the universe, we
remain stymied in two key ways.
First, we can make no reasoned conjecture about the
frequency, or even the existence, of life elsewhere in the universe. As an
optimist by temperament and as a betting man, I allow that certain features
of the natural world would lead me to place my chips on yes if someone forced
me to wager. But I also know the difference between a pure flutter based on
hope and a smart play based on genuine probabilities.
The fact that fossilized life of the simplest bacterial
grade appears in some of the most ancient rocks on Earth suggests that an
origin of life in these conditions may be nearly inevitable, since incredibly
improbable events should not occur so quickly. But my skeptical side retorts
that good luck in one try proves nothing. I may win the lottery the first time
I buy a ticket, and I might flip 10 heads in a row on my first sequence of
tosses.
I might also argue that since our immense universe contains
gazillions of galaxies filled with appropriate stars and planets, and since
life did emerge on the one and only planet we really know, how can we deny
that a sizable proportion of these other planets must also contain life? Yet a
logical fallacy dooms this common argument because either alternative can be
reconciled with the positive result that I must obtain for the only place I
can sample—our Earth. For if all appropriate planets generate some form
of life, then I should not be surprised that I have found living things on my
own world. But if life really exists on my planet alone, then I must still
record a positive result from this only possible sample. After all, I knew
the answer for the earth before I ever formulated my scheme for sampling.
Unfortunately, we are stymied by the fact that our knowledge
about life must, at least for now, be limited to studies of a single experiment
on Earth. All earthly life shares a remarkably complex set of biochemical
features, but does this commonality record the only conceivable building
blocks for any entity that we would call "alive"? Or do all earthly creatures
share these features only because we have inherited these properties from a
common ancestor that used one configuration among a million alternatives
unknown to us but quite conceivable and workable? Indeed, would we, in our
carbon-based parochialism, even recognize otherworldly forms of
life—pulsating sheets of silica, perhaps—well beyond our ken?
The architect of this conceptual prison built only two
doors leading to a solution, with the path to each door marked by the same
sign: FIND A REPLICATE! On one path, we make the replicates ourselves by
gaining such an improved understanding of the nature of things that we can
define the set of all conceivable living forms and then test their properties
by chemical synthesis in our laboratory.
As a natural historian at heart, however, I confess my
strong preference for the second path of exploration: a search for possible
natural occurrences elsewhere. This Columbian path has served us so well
before, and nature's products do tend to outshine our own poor workmanship
by manifesting things undreamed of in all our philosophy. So let us seek
nature's own replicate—on Mars or a few other potential places in our
solar system, if we really luck out (and are willing to content ourselves
with simple things at bacterial grade and unfit for mutual conversation);
or elsewhere, despite daunting distances (beyond any possibility for
two-way conversation during human lifetimes) but promising—in the most
exciting and improbable long shot in all human history—a potential
insight soaring well beyond our meager powers of imagination.
[
Stephen Jay Gould, "Will We Figure Out How Life Began?" April 10, 2000;
Time
magazine, 155 (14): 92-93. ]
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