Adelaide
ARE we alone in the Universe? When I was a student in the 1960s I was
convinced that the answer was 鈥渘o鈥�. This put me at odds with the prevailing
scientific view. The orthodox position at that time was summed up by the French
biologist Jacques Monod when he wrote of the 鈥渦nfeeling immensity of the
Universe鈥�, and declared that we had emerged from it alone and by pure chance. It
was an opinion echoed by many other leading scientists. American palaeontologist
George Gaylord Simpson, one of the giants of modern biology, described attempts
to search for life elsewhere in the Universe鈥攅specially intelligent
life鈥攁s 鈥渁 gamble of the most adverse odds in history鈥�.
Thirty years on, there has been a remarkable U-turn. Take Christian de Duve
who, like Monod, is a Nobel prizewinning biologist. In his book Vital
Dust, published in 1995, de Duve suggests that life is 鈥渁 cosmic
imperative鈥� bound to arise wherever conditions allow. His stance is shared by
many at NASA, whose astrobiology programme is dedicated to seeking out alien
life forms. Meanwhile, a team of enthusiastic astronomers sponsored by the SETI
Institute in California is sweeping the sky with radio telescopes in the hope of
stumbling across a message from ET. Journalists, Hollywood producers and
schoolchildren likewise assume that the Universe is teeming with life.
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This shift in opinion has little to do with advances in understanding. True,
we now have concrete evidence of planets in other star systems, but most
astronomers believed all along that they were there. Biochemists have inched
forward in their attempts to synthesise the building blocks of life, but
creating life in a test tube remains a distant dream. We may soon discover
evidence for past life on Mars, but if so it will almost certainly have arrived
there from Earth, in rocks blasted off our planet by large asteroid impacts
(快猫短视频, 12 September 1998, p 24).
Yet the question of whether life is widespread in the Universe is important.
Researchers are making plans to search for Earth-like planets around other
stars, chiefly because they hope to find alien life there
(see 鈥淎lien haven鈥�). The
assumption that life should arise inevitably given Earth-like conditions is
known as biological determinism. But it is hard to find any support for it in
the known laws of physics, chemistry or biology. If we relied solely on these
laws to explain the workings of the Universe, it would be reasonable to conclude
like Monod that life can only have arisen by sheer good luck鈥攁nd that it
is therefore exceedingly unlikely to be found elsewhere. But those hoping to
encounter aliens need not despair: an exciting new field of research may yet
justify the theory of biological determinism, and thereby boost our chances of
finding neighbours somewhere in the cosmos.
The idea of biological determinism received a fillip in 1953, when Harold
Urey and Stanley Miller at the University of Chicago tried to recreate in a test
tube what they believed to be the conditions of primeval Earth. They found that
amino acids鈥攖he building blocks of proteins鈥攚ere part of the
chemical sludge formed when electricity was discharged through a mixture of
gaseous methane, ammonia, water vapour and hydrogen. The Miller-Urey experiment
was hailed as the first step towards the creation of life in the laboratory:
many chemists envisaged 鈥渄estination life鈥� to lie at the end of a long road down
which a chemical soup zapped with energy would be inexorably conveyed by the
passage of time.
Uphill struggle
But this idea did not stand up to scrutiny. Making the building blocks of
life is easy鈥攁mino acids have been found in meteorites and even in outer
space. But just as bricks alone don鈥檛 make a house, so it takes more than a
random collection of amino acids to make life. Like house bricks, the building
blocks of life have to be assembled in a very specific and exceedingly elaborate
way before they have the desired function. To form proteins, many amino acids
must link together in long chains in the right order. In energy terms that is an
鈥渦phill鈥� process.
In itself this is not a problem as there were plentiful energy sources on the
early Earth. The problem is that simply throwing energy willy-nilly at amino
acids will not create delicate chain molecules with highly specific sequences,
but a tarry mess鈥攊n the same way that putting a stick of dynamite under a
pile of bricks won鈥檛 make a house. Somehow the energy has to be fed into the
system in a contrived and particular manner. In a living organism this step is
under the control of the cell鈥檚 molecular machinery, with its intricate
specifications, but in a jumbled prebiotic chemical soup, the amino acids would
have to take pot luck. So while amino acids are written into the laws of nature,
large and highly specialised molecules such as proteins are certainly not.
We now know that the secret of life lies not with the chemical ingredients as
such, but with the logical structure and organisational arrangement of the
molecules. So DNA is a genetic databank, and genes are instructions for making
customised proteins and, indirectly, other biological molecules. Like a
supercomputer, life is an information-processing system, which implies a special
sort of organised complexity. It is the information content, or software, of the
living cell that is the real mystery, not the hardware components.
Nothing better illustrates the computational prowess of life than the genetic
code. All known life is based on a deal struck between nucleic acids and
proteins鈥攖wo classes of molecule that from a chemical point of view are
scarcely on nodding terms. The nucleic acids DNA and RNA store instructions, and
proteins do most of the work. Together these molecules perform life鈥檚 many
miracles, but on their own they are helpless. To manufacture proteins, nucleic
acids employ a clever intermediary to form a coded information channel. It
works like this. DNA, the famous double helix, is built like a ladder with four
different kinds of rung. The information is stored in the sequences of these
rungs, just as an instruction manual records information in sequences of
letters. Proteins are built from 20 different amino acids, and the right protein
is made only if the amino acids are linked together in the right order.
To translate from the four-letter alphabet used by DNA into the twenty-letter
system used by proteins, all Earth life uses the same code. The key question
when it comes to the inevitability鈥攐r otherwise鈥攐f life is how this
ingenious system of coding emerged? How did stupid atoms spontaneously write
their own software, and where did the very peculiar form of information needed
to get the first living cell up and running come from?
Nobody knows, but scientists have traditionally divided into two camps on the
issue. In one group are those who believe it all happened by chance鈥攖hat
life is the result of a stupendous chemical fluke. That was Monod鈥檚 view. It is
easy to work out the odds against a random chemical mixture just happening to
shuffle the appropriate molecules into the elaborate arrangement needed. The
numbers are breathtakingly huge. If life as we know it arose by chance, it will
have happened only once in the observable Universe.
By contrast, biological determinists assume that chance is secondary, and
that the right sorts of molecule obligingly form as a result of the laws of
nature. American biogenesis pioneer Sidney Fox, for example, claimed that
chemistry prefers to link up amino acids in precisely the right combinations to
make them biologically functional. If so, it is as if there is an in-built
bias鈥攅ven a conspiracy鈥攊n nature to create life-encouraging
substances. But is it credible that the laws of physics and chemistry contain a
blueprint for life? How would the crucial information content of life be encoded
in those laws?
To address this question, we need to think more carefully about the nature of
the information that underpins living things. One important observation is that
a structure that is rich in information tends to lack patterns. This property is
illustrated most clearly by a branch of mathematics known as algorithmic
information theory, which seeks to quantify the complexity of information by
treating it as the output of a computer program, or algorithm. Consider the
binary sequence 10101010101010101010 . . . This can be generated by the simple
command 鈥淧rint 10 n times.鈥� The input instructions are far shorter than
the output sequence, reflecting the fact that the output contains a repeating
pattern, which is easy to describe compactly. For this reason, the output has
very little information content. By contrast, an apparently random sequence
such as 110101001010010111. . . cannot be condensed into a simple set of
instructions, so it has a high information content. If the job of DNA is to
store information efficiently, it had better not contain too many patterns in
the sequence of 鈥渞ungs鈥�, since patterns represent informational redundancy.
Biochemists confirm this expectation. The genomes of organisms that have been
sequenced so far mostly look like random jumbles of the four constituent
letters.
The higgledy-piggledy nature of genome sequences runs counter to biological
determinism. The laws of physics can be used to predict ordered structures, but
not random ones. A crystal, for instance, is simply a regular array of atoms
with a periodic structure, like the repeating binary sequence given above, and
is thus almost devoid of information. The construction of crystals is built into
the laws of physics, as their periodic forms are determined by the mathematical
symmetries inherent in those laws. But the random sequences of amino acids in
proteins, or the series of 鈥渞ungs鈥� in the DNA ladder, cannot be 鈥渋n鈥� the laws of
physics, any more than houses are.
Nor can it be 鈥渋n鈥� the laws of chemistry. A direct illustration of this fact
comes from examining the structure of DNA. Each rung of the ladder is made up of
two segments, which couple together snugly like a lock and key. Ultimately,
chemistry determines the nature of the bonds that hold together the segments,
and also the forces that attach them to the sides of the ladder. However, there
are no chemical bonds between successive rungs. Chemistry doesn鈥檛 care about the
order of the rungs, and life is free to change them on a whim. Just as the
sequence of letters in an instruction manual is independent of the chemistry of
the paper and ink, so the 鈥渓etters鈥� in DNA鈥攚hich make up the
information鈥攁re independent of the chemical properties of nucleic acid. It
is this ability of life to free itself from the strictures of chemistry that
bestows upon it such power and versatility. Biological determinism would imply a
chemical straitjacket that would serve only to inhibit, not enhance, biological
creativity.
If life represents an escape from chemistry, we cannot appeal to chemistry to
explain life. But where else might an explanation lie? Life is ultimately about
complex information processing, so it makes sense to seek a solution in the
realm of information theory and complexity theory. Since biological information
is not encoded in the laws of physics and chemistry (at least as currently
known), where does it come from? There seems to be agreement that information
cannot come into existence spontaneously (except perhaps in the big bang), so
the information content of living systems must somehow originate in their
environment. Although there is no known law of physics able to create
information from nothing, there might be some sort of principle that could
explain how information can be garnered from the environment and accumulated in
macromolecules.
One way to do this is by Darwinian evolution. Life on Earth started with
simple organisms possessing short genomes with a relatively low information
content. More complex organisms have longer genomes storing more information.
The added information has flowed from the environment into the genomes by the
process of natural selection: whenever a selection among alternative genomes is
made鈥攁ccording to the degree of 鈥渇itness鈥� they confer on their
owners鈥攊nformation is gained. So Darwinism can explain how organisms
acquire information. But Darwinism kicks in only when life is already under way.
How can we appeal to natural selection in the prebiotic phase?
Some biochemists believe that a form of molecular Darwinism is the answer.
They envisage replicating molecules in some sort of chemical soup. Although bare
replicating molecules may not satisfy most people鈥檚 intuitive definition of
life, they can still undergo a type of Darwinian evolution if they are subject
to variation and selection. Proponents of this Darwinism-all-the-way-down theory
suppose that the first replicator molecule was simple enough to form purely by
chance.
The trouble is that the only experience we have of replicating molecules is
of those used by life. It is extremely unlikely that DNA would form by chance.
Even its simpler cousin, RNA, is hard to make in long enough strands to be
biologically potent. And shorter nucleic acid molecules tend to make more errors
when replicating. If the error rate gets too high, information leaks away faster
than selection can inject it, and evolution grinds to a halt. Far from
accumulating information, an error-prone molecule will shed it.
So for molecular Darwinism to work, nature must obligingly providereplicators simple enough to form by chance, deft enough to replicate accurately
and with a huge range of variants鈥攚hich are also good
replicators鈥攆or selection to act upon. These need not be nucleic acids,
but to explain life as we know it they would eventually have to make nucleic
acids and hand over the replicating function to them. In effect, molecular
Darwinism still smuggles in biological determinism. Not only must the laws of
nature imply the existence of molecules possessing all the above properties, but
the evolutionary pathway that the population of replicators follows must also
lead to nucleic acids. Otherwise life as we know it would still be a tremendous
fluke.
So should we concede that life is the result of an exceedingly unlikely
chemical accident, a chance event unique in the entire Universe? Not
necessarily. A type of biological determinism may still be true, even if life
isn鈥檛 written into the familiar laws of physics, chemistry and evolutionary
theory. It may be that these laws can account for life鈥檚 hardware, that is, the
raw materials, but the vital software, or informational component, derives from
the laws of information theory.
The concept of 鈥渋nformation鈥� is admittedly rather woolly, though this is
usual when a subject is in its infancy. Two centuries ago, energy was an equally
vague notion. 快猫短视频s intuitively recognised it as significant in physical
processes, but it lacked mathematical rigour. Today, we accept energy as a real
and fundamental quantity, because it is well understood. Information remains
bewildering, partly because it crops up in different guises in so many
scientific fields. In relativity theory, it is information that is forbidden to
travel faster than light. In quantum mechanics, the state of a system is
described by its maximum information content. In thermodynamics, information
falls as entropy rises. In biology, a gene is a set of instructions containing
the information needed to execute some task.
What we know about information comes mainly from the realm of human
discourse. A landmark study in information theory was an analysis of
communication over noisy radio channels conducted by American electrical engineer
Claude Shannon during the Second World War. But nobody has yet written down the
equivalent of Newton鈥檚 laws for informational dynamics. 快猫短视频s can鈥檛 even
agree on whether information is invariably conserved in physical processes. For
years, debate has raged over what happens to the information in a star when it
collapses to form a black hole, which subsequently evaporates. Is the
information irreversibly lost, or does it somehow get back out again?
One new area of research, however, offers a tantalising pointer. Until
recently, biochemists treated life鈥檚 molecules as little blocks that stick
together. In reality, molecular structure and bonding are subject to quantum
mechanics. Now physicists have extended the concept of information to the
quantum domain, and made some extraordinary discoveries. One of these is the
ability of quantum systems to process information exponentially faster than
classical systems鈥攁 property that lies behind the quantum computer.
The riddle of biogenesis is essentially computational in
nature鈥攄iscovering a very special type of molecular system from among a
vast decision tree of chemical alternatives, most branches of which represent
biological duds. Could it be that the key initial steps in 鈥渋nforming鈥� matter
and setting it on the road to life lie in the offbeat realm of quantum physics?
If so, biological determinism might at last receive a convincing theoretical
underpinning, justifying the popular belief that we inhabit a bio-friendly
Universe in which we are not alone.