The origin of life is one of the great outstanding mysteries of
science.
How did a non-living mixture of molecules transform themselves
into a
living organism? What sort of mechanism might be responsible?
A century and a half ago,
Charles Darwin
produced a convincing explanation
for how life on Earth evolved from
simple microbes to the complexity of the
biosphere today, but he
pointedly left out how life got started in the first place.
"One might
as well speculate about the origin of matter," he quipped.
But that did
not stop generations of scientists from investigating the puzzle.
The
problem is, whatever took place happened billions of years ago, and all
traces long ago vanished – indeed, we may never have a blow-by-blow
account of the process. Nevertheless we may still be able to answer the
simpler question of whether life's origin was a freak series of events
that
happened only once, or an almost inevitable outcome of
intrinsically
life-friendly laws. On that answer hinges the question of
whether we are
alone in the universe, or whether our galaxy and others
are teeming with life.
Most research into life's murky origin has
been carried out by chemists.
They've tried a variety of approaches in
their attempts to recreate the
first steps on the road to life, but
little progress has been made. Perhaps that
is no surprise, given life's
stupendous complexity. Even the simplest bacterium
is incomparably more
complicated than any chemical brew ever studied.
But a more
fundamental obstacle stands in the way of attempts to cook up
life in
the chemistry lab. The language of chemistry simply does not mesh
with
that of biology. Chemistry is about substances and how they react,
whereas biology appeals to concepts such as information and
organisation.
Informational narratives permeate biology. DNA is
described as a genetic
"database", containing "instructions" on how to
build an organism. The genetic
"code" has to be "transcribed" and
"translated" before it can act. And so on.
If we cast the problem of
life's origin in computer jargon, attempts at chemical
synthesis focus
exclusively on the hardware – the chemical substrate of life
– but
ignore the software – the informational aspect. To explain how life
began
we need to understand how its unique management of information
came about.
In the 1940s, the mathematician
John von Neumann
compared life to a
mechanical constructor, and set out the logical
structure required for a
self-reproducing automaton to replicate both
its hardware and software.
But Von Neumann's analysis remained a
theoretical curiosity. Now a new
perspective has emerged from the work
of engineers, mathematicians and
computer scientists, studying the way
in which information flows through
complex systems such as communication
networks with feedback loops, logic
modules and control processes. What
is clear from their work is that the dynamics
of information flow
displays generic features that are independent of the specific
hardware
supporting the information.
Information theory has been
extensively applied to biological systems at many
levels from genomes to
ecosystems, but rarely to the problem of how life actually
began. Doing
so opens up an entirely new perspective on the problem. Rather than
the
answer being buried in some baffling chemical transformation, the key
to life's
origin lies instead with a transformation in the organisation
of information flow.
Sara Walker, a Nasa astrobiologist working at
Arizona State University, and I
have proposed that the significant
property of biological information is not its
complexity, great though
that may be, but the way it is organised hierarchically.
In all physical
systems there is a flow of information from the bottom upwards,
in the
sense that the components of a system serve to determine how the system
as a whole behaves. Thus if a meteorologist wants to predict the
weather, he may
start with local information, such as temperature and
air pressure, taken at various
locations, and calculate how the weather
system as a whole will move and change.
In living organisms, this
pattern of bottom-up information flow mingles with the inverse
- top-down information flow – so that what happens at the local level can
depend
on the global environment, as well as vice versa.
To take a
simple example; whether a cell expresses a gene can depend on
mechanical stresses or electric fields acting on the whole cell by its
environment.
Thus, a change in global information (a pattern of force)
at the macroscopic level
translates into a change in local information
movement at the microscopic level
(switching on a gene). More generally,
a range of signals received from its environment
help to dictate how a
cell's DNA is distributed and transcribed. Walker and I propose
that the
key transition on the road to life occurred when top-down information
flow first predominated. Based on simple mathematical models, we think
it may have happened
suddenly, analogously to a heated gas abruptly
bursting into flame.
There is a second distinctive way in which
life handles information processing. The
language of genes is digital,
consisting of discrete bits, cast in the language of a
four-letter
alphabet. By contrast, chemical processes are continuous. Continuous
variables
can also process information – so-called analogue computers
work that way – but less reliably than digital. Whatever chemical system
spawned life, it had to feature a transition from analogue to digital.
The
way life manages information involves a logical structure that differs
fundamentally
from mere complex chemistry. Therefore chemistry alone
will not explain life's origin,
any more than a study of silicon, copper
and plastic will explain how a computer can
execute a program. Our work
suggests that the answer will come from taking
information seriously as
a physical agency, with its own dynamics and causal
relationships
existing alongside those of the matter that embodies it – and that
life's origin can ultimately be explained by importing the language and
concepts
of biology into physics and chemistry, rather than the other
way round.
