Can
quantum biology help us to understand what distinguishes a bunch of
molecules from a living organism?
In
the beginning, during the first billion years (Gyr) after the Earth
was formed about 4.5 Gyr ago, intense meteor bombardment left little
remains of the original crustal rocks. High energy collisions with
meteorites up to 500 km in diameter periodically disintegrated newly
formed continental crust and vapourised early oceans, annihilating
primitive life forms possibly existing near the surface.
Gene
sequencing suggests that the most primitive known domains of life,
namely bacteria and archaea, have been evolving separately for as
long as 4 Gyr. Deep-sea volcanic vents are candidates for supporting
such early life. The oldest fossils of single-celled microbes are
dated at around 3.5 Gyr, while the oldest evidence of the more
complex multicellular precursors of plants and animals is more
recent, at around 2 Gyr. Structurally, it appears that this
complexity in fact arose as the happy endosymbiotic result of the
invasion of the archaean cell by bacteria!
In
the meantime, life has become so prolific and so complex that human
life forms have taken up the study of life and its origins. But are
we any closer to an answer? Can we state exactly what it is that
distinguishes a bunch of molecules from a living organism?
An
intriguing experiment by Miller and Urey in 1953 showed that
electrical activity in a gaseous mixture of methane, ammonia, water
and hydrogen can produce amino acids: the building blocks of
proteins. In spite of more recent and remarkable work in viable DNA
design, no experiment has yet been able to synthesise from basic
components an object that has the characteristics of a living cell.
All
living systems are made up of molecules, and the properties of
molecules are given by quantum mechanics, our most successful and
fundamental theory to date. Living systems are necessarily open
systems constantly exchanging energy and matter with the environment
in order to maintain the non-equilibrium state synonymous with
living. While living systems are therefore fundamentally open quantum
systems, the level of complexity typical of biological systems poses
a huge computational challenge to such a fundamental description.
Furthermore, many of the processes associated with life are
sufficiently described by Newtonian physics.
Quantum
biology is the applied science of open quantum systems to those
aspects of biology where a description in terms of Newtonian physics
is insufficient. An important question is whether quantum theory can
add anything to biology: We know that molecules are ultimately
described by quantum chemistry, but can such a description help us to
understand life itself?
The
most well-established area in quantum biology is the study of aspects
of one of life's oldest processes: photosynthesis. While evidence of
quantum mechanical tunneling in electron transfer in purple bacteria
was first reported almost half a century ago, more recently the
detection of quantum coherence in energy transfer in green sulphur
bacteria and marine algae has contributed to a revival of interest in
the possibility that the optimality of some biological processes is
due to a sustenance of quantum effects in the warm, wet and noisy
environments typical of living systems.
As
theorists, we are working hard to keep up. Our research in Durban,
with collaborators in Singapore and Amsterdam, has involved the
application of open quantum systems models of energy transfer to the
photosynthetic process, showing how interaction with an environment
can in fact enhance transport efficiency. More recently, we have
proposed that quantum
spin plays a direct role in reducing the
yield of potentially destructive states
during charge transfer in photosynthesis, constituting a new example
of a quantum mechanical protective mechanism in a living organism.
Given
that the simplest living systems exhibit functional complexity of a
quantum nature when probed at the limits of our instrumentation, that
far more complex animals are able to sense subtle changes in their
environments with an accuracy described by quantum mechanics, should
come as no surprise. The proposal that navigation in the Earth's
magnetic field, as well as our senses of vision and smell, and also
our cognition, require quantum mechanical description, are exciting
developing areas of quantum biology.
The
highest achievement of quantum biology would be a contribution to a
scientific understanding of what distinguishes a living system from
the inanimate matter from which it is constructed, i.e. a theory of
life. The test of such a theory would be the synthesis of life
itself. In the absence of such a theory and its confirmation, outside
of famous works of fiction, quantum biology will, for now, have to
fulfill a more practical role.
The
primary importance of the field of quantum biology, in its present
state, lies in the identification and mimicry of the ingenious feats
of engineering taking place in systems ranging from bacteria to
birds. If non-trivial quantum effects on a macroscopic scale play a
role in getting the job done better in certain processes perfected
over billions of years at physiological temperatures and in immensely
complex systems, then there exists before our very eyes a wealth of
information in the biological world from which to draw inspiration
for our own technologies.
Synthetic
biology is gathering momentum to become the next big thing in
science, with biologically-inspired quantum artificial photosynthetic
systems
promising to contribute to the development of the kind of renewable
energy technologies essential for our continued existence on this
planet (and perhaps others!),
and this is just the beginning.
As
far as understanding what life is, however, we are limited by a lack
of precise knowledge of the conditions under which life emerged on
Earth, in a possibly singular event. Barring the sudden discovery of
evidence of life on Mars by the Curiosity rover or a roving Mars One
colonist, for now, we will have to be satisfied with a definition of
life as the continual state of change preceding death, and with the
knowledge that the rabbit hole goes as least as deep as we are
prepared to venture.
