Katherine Taylor
for Quanta Magazine

Why does life exist?

Popular hypotheses credit a primordial soup, a bolt of lightning,
and a colossal stroke of luck.

But if a provocative new theory is correct, luck may have little
to do with it. Instead, according to the physicist proposing the
idea, the origin and subsequent evolution of life follow from the
fundamental laws of nature and “should be as unsurprising as
rocks rolling downhill.”

From the standpoint of physics, there is one essential difference
between living things and inanimate clumps of carbon atoms: The
former tend to be much better at capturing energy from their
environment and dissipating that energy as heat.

Jeremy
England, a 31-year-old assistant professor at the
Massachusetts Institute of Technology, has derived a mathematical
formula that he believes explains this capacity. The formula,
based on established physics, indicates that when a group of
atoms is driven by an external source of energy (like the sun or
chemical fuel) and surrounded by a heat bath (like the ocean or
atmosphere), it will often gradually restructure itself in order
to dissipate increasingly more energy. This could mean that under
certain conditions, matter inexorably acquires the key physical
attribute associated with life.

Kristian
Peters

“You start with a random clump of atoms, and if you shine light
on it for long enough, it should not be so surprising that you
get a plant,” England said.

England’s theory is meant to underlie, rather than replace,
Darwin’s theory of evolution by natural selection, which provides
a powerful description of life at the level of genes and
populations. “I am certainly not saying that Darwinian ideas are
wrong,” he explained. “On the contrary, I am just saying that
from the perspective of the physics, you might call Darwinian
evolution a special case of a more general phenomenon.”

His idea, detailed in a paper and further elaborated
in a talk he is delivering at universities
around the world, has sparked controversy among his colleagues,
who see it as either tenuous or a potential breakthrough, or
both.

England has taken “a very brave and very important step,” said
Alexander Grosberg, a professor of physics at New York University
who has followed England’s work since its early stages. The “big
hope” is that he has identified the underlying physical principle
driving the origin and evolution of life, Grosberg said.

“Jeremy is just about the brightest young scientist I ever came
across,” said Attila Szabo, a biophysicist in the Laboratory of
Chemical Physics at the National Institutes of Health who
corresponded with England about his theory after meeting him at a
conference. “I was struck by the originality of the ideas.”

Others, such as Eugene Shakhnovich, a professor of chemistry,
chemical biology and biophysics at Harvard University, are not
convinced. “Jeremy’s ideas are interesting and potentially
promising, but at this point are extremely speculative,
especially as applied to life phenomena,” Shakhnovich said.

England’s theoretical results are generally considered valid. It
is his interpretation — that his formula represents the driving
force behind a class of phenomena in nature that includes life —
that remains unproven. But already, there are ideas about how to
test that interpretation in the lab.

“He’s trying something radically different,” said Mara Prentiss,
a professor of physics at Harvard who is contemplating such an
experiment after learning about England’s work. “As an organizing
lens, I think he has a fabulous idea. Right or wrong, it’s going
to be very much worth the investigation.”

Courtesy of Jeremy
England

At the heart of England’s idea is the second law of
thermodynamics, also known as the law of increasing entropy or
the “arrow of time.” Hot things cool down, gas diffuses through
air, eggs scramble but never spontaneously unscramble; in short,
energy tends to disperse or spread out as time progresses.
Entropy is a measure of this tendency, quantifying how dispersed
the energy is among the particles in a system, and how diffuse
those particles are throughout space. It increases as a simple
matter of probability: There are more ways for energy to be
spread out than for it to be concentrated.

Thus, as particles in a system move around and interact, they
will, through sheer chance, tend to adopt configurations in which
the energy is spread out. Eventually, the system arrives at a
state of maximum entropy called “thermodynamic equilibrium,” in
which energy is uniformly distributed. A cup of coffee and the
room it sits in become the same temperature, for example.

As long as the cup and the room are left alone, this process is
irreversible. The coffee never spontaneously heats up again
because the odds are overwhelmingly stacked against so much of
the room’s energy randomly concentrating in its atoms.

Although entropy must increase over time in an isolated or
“closed” system, an “open” system can keep its entropy low — that
is, divide energy unevenly among its atoms — by greatly
increasing the entropy of its surroundings. In his influential
1944 monograph “What Is Life?” the eminent quantum physicist
Erwin Schrödinger argued that this is what living things must do.
A plant, for example, absorbs extremely energetic sunlight, uses
it to build sugars, and ejects infrared light, a much less
concentrated form of energy. The overall entropy of the universe
increases during photosynthesis as the sunlight dissipates, even
as the plant prevents itself from decaying by maintaining an
orderly internal structure.

Life does not violate the second law of thermodynamics, but until
recently, physicists were unable to use thermodynamics to explain
why it should arise in the first place. In Schrödinger’s day,
they could solve the equations of thermodynamics only for closed
systems in equilibrium. In the 1960s, the Belgian physicist Ilya
Prigogine made progress on predicting the behavior of open
systems weakly driven by external energy sources (for which he
won the 1977 Nobel Prize in chemistry). But the behavior of
systems that are far from equilibrium, which are connected to the
outside environment and strongly driven by external sources of
energy, could not be predicted.

This situation changed in the late 1990s, due primarily to the
work of Chris Jarzynski, now at the University of Maryland, and
Gavin Crooks, now at Lawrence Berkeley National Laboratory.
Jarzynski and Crooks showed that the entropy produced by a
thermodynamic process, such as the cooling of a cup of coffee,
corresponds to a simple ratio: the probability that the atoms
will undergo that process divided by their probability of
undergoing the reverse process (that is, spontaneously
interacting in such a way that the coffee warms up). As entropy
production increases, so does this ratio: A system’s behavior
becomes more and more “irreversible.” The simple yet rigorous
formula could in principle be applied to any thermodynamic
process, no matter how fast or far from equilibrium. “Our
understanding of far-from-equilibrium statistical mechanics
greatly improved,” Grosberg said. England, who is trained in both
biochemistry and physics, started his own lab at MIT two years
ago and decided to apply the new knowledge of statistical physics
to biology.

Using Jarzynski and Crooks’ formulation, he derived a
generalization of the second law of thermodynamics that holds for
systems of particles with certain characteristics: The systems
are strongly driven by an external energy source such as an
electromagnetic wave, and they can dump heat into a surrounding
bath. This class of systems includes all living things. England
then determined how such systems tend to evolve over time as they
increase their irreversibility. “We can show very simply from the
formula that the more likely evolutionary outcomes are going to
be the ones that absorbed and dissipated more energy from the
environment’s external drives on the way to getting there,” he
said. The finding makes intuitive sense: Particles tend to
dissipate more energy when they resonate with a driving force, or
move in the direction it is pushing them, and they are more
likely to move in that direction than any other at any given
moment.

“This means clumps of atoms surrounded by a bath at some
temperature, like the atmosphere or the ocean, should tend over
time to arrange themselves to resonate better and better with the
sources of mechanical, electromagnetic or chemical work in their
environments,” England explained.

Courtesy of Michael
Brenner/Proceedings of the National Academy of
Sciences

Self-replication (or reproduction, in biological terms), the
process that drives the evolution of life on Earth, is one such
mechanism by which a system might dissipate an increasing amount
of energy over time.

As England put it, “A great way of dissipating more is to make
more copies of yourself.”

In a September paper in the Journal of
Chemical Physics, he reported the theoretical minimum amount of
dissipation that can occur during the self-replication of RNA
molecules and bacterial cells, and showed that it is very close
to the actual amounts these systems dissipate when replicating.

He also showed that RNA, the nucleic acid that many scientists
believe served as the precursor to DNA-based life, is a
particularly cheap building material. Once RNA arose, he argues,
its “Darwinian takeover” was perhaps not surprising.

The chemistry of the primordial soup, random mutations,
geography, catastrophic events and countless other factors have
contributed to the fine details of Earth’s diverse flora and
fauna. But according to England’s theory, the underlying
principle driving the whole process is dissipation-driven
adaptation of matter.

This principle would apply to inanimate matter as well. “It is
very tempting to speculate about what phenomena in nature we can
now fit under this big tent of dissipation-driven adaptive
organization,” England said. “Many examples could just be right
under our nose, but because we haven’t been looking for them we
haven’t noticed them.”

Scientists have already observed self-replication in nonliving
systems. According to new research led by Philip Marcus of the
University of California, Berkeley, and reported in Physical Review Letters in
August, vortices in turbulent fluids spontaneously replicate
themselves by drawing energy from shear in the surrounding fluid.
And in a paper in Proceedings of the National
Academy of Sciences, Michael Brenner, a professor of applied
mathematics and physics at Harvard, and his collaborators present
theoretical models and simulations of microstructures that
self-replicate. These clusters of specially coated microspheres
dissipate energy by roping nearby spheres into forming identical
clusters. “This connects very much to what Jeremy is saying,”
Brenner said.

Besides self-replication, greater structural organization is
another means by which strongly driven systems ramp up their
ability to dissipate energy. A plant, for example, is much better
at capturing and routing solar energy through itself than an
unstructured heap of carbon atoms. Thus, England argues that
under certain conditions, matter will spontaneously
self-organize. This tendency could account for the internal order
of living things and of many inanimate structures as well.
“Snowflakes, sand dunes and turbulent vortices all have in common
that they are strikingly patterned structures that emerge in
many-particle systems driven by some dissipative process,” he
said. Condensation, wind and viscous drag are the relevant
processes in these particular cases.

“He is making me think that the distinction between living and
nonliving matter is not sharp,” said Carl Franck, a biological physicist at Cornell
University, in an email. “I’m particularly impressed by this
notion when one considers systems as small as chemical circuits
involving a few biomolecules.”

Wilson Bentley

England’s bold idea will likely face close scrutiny in the coming
years.

He is currently running computer simulations to test his theory
that systems of particles adapt their structures to become better
at dissipating energy. The next step will be to run experiments
on living systems.

Prentiss, who runs an experimental biophysics lab at Harvard,
says England’s theory could be tested by comparing cells with
different mutations and looking for a correlation between the
amount of energy the cells dissipate and their replication rates.

“One has to be careful because any mutation might do many
things,” she said. “But if one kept doing many of these
experiments on different systems and if [dissipation and
replication success] are indeed correlated, that would suggest
this is the correct organizing principle.”

Brenner said he hopes to connect England’s theory to his own
microsphere constructions and determine whether the theory
correctly predicts which self-replication and self-assembly
processes can occur — “a fundamental question in science,” he
said.

Having an overarching principle of life and evolution would give
researchers a broader perspective on the emergence of structure
and function in living things, many of the researchers said.
“Natural selection doesn’t explain certain characteristics,” said
Ard Louis, a biophysicist at Oxford University, in an email.
These characteristics include a heritable change to gene
expression called methylation, increases in complexity in the absence of natural
selection, and certain molecular changes Louis has recently
studied.

If England’s approach stands up to more testing, it could further
liberate biologists from seeking a Darwinian explanation for
every adaptation and allow them to think more generally in terms
of dissipation-driven organization. They might find, for example,
that “the reason that an organism shows characteristic X rather
than Y may not be because X is more fit than Y, but because
physical constraints make it easier for X to evolve than for Y to
evolve,” Louis said.

“People often get stuck in thinking about individual problems,”
Prentiss said. Whether or not England’s ideas turn out to
be exactly right, she said, “thinking more broadly is where many
scientific breakthroughs are made.”

Emily Singer

contributed
reporting.