Jeremy England, a 31-year-old
physicist at MIT, thinks he has found the underlying physics driving the origin
and evolution of life.
Katherine Taylor for Quanta Magazine
January 22, 2014
Comments (43)
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
Cells from the moss Plagiomnium affine with visible
chloroplasts, organelles that conduct photosynthesis by capturing sunlight.
“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 recent 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
A computer simulation by Jeremy England and colleagues
shows a system of particles confined inside a viscous fluid in which the
turquoise particles are driven by an oscillating force. Over time (from top to
bottom), the force triggers the formation of more bonds among the particles.
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-Replicating Sphere Clusters: According to new research at Harvard, coating the
surfaces of microspheres can cause them to spontaneously assemble into a chosen
structure, such as a polytetrahedron (red), which then triggers nearby spheres
into forming an identical structure.
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 appearing online this week 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
If a new theory is correct, the same physics it
identifies as responsible for the origin of living things could explain the
formation of many other patterned structures in nature. Snowflakes, sand dunes
and self-replicating vortices in the protoplanetary disk may all be examples of
dissipation-driven adaptation.
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.”
Correction: This article was revised on January 22,
2014, to reflect that Ilya Prigogine won the Nobel Prize in chemistry, not
physics.
