Researchers at the University of Tokyo have for the first time been able to
create an RNA molecule that replicates, diversifies, and develops complexity,
following Darwinian evolution. This has provided the first empirical evidence
that simple biological molecules can lead to the emergence of complex lifelike
systems.
We just received more evidence that life on Earth may have started with RNA,
with scientists in Japan creating RNA that can replicate, diversify, and
develop complexity all on its own.
Long before Earth had its first budding cells of primordial ooze, it was
awash with a churning organic soup that sat on the brink of something
profound.
That thin line between complex chemistry and the evolution of life
represents a pivotal moment in the emergence of biology. Unfortunately, for
all of its importance, we know very few details about exactly how it
happened.
An experiment conducted by the scientists from the University of Tokyo has
now reinforced the view that RNA's unique talents have what it takes to
explain how life bubbled forth billions of years ago, backing up what's
known as the 'RNA world' hypothesis.
But the research also shows that it might not have happened exactly as we
thought.
Their work shows how a molecule that remains crucial to the survival and
reproduction of every living thing today can inch its way towards an
evolving system if it works as a team.
"We found that the single RNA species evolved into a complex replication
system: a replicator network comprising five types of RNAs with diverse
interactions, supporting the plausibility of a long-envisioned evolutionary
transition scenario," says evolutionary biologist Ryo Mizuuchi.
Stripped to its barest essentials, life is made up of molecules that can
make imperfect copies of themselves, churning out a virtually limitless
population of variants which might (or might not) hold it together long
enough to make copies themselves.
The search for life's origin has in effect been a hunt for candidates that
can carry out this replication task without a supporting cast of highly
specialized organic materials, such as DNA or proteins, to assist.
RNA has long been a frontrunner in this search. It's ubiquitous throughout
the biosphere today, could have been present on ancient Earth as a result of
non-biological processes, can preserve a large amount of information, and
act as a dynamic physical unit.
This means it could potentially make structures that can physically build
new molecules that can in turn build new structures. If this process is
imperfect, some of the 'replicator' structures will do the job faster or
more efficiently than others, becoming the dominant form of RNA ... at
least, until something even better comes along.
As alluring as this idea is, we've known for decades that self-constructed
units of individual RNA molecules are just too simple and too unstable for
such a scenario. Even its deoxygenated sibling, DNA, lacks the grit to hold
itself together long enough for natural selection to get off to a flying
start.
That doesn't mean multiple strands acting as a team couldn't perform the job
instead. Having a handful of different replicative units acting on a
population level just might solve this information problem easily.
Various replicators have been designed around RNA, DNA, and even proteins to
show how this might feasibly work, with researchers going to lengths to
build in functionality that allows the molecule structures to cooperate and
make copies at a suitable rate.
While they can sustain replication, until now none have become more complex
over time, leaving open the question of whether RNA is capable of evolving.
Mizuuchi's team have cracked the right design of RNA molecules to create
individual replicator molecules that can operate collectively to not only
preserve information and change over time, but to do so in such a way that
the solution becomes more complex over successive generations.
Their experiment used cloned lengths of RNA in water droplets suspended in
oil which underwent more than a hundred rounds of replication, with each
round being tested and analyzed.
"Honestly, we initially doubted that such diverse RNAs could evolve and
coexist," says Mizuuchi.
"In evolutionary biology, the 'competitive exclusion principle' states that
more than one species cannot coexist if they are competing for the same
resources. This means that the molecules must establish a way to use
different resources one after another for sustained diversification. They
are just molecules, so we wondered if it were possible for nonliving
chemical species to spontaneously develop such innovation."
The proof-of-concept demonstrates this is possible, so long as the RNA don't
compete with one another for resources, but rely on one another in a sort of
host-parasite manner. If even one RNA replicator is removed, the others go
extinct.
While we can be more confident that an 'RNA world' scenario is plausible, it
falls short of showing this is how life bloomed on Earth billions of years
ago. For that we'd need diverse bodies of evidence, from geology to
astrophysics, to build a convincing case.
Nonetheless, it's a solid step forward in our search for chemical models of
evolution that are capable of transforming primordial goop into a dazzling
array of biodiversity that continues to become more complex to this very
day.
Reference:
Evolutionary transition from a single RNA replicator to a multiple
replicator network by Ryo Mizuuchi, Taro Furubayashi and Norikazu Ichihashi,
18 March 2022, Nature Communications.