 Could life on earth owe its origins to an icy environment?
At last we have a full science-based theory on how life on Earth could have originated
In
follow-up work to Miller and Urey's groundbreaking study look at the
synthesis of organic compounds in a primordial environment, it
was shown that
RNA monomeric bases could form under conditions similar to those of a
prehistoric Earth. More recent work has shown how such
individual bases, floating in a water environment, could link
together into chains.
But none of that explains how the
life made the jump from RNA to DNA and how it added a protective and
sustaining soup of organic compounds along the way. The answer
may lie in a little known concept -- RNA can act as an enzyme.
It
is now generally regarded in most of the biochemistry community as
sufficiently proven that RNA acts as the enzyme to make the proteins
in our body (the so called "ribozyme"). Thus it
doesn't take a huge leap of logic to think that RNA enzymes, despite
their lesser catalytic prowess, could slowly generate sugars,
proteins, phospholipids and other key macromolecules. In fact,
a number of RNA enzymes that generate various organic molecule types
have been discovered -- including enzymes to accomplish
self-replication of the enzymes themselves.
A
critical question that remained unanswered, though, was how the
ancient RNA enzymes could survive. RNA naturally undergoes
hydrolysis reactions in water that can break its chains. While
occurring at a low rate, the large number of the phosphodiester
linkages in a long RNA chain make it virtually inevitable that and
RNA molecule would break apart in days, if not months. So how
did our potential RNA ancestors escape destruction?
Now
researchers at the MRC Laboratory of Molecular Biology in the United
Kingdom think they have cracked that puzzle. By placing RNA
inside liquid pockets of water encased inside cooling ice, they found
that RNA enzymes could function and at the same time escape
degradation. At these cooler temperatures the energy barrier
was presumably too high for uncatalyzed hydrolysis of the
phosphodiester linkages to occur -- thus safeguarding the RNA.
But with sufficient ions (added via dissolving commonly occurring
salts like magnesium-halogen salts in water), the RNA enzyme could
lower the energy barrier of the desirable reactions and survive and
self-replicate.
Thus the origin of life on Earth might not
have been in a deep-sea vent or open ocean, but in a cold muddy
puddle in the icy north or south, which contained a mix of water and
organic byproducts of freed carbon from the Earth's crust.
The
study's lead author Philipp Holliger explains, "It’s like the
tortoise and the hare problem. The tortoise is slower, but
it keeps on going, rather than falling apart. One thing that was
available at the beginning of the Earth was time."
Over
time this life form could have built up an arsenal of useful
chemicals -- evolution at its most basic microscopic form. The
most critical developments would have been the creation of a
protective phospholipid bilayer, the creation of protein enzymes to
offer faster catalysis, and last, but not least, the switch to the
more chemically stable DNA. Once a self-replicating
RNA-lifeform gained these adaptations, it would at last have been
ready to venture into warmer climates and begin to survive and
reproduce, capturing the sun's power to fix energy in carbon-based
molecules.
From there a long
evolutionary road lay ahead, eventually
reaching man and our zoological peers in the modern world.
So
is the theory true? We may never know. But it appears
that science has at last provided a somewhat plausible explanation as
to how life could have made the leap from carbon compounds to a
complex living system.
The new study is published in
the September 21 edition of the journal Nature
Communications.
It builds upon this previous
2004 study, published in Astrobiology,
which suggests that RNA enzymes could have functioned in an icy
environment.
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