Life may have come from warm ponds as meteorites crashed into them

 In Science


meteor showerREUTERS/Ognen Teofilovski

  • Charles Darwin once theorized that the origin of life —
    known as abiogenesis — could have happened as precursor
    compounds came together in “warm little ponds.”
  • A new study provides evidence for that theory, modeling
    how meteorites may have delivered compounds that could
    have led to the formation of RNA (a cousin of DNA) in ponds all
    over the Earth.
  • Proponents of the other major life-origin story, the
    idea that life comes from hydrothermal vents deep in the ocean,
    are not convinced.

The biggest question about life is an obvious one, but the answer
is hotly debated.

How did it all begin?

The most well-known of biologists, Charles Darwin, once
theorized in a private letter
to his friend Joseph Lee Hooker
that life — the very first molecules of it — could have emerged
from a “warm little pond” where some precursor components
underwent a chemical reaction, creating compounds that would
later develop into the forms of life as we know them today.

The other main theory is that life could have first emerged
underwater at ultrahot
hydrothermal vents
, where cold seawater is heated to searing
temperatures by volcanic activity deep in the ocean, providing
enough energy to transform chemicals and other particles.

This week, the warm little ponds theory got a boost.

In a study published Monday in the journal Proceedings
of the National Academy of Sciences
, researchers wrote that
they had mathematically modeled a way that meteorites, which
smacked into early Earth far more regularly than they do now,
could have delivered organic materials called nucleobases to warm
little ponds all over the earth. These nucleobases would have
served
 as the building blocks for RNA (a cousin of DNA),
which many scientists think was the first sort of “life” to
emerge, since it can both store information and help catalyze
chemical reactions that would lead to the formation of other
organic compounds.

Scientists say that for life to exist, molecules have to be
able to trigger reactions that lead to the creation of new
molecules. RNA has properties that could allow it to do that
without already having complex proteins, which DNA requires for
the replication process — a key argument for why RNA would
come first.

The authors say their simulation is the first to model this
origin story in such a complete way, helping demonstrate
that the warm little pond story might be the correct one.

“No one’s actually run the calculation before. It’s pretty
exciting,” Ben Pearce of McMaster University, lead author of the
new study, said in a
press release
.


warm little pond
A
warm little pond on present day Earth, on the Bumpass Hell trail
in Lassen Volcanic National Park in California. The warm little
ponds that are prime candidates for the birthplace of life on
Earth probably looked somewhat similar.


B. K. D.
Pearce



A model for life formation

Pearce and co-authors created a model of what the Earth looked
like as the planet changed geologically in its early stages. All
over the world, meteorites that hit the planet somewhere between
4.5 and 3.7 billion years ago could have delivered nucleobases to
those warm little ponds, thousands of which could have been found
all over early continents still emerging from the ocean.

During dry seasons, these ponds would shrink, bringing chemicals
together; they’d expand during wet seasons. Wet and dry cycles
help explain polymerization, the process by which compounds
develop into chains or three-dimension structures, the authors
wrote. The shrunken pools during dry cycles would bring compounds
close together, allowing them to bond, which is one of the
reasons they think that chemicals coming together in more open
water near hydrothermal vents is a less plausible explanation.

But because of rapidly changing environmental conditions during
these cycles, the authors believe RNA must have formed
within a few years of meteorite impact. Otherwise, UV from
sunlight during dry periods could have destroyed the
compounds.

The authors think this happened at least 4.17 billion years ago,

millions of years before
we have clear evidence of life
forming on the planet. That evidence dates to about 3.5 billion
years ago — though another study this
week
also made the claim that life existed at least 3.95
billion years before now.

There are several reasons the authors believe this model could
help solve the question of the origin of life, known as
“abiogenesis.” Their model accounts for meteorites as the source
material for the RNA precursors; it can help explain how some of
the necessary reactions would have occurred; and they say their
timeline fits with the early geologic conditions of the planet.

“Because there are so many inputs from so many different fields,
it’s kind of amazing that it all hangs together. Each step led
very naturally to the next,” said study co-author Ralph
Pudritz in the release. “To have them all lead to a clear picture
in the end is saying there’s something right about this.”


hydrothermal vent noaa oar nurp
A hydrothermal vent or
“black smoker chimney.”


OAR/National
Undersea Research Program (NURP); NOAA



Unanswered questions

But the question is still far from solved.

“Yeah, that’s a possibility, but it’s certainly far, far, far
from being the only possibility, and it’s far, far, far from
being true,” Jan Amend,
a geochemist at the University of Southern California who studies
hydrothermal vents,
told Newsweek
.

Amend disagreed with the necessity of the wet-dry cycle, pointed
out the meteorite impacts could have destroyed the precursor
compounds, and said that even meteorites probably couldn’t have
delivered the all the necessary materials.

Other scientists prefer the hydrothermal vent explanation because
they believe it’s the only way there could have been sufficient
energy for the life-forming reactions to occur.

This new study at least advances the warm pond theory, even if
the question remains far from solved.

“Based on what we know about planet formation and the chemistry
of the solar system, we have proposed a consistent scenario for
the emergence of life on Earth. We have provided plausible
physical and chemical information about the conditions under
which life could have originated,” co-author Dmitry Semenov of
the Max Planck Institute for Astronomy said in the news release.
“Now it’s the experimentalists turn to find out how life could
indeed have emerged under these very specific early conditions.”

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