Scientists at the Foundation for Applied Molecular Evolution announced today that ribonucleic acid (RNA), an analog of DNA that was likely the first genetic material for life, spontaneously forms on basalt lava glass. Such glass was abundant on Earth 4.35 billion years ago. Similar basalts of this antiquity survive on Mars today.
"Communities studying the origins of life have diverged
in recent years," remarked Steven Benner, a co-author of the study
appearing online in the journal Astrobiology.
"One community re-visits classical questions with
complex chemical schemes that require difficult chemistry performed by skilled
chemists," Benner explained. "Their beautiful craftwork appears in
brand-name journals such as Nature and Science." However, precisely
because of the complexity of this chemistry, it cannot possibly account for how
life actually originated on Earth.
In contrast, the Foundation study takes a simpler approach.
Led by Elisa Biondi, the study shows that long RNA molecules, 100-200
nucleotides in length, form when nucleoside triphosphates do nothing more than
percolate through basaltic glass.
"Basaltic glass was everywhere on Earth at the
time," remarked Stephen Mojzsis, an Earth scientist who also participated
in the study. "For several hundred million years after the Moon formed,
frequent impacts coupled with abundant volcanism on the young planet formed
molten basaltic lava, the source of the basalt glass. Impacts also evaporated
water to give dry land, providing aquifers where RNA could have formed."
The same impacts also delivered nickel, which the team
showed gives nucleoside triphosphates from nucleosides and activated phosphate,
also found in lava glass. Borate (as in borax), also from the basalt, controls
the formation of those triphosphates.
The same impactors that formed the glass also transiently
reduced the atmosphere with their metal iron-nickel cores. RNA bases, whose
sequences store genetic information, are formed in such atmospheres. The team
had previously showed that nucleosides are formed by a simple reaction between
ribose phosphate and RNA bases.
"The beauty of this model is its simplicity. It can be
tested by highschoolers in chemistry class," said Jan Špaček, who was not
involved in this study but who develops instrument to detect alien genetic
polymers on Mars. "Mix the ingredients, wait for a few days and detect the
RNA."
The same rocks resolve the other paradoxes in making RNA in
a path that moves all of the way from simple organic molecules to the first
RNA. "For example, borate manages the formation of ribose, the 'R' in
RNA," Benner added. This path starts from simple carbohydrates that could
"not not" have formed in the atmosphere above primitive Earth. These
were stabilized by volcanic sulfur dioxide, and then rained to the surface to
create reservoirs of organic minerals.
Thus, this work completes a path that creates RNA from small
organic molecules that were almost certainly present on the early Earth. A
single geological model moves from one and two carbon molecules to give RNA
molecules long enough to support Darwinian evolution.
"Important questions remain," cautions Benner.
"We still do not know how all of the RNA building blocks came to have the
same general shape, a relationship known as homochirality." Likewise, the
linkages between the nucleotides can be variable in the material synthesized on
basaltic glass. The import of this is not known.
Mars is relevant to this announcement because the same
minerals, glasses, and impacts were also present on Mars of that antiquity.
However, Mars has not suffered continental drift and plate tectonics that
buried most rocks from Earth older than 4 billion years. Thus, rocks from the
relevant time remain on the surface of Mars. Recent missions to Mars have found
all of the needed rocks, including borate.
"If life emerged on Earth via this simple path, then it
also likely emerged on Mars," said Benner. "This makes it even more
important to seek life on Mars as soon as we can."
Reference: Research Paper, Proceedings of the National Academy of Sciences, Astrobiology