One
of the greatest mysteries about the origin of life is how the necessary
ingredients consistently came together in a workable way. On a planet full of
raw chemical materials, what happy accident of nature led to the first tiny
glimmer of life?
To
Alexander Graham Cairns-Smith, that glimmer may owe something to the sparkle of
a crystal.
Cairns-Smith,
an organic chemist at the University of Glasgow, sees a significant
relationship between the structure of DNA molecules and the structure of
certain kinds of mineral crystals. He says that while patterned structures that
replicate themselves are common in the inorganic world of
crystals, it is a rare quality in the organic world — DNA
and RNA are the only organic molecules we know of that strongly exhibit this
characteristic.
The
four bases that
help make up the DNA molecule — adenine, cytosine, guanine, and thymine — do not repeat
endlessly in strict order (such as ACGTACGTACGT...). Instead, the pattern is varied,
like a barcode. This variability in DNA leads to the differences between
organisms, and the copying of such complex sequences is the basis of heredity. Cairns-Smith
sees this ability to print off reliable copies of sequences as an important point
of similarity between certain kinds of inorganic crystals and DNA.
"One
of the miracles of life, to my mind, is the accuracy with which DNA gets itself
replicated in the cell," he says. "It has to be that unbelievably
accurate, otherwise we'd all die out in no time."
Life's
First Barcode?
In
1949, the Irish scientist J.D. Bernal suggested that clay minerals may have created
a meeting place for life's first molecules. Such a scenario could explain how
the randomly dispersed molecules of life managed to come together in the
diffuse primordial soup.
Cairns-Smith's
idea takes Bernal's theory a step further. In his view, clay
mineral layers not only attracted certain chemicals from the environment to
their surfaces, the mineral layers also acted as the first genetic information
carriers, much as the base pairs in DNA do today.
"The
objects that I'm particularly interested in are mixed-layered crystals, in
which the crystal structure consists of beautifully formed layers packed on top
of each other, but with an arbitrary sequence," says Cairns-Smith. "In
that respect, they're like a DNA molecule, which has base pairs, little
platelets inside it which are stacked on top of each other. It is the sequence
of this stacking which creates the information."
Cairns-Smith
doesn't think the clay mineral crystals were "alive" anymore than a
DNA sample is thought to be alive. Instead, by acting as the first genetic
materials for early life, clay mineral crystals created a link between the
worlds of inorganic and organic chemistry.
At
some point, life launched free of its inorganic genetic origins — the organic
substances that evolved from chemical interactions on the mineral layers became
stable enough to live apart from their birthplace, and complex enough to
replicate themselves into the future.
Some
mineral layer combinations probably worked better than others when it came to marshalling
the organic molecules that were to eventually become genetic materials. One of
his favorite contenders for life's early mineral template is authigenic
chlorite, which can create complicated shapes that resemble brussels sprouts. Such
chlorite crystals growing inside sandstone often coat the sand grains and do
not block the flow of solutions within the rock — a potentially important
quality for the very first evolving systems. However, rather than one
particular mineral layer sequence leading to life, Cairns-Smith thinks many
different mixed layer structures might have contributed to life's evolution.
Cairns-Smith
first came up with his theory in the 1960s, and he has promoted it in several
books and articles. Some scientists find the idea intriguing, but others
discount it because life today shows no evidence of having inorganic origins. Most
scientists who work on the problem of life's origin instead focus on how simple
sugars, amino acids, and other organic chemicals came together to form nucleic
acids and proteins.
Cairns-Smith
thinks such a chain of events was improbable on the early Earth — the nucleic
acid and protein system of life is too complex to have sprung outright from
simple ingredients. Even the RNA world hypothesis, which envisions RNA playing dual
roles that today are carried out by DNA and proteins, is a relatively advanced
and sophisticated process.
"A
simpler kind of evolution came first, and then what are now the molecules of
life came to be produced in a consistent way," says Cairns-Smith. "Of
course there was no foresight here, but as soon as an evolutionary process was
underway, the world would have changed and nature would have had a new set of
toys to play with."
If
crystals provide the order life needs to arise, that has implications for the possibility
for life
beyond Earth. Mineral crystals should be common on rocky worlds, even if those
planets have chemical environments quite different from our own. If
Cairns-Smith's theory is correct, then the spark of life may be shimmering on
crystal surfaces throughout the universe.
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