Top: Representation of mineral crystal layers, a possible primitive gene? Middle: Variations in genetic “barcodes”. Bottom: A consortium of chemically different primitive genetic materials.
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.