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.