A rusty path to life's origin.

A rusty path to life's origin

One of the more notable gaps in theories concerning theorigin of life on earth is the step from simple molecules, such as formaldehyde, cyanides and various amino acids, to complex polymers that can replicate themselves. A variety of credible, experimentally tested models have been proposed for the initial and final stages (SN: 1/31/81, p.72; 4/21/84, p.247). What's missing, says Gustaf Arrhenius of the Scripps Institution of Oceanography in La Jolla, Calif., is a selective process for concentrating and converting a wide range of organic molecules into large, organized structures with the right biochemical properties.

Some scientists have suggested that amino acids, under theright conditions, spontaneously organize themselves into microscopic packets that have some of the electrical properties of primitive cells (SN: 6/30/84, p.408). A more commonly held view is that the surfaces of minerals may have served as templates for the organization and combination of the molecular starting materials. Most of the attention, so far, has focused on clay minerals, usually aluminum silicates (SN: 9/12/81, p.166). However, Arrhenius proposes that common minerals such as iron oxide hydroxides (rustlike compounds), manganates and iron-manganese phosphates are more likely candidates.

Unlike silicates, says Arrhenius, hydrous iron and manganeseminerals grow and dissolve readily in response to small changes in acidity or electric potential. This allows these minerals periodically to release any polymers that may have formed. Clay minerals, he contends, lock up any internally synthesized polymers and "would thus seem to lead to a dead end from the point of view of molecular function and evolution.' Furthermore, compounds containing transition elements such as iron and manganese are known to be generally more active than aluminosilicates as catalysts.

As a first step, Arrhenius and his group investigated the effectof ultraviolet light on two reaction systems, one consisting of iron sulfate dissolved in water saturated with carbon dioxide and the other a suspension of fine iron carbonate particles in water. In both cases, ferric oxide hydroxide appeared as a solid product along with some organic matter.

This demonstrates for the first time, says Arrhenius, "thatferrous carbonate in aqueous suspension is capable of producing organic compounds in [ultraviolet] radiation.' However, the researchers have not yet been able to identify the specific carbon-containing compounds mixed in with the solid hydroxide product. More recent irradiation experiments show that a similar process occurs with manganese carbonate. Furthermore, analyses of natural, crystalline specimens of iron oxide hydroxide show the presence of organic matter, indicating that incorporation of organic compounds into the mineral may be a commonly occurring process in nature.