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Biological Chemistry

Comet collisions may have helped seed life on Earth, lab experiment shows

Researchers produce ribose—a sugar important to sustaining life—in the icy conditions experienced by comets in outer space

by Sarah Everts
April 7, 2016 | APPEARED IN VOLUME 94, ISSUE 15

By exposing methanol, water, and ammonia—simple compounds found in the ice of comets—to conditions found in outer space—ultraviolet radiation and icy cold temperatures—researchers have formed a diversity of organic molecules in the lab. One of these molecules, the five-carbon sugar ribose, typically appears in the backbone of ribonucleic acid (RNA), a candidate for the first self-replicating genetic material used by life on Earth, explains Uwe Meierhenrich, a physical chemist at the University of Nice, who led the research (Science 2016, DOI: 10.1126/science.aad8137).

If Meierhenrich’s lab model of interstellar ice holds true in outer space, the new results support the theory that organic material required to seed life on Earth could have come from a series of comet collisions.

“When you think of the origin of life on Earth, it’s very intriguing that sugars, particularly those that form an essential part of the RNA backbone, might be right there on small icy grains waiting to be delivered to our planet,” comments Michael P. Callahan, who studies origin-of-life chemistry at Boise State University. “It is amazing to see the diversity of relatively complicated sugars coming from simple ice mixtures,” he adds.

Meierhenrich and his team propose that ribose is formed in their comet model through a formose reaction, a process that involves the formation of sugars from formaldehyde building blocks. They suggest that formaldehyde first condenses with itself in an autocatalysis reaction to form glycolaldehyde, thereby kicking off a series of reactions that eventually results in ribose and other five-carbon sugars.

If sugars such as ribose can be synthesized under interstellar ice conditions, these types of compounds may be commonly produced by comets, meteorites, and interstellar dust and gas particles, the researchers suggest. Preliminary data acquired from the Rosetta mission suggest the comet 67P/Churyumov-Gerasimenko carries a variety of complex organic molecules including acetone, propanal, and acetamide that waft off the interstellar body. “Just imagine what we’d find if we could take a core sample from the comet,” Meierhenrich says.

Even if ribose is common on comets, there are still many outstanding questions about how life may have been seeded by organic material originating from outer space, Callahan adds. For example, RNA is composed of d-ribose, so an enantiometric excess of the sugar must have been created by polarized light or some other interstellar condition. In addition, coupling nucleobases with ribose—a reaction needed to form RNA—is a chemical challenge because it’s thermodynamically unfavorable. “So even if you have a source of sugars,” Callahan adds, “the chemistry that occurs next is still mysterious.”

Made in the lab
 
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