One nitrogen atom, three hydrogen atoms. That's all it takes to make the basic ammonia molecule. This simple compound was one of the most important building blocks for the origin of life, scientists believe, providing the nitrogen that is crucial to many organic compounds. They just don't know for sure how so much of it could form under the conditions of the early Earth. In a new study this week, Sandra Pizzarello and colleagues tie the ammonia surplus to one of the more fascinating theories about the rise of life—that some of its basic components seeded the Earth from space on board meteorites that pounded the planet's surface. Pizzarello's team analyzed a particular meteorite found in Antarctica. Its name is Graves Nunataks (GRA) 95229, and it was discovered in 1995. But its important characteristic is that the it belongs to a class of meteorites called carbonaceous chondrites that are full of organic materials. In the lab, the researchers tried to simulate how those materials in GRA 95299 might have reacted when they reached the younger Earth.
Pizzarello and her co-authors subjected a sample of the meteorite ... to temperatures of 300 degrees Celsius at high pressures in the presence of water to simulate hydrothermal conditions on the meteorite's parent asteroid or on Earth. Under heat and pressure, GRA 95229 released almost nothing but ammonia, in amounts that constitute roughly 1 percent by mass of the type of meteoritic material examined. Its parent asteroid, the authors speculate, must have been rich in ammonia. [Scientific American]
Why does this ancient blast of ammonia matter so much? Consider the nitrogen that makes up about 80 percent of today's atmosphere
: It's lounging around up there in the non-reactive form N2. Once nitrogen has bonded to itself in that way, it's pretty loath to break up. But for nitrogen to have been taken up and used in biochemistry at the dawn of life, it would've needed to be in reduced form—that is, bonded to carbon or to hydrogen (as it is in ammonia). At first, researchers like Stanley Miller
who were trying to crack the origin of life
problem didn't consider this an issue:
To participate in most biochemical processes, that molecule has to be reduced, which typically involves bonding to hydrogen or carbon. In Miller's time, the newly formed Earth's atmosphere was thought to promote chemical reduction, and thus would provide a healthy supply of ammonia. In more recent decades, however, views on the early Earth have shifted, and it's now thought to have been chemically neutral, and might even have a tendency towards oxidation, the opposite reaction. That has left researchers considering alternate sources for biochemical precursors. [Ars Technica]
And one of those "alternate sources" is arrival by meteorite. The Pizzarello study, which appears in the Proceedings of the National Academy of Sciences, is just the most recent to cite evidence for the idea that meteorite strikes ignited the rise of life. Previous studies have suggested that not only the organic materials in meteorites but also their energy of impact
could have helped life get going. Forty years after the famed Murchison meteorite's 1969 landing in Australia, scientists showed that it contained nucleobases that came from space
, and demonstrated that those compounds weren't soil contaminants. However, as DISCOVER blogger Phil Plait pointed out
about studies like these a few years ago, one must be cautious. The fact that organically friendly material came to Earth from space doesn't mean that those compounds weren't already here. Given that the young Earth may have had a dearth of useful nitrogen, though, this study provides a tantalizing solution to that problem.
"What is important is the finding of abundant ammonia. Nitrogen is an indispensable ingredient for the formation of the biopolymers, such as DNA, RNA and proteins, on which life depends, and any theory that tries to explain life's origin has to account for a supply of 'usable' nitrogen," Professor Pizzarello said. "Therefore, its direct delivery as ammonia and in relatively large amounts from the nearby asteroids could have found a 'prebiotic venue' on the early Earth." [The Independent]
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