In the wake of Stanley Miller's classic experiments, it seemed that one aspect of the origin of life was sorted out: the Earth's early atmosphere could easily produce some of the chemicals that are typical of modern life, providing a healthy supply of building blocks for the first pre-biotic chemistry. One of the components of Miller's experiments, however, was ammonia, a nitrogen bonded with three hydrogens; since his time, questions have been raised as to whether the early atmosphere could have contained any ammonia whatsoever. Now, an analysis of material in an Antarctic meteorite suggests that ammonia could have been delivered to the Earth from outer space.
Nitrogen makes up the largest fraction of the Earth's atmosphere, but it's largely present as the unreactive N2 molecule. 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.
One possible source includes extraplanetary delivery, with organic molecules arriving from space. Abundant organic materials have been found on some comets and a few types of meteors, but many of these are complex, stable end products, not the simple precursor molecules that could feed into early biochemistry. This is especially true of the most famous carbon-rich meteorite, the Murchison.
In the new paper, the authors look at a different class of meteorites found in Antarctica. These contain a collection of accessible components that include ammonia and amino acids, but also a large mix of insoluble organic molecules that have been tough to characterize. To get around this, the authors put the material in water, and then treated it with heat and high pressures, which released a great deal of ammonia. This suggests that the potential contribution of these meteorites to the early Earth's biochemistry is much larger than earlier work would have shown.
"An abundant exogenous delivery of ammonia, therefore, might have been significant in aiding early Earth’s molecular evolution toward prebiotic syntheses," the authors conclude, saying it "would make a reasonable case for exobiology." That's a pretty loose definition of exobiology, which generally refers to the process of life forming somewhere other than the Earth. But it does place an emphasis on the potential role of materials and processes that aren't inherent to our planet. This isn't the first paper to point that out; another recent study indicates that the impact of a comet on the atmosphere can produce lots of reduced nitrogen. It seems possible that life originated on Earth, but may have needed an assist from the rest of our solar system.
PNAS, 2011. DOI: 10.1073/pnas.1014961108 (About DOIs).
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