Does Discovery of Ammonia in Meteorites Breathe Life into the Origin of Life?
I remember the locker room pranks we used to play on each other during high school. One of them involved breaking an ammonia capsule and placing it under some unsuspecting person’s nose. Even though these capsules are meant to revive someone who has been knocked out, I never saw one used in that manner. But I saw plenty of them broken open as part of a practical joke.
Recently, researchers from Arizona State University (ASU) have discovered ammonia in the meteorite Grave Nunataks 95229.1 These researchers think this discovery might break through the “fog” and bring back to life an evolutionary explanation for the origin of life. Critical analysis of this discovery suggests otherwise.
Meteorites and the Origin of Life
Researchers discovered that carbonaceous chondrites, a class of meteorites, contain diverse organic materials, including amino acids. Thus, some scientists have suggested that meteorites may have delivered the raw materials needed to begin the origin-of-life process on early Earth.
Additionally, researchers discovered that the suite of organic materials found in the various carbonaceous chondrites differ From one another. Presumably, these different materials reflect not only the chemical resources available for the origin of life, but also the environments and physicochemical processes found in the early solar system.
Analysis of Grave Nunataks
To gain greater understanding of carbonaceous chondrite chemistry, researchers recently analyzed the Grave Nunataks meteorite’s insoluble organic fraction—the most abundant organic material in carbonaceous chondrites, consisting of complex, poorly defined insoluble compounds.
The investigators heated the insoluble material in water at high temperature and pressure to mimic the hydrothermal environment likely to exist on the asteroid and on early Earth. After heating, they observed a release of high levels of ammonia, in addition to other organic materials. It is currently not clear if this ammonia is free within the meteorite or chemically bound to the organic materials and becoming liberated through hydrolysis.
Ammonia and the Origin of Life
The release of ammonia from the insoluble organic fraction is an exciting result for origin-of-life investigators because it may reveal a possible source of ammonia for the chemical evolution of life.
Scientists do not believe that ammonia was present in early Earth’s atmosphere. Part of the problem is due to photochemical destruction of this compound by the ultraviolet radiation emanating from the Sun.
The latest research confirms that early Earth’s atmosphere was indeed devoid of ammonia as well as other reducing gases such as water, methane, and hydrogen. A reducing gas mixture is needed for chemical evolutionary pathways to produce amino acids and other biomolecules—such as nucleobases. The atmosphere consisted, instead, of nitrogen, carbon dioxide, and water. This combination of gases will not support the production of life’s building blocks.
In the face of this difficulty, the ASU scientists suggest that ammonia may have been delivered via meteorite, thus circumventing these problems for evolution. However, a thoughtful analysis of this proposal raises questions about its validity.
Given the observed chemical disparity of carbonaceous chondrites, it is not clear if Grave Nunataks is representative or simply an anomaly. If the latter, then it is not likely meteorites delivered substantial amounts of ammonia to early Earth. If the previous, then the meteorite’s ammonia level is relatively low (on the order of ten grams per kilogram of the insoluble organic material), which in turn constitutes less than one percent of the meteorite’s mass. In other words, a meteorite weighing 100 kilograms would yield only about 10 grams of ammonia. Given such a low level, it is debatable if meteorite delivery could have provided the amounts of ammonia necessary for chemical evolution.
And, as discussed in my books Origins of Life and Creating Life in the Lab, the presence of early Earth’s building-block materials is not enough to explain the origin of life. These materials must be able to combine in a stepwise manner to form more-complex chemical entities that in turn could evolve into the first life-forms. While origin-of-life researchers have indentified pathways that, in principle, could lead to life, they have failed to show that these pathways could generate life under early Earth’s conditions. That is, these putative prebiotic reactions lack geochemical relevance. (See here and here for articles that illustrate this point.)
The bottom line is this: It is questionable if the ammonia in the Grave Nunataks meteorite is capable of reviving the evolutionary origin-of-life paradigm. It seems the release of this compound from the insoluble organic material is just a prank that nature has played on the origin-of-life research community.
- Sandra Pizzarello et al., “Abundant Ammonia in Primitive Asteroids and the Case for a Possible Exobiology,” Proceedings of the National Academy of Sciences, USA 108, no. 11 (March 15, 2011). Published electronically February 28, 2001. doi: 10.1073/pnas.1014961108.