20.4: Origins of Organic Molecules in a NON-Reducing Atmosphere

A prebiotic non-reducing atmosphere is based on several assumptions: (1) The early Earth would have had insufficient gravity to hold \(\rm H_2\) and other light gasses; (2) as a result, “outgassing” would have resulted in a loss of \(\rm H_2\) and other reducing agents from the atmosphere; (3) The geological evidence suggests that the Earth’s oceans and crust formed early in the Hadean Eon, just a few hundred million years after formation of the planet; (4) studies of 4.4-billion-year-old (early Hadean Eon) Australian zircon crystals suggest that their oxidation state is the same as modern day rocks, meaning that the early Hadean atmosphere was largely \(\rm N_2\) and \(\rm CO_2\), a distinctly non-reducing one! Solid geological evidence of cellular life dates to 3.5-3.95 billion years ago (i.e., the Archaean Eon). Softer evidence of microbial life exists in the form of graphite and other possible footprints of life as old as 4.1 billion years ago, near the end of the Hadean Eon. If this is true, the discovery of an oxidizing Hadean atmosphere at least 3 billion years earlier (4.4 billion years ago) argues for the origins of life in a non-reducing atmosphere. Therefore, regardless of whether life began 3.5 or even 4.1 billion years ago, the evidence suggests that life’s beginnings had to contend with a non-reducing environment. A colorized image of the Australian zircon is shown in Figure 20.7.

Figure 20.7: The oxidation state of this Australian zircon crystal supports an oxidizing atmosphere on Earth as far back as 4.4 billion years ago. Photo Courtesy of J. W. Valley et al.

Before we look more closely at other evidence of life origins under non-reducing conditions, let’s consider the Panspermia, the possibility that life came to Earth from extraterrestrial sources and a related hypothesis that prebiotic organic molecules came from extraterrestrial sources. Then we’ll return to the question of how cells might have formed in localized, favorable terrestrial environments.

20.4.1 Panspermia – an Extraterrestrial Origin of Earthly Life

Panspermia posits that life itself arrived on our planet by hitchhiking on comets or meteorites. Since these are unlikely to have sustained life in space, they must have been a kind of interstellar ‘mailbox’ into which dormant life forms were deposited. The cells in the mailboxes must have been cryptobiotic. Examples of cryptobiosis exist today (e.g., bacterial spores, brine shrimp!). Once delivered to Earth’s life-friendly environment, such organisms would emerge from dormancy, eventually populating the planet. There is however, no evidence of dormant or cryptobiotic life on comets or meteorites, and no hard evidence to support Panspermia. On the other hand, there is evidence at least consistent with an extraterrestrial source of organic molecules, and even more to support more terrestrial origins of life. In any case, notions of Panspermia (and even extraterrestrial sources of organic molecules) just beg the question of the conditions that would have led to the origins of life elsewhere!

Not a favored scenario, panspermia is still intriguing in the sense that it is in line with a likelihood that organic molecules formed soon after the Big Bang. Moreover, if ready-made organic molecules and water were available, we can expect (and many do!) that there is life on other planets. This possibility has stimulated discussion and serious funding of programs looking for signs of extraterrestrial life. It supported the earlier Search for Extraterrestrial Intelligence (SETI) program based on the assumption that life not only exists elsewhere, but that it evolved high level communication skills (and why not?)! More recently NASA funded Rover’s search for signs of water on Mars and Cassini’s recent discovery of water on Enceladus, one of Saturn’s moons. For a fascinating story about meteorites from Mars that contain water and that are worth more than gold, check out Martian Obsession. For one about Titan (a moon of Saturn’s), see Life on Titan?.

And then there is this! Maybe, just maybe we have exported earthly life to comets when they bumped into us and skittered away instead of crashing and burning. And maybe, just maybe they scraped up a few upper atmospheric microbes along their way to other outerspace encounters (upper-atmosphere microbial colonies). And maybe, just maybe such encounters could have led to exoplanetary life. Read about the possibilities and realities at. Maybe, just maybe.... Finally, maybe, just maybe, our attempt to share the gift of life can explain organic molecules on some space objects that come near or actually visit us from time to time!

CHALLENGE

We spend a lot of money on our space programs, designing experiments to probe for water and life on Mars, other planets, and/or their moons. If life is discovered there some day, would it argue for multiple origins of life on earth? Do you think we should be spending the money!!?

20.4.2 Extraterrestrial Origins of Organic molecules

Even if life did not come to us ready-made, could organic molecules have arrived on Earth from outer space? They are abundant, for example in interstellar clouds, and could have become part of the Earth as the planet formed around 4.8 billion years ago. This suggests that there was no need to create them de novo. Meteorites, comets, and asteroids are known to contain organic molecules and could have brought them here during fiery impacts on our planet. Comet and meteorite bombardments would have been common 3.8 or more billion years ago. In this scenario the question of how (not on Earth!) free energy and inorganic molecular precursors reacted to form organic molecules…, is moot! A related hypothesis suggests that those fiery hits themselves provided the free energy necessary to synthesize organic molecules from inorganic ones… a synthesis-on-arrival scenario. With this hypothesis on the one hand, we are back to an organic oceanic primordial soup. On the other, some have suggested that organic molecules produced in this way (not to mention any primordial life forms) would likely have been destroyed by the same ongoing impacts by extraterrestrial bodies; witness the relatively recent dinosaur extinction by an asteroid impact off the coast of Mexico some 65.5 million years ago.

350 Life Origins in a Non-Reducing Atmosphere?