Sea Vents Closed as Life-Origin Site

Sea Vents Closed as Life-Origin Site

A mile and a half down, beyond the reach of sunlight, volcanic geysers spew 400°C water into the oceans. Life teems around these deep, hidden vents—pink sea cucumbers, eyeless shrimp, tubeworms, and more—creating underwater communities.

Hydrothermal vents on the ocean floor now play an important role in origin-of-life explanations. Microbes thriving there suggest to some origin-of-life researchers that life got its start at high temperatures.1 Laboratory experiments indicating that prebiotic molecules can form under deep-sea hydrothermal vent conditions further fuel these speculations.2

The appeal to deep-sea hydrothermal vents as a source of prebiotic molecules becomes even more important as researchers concede that Earth’s early atmosphere could not support prebiotic molecule formation.3 Without a source of prebiotic molecules, naturalistic origin-of-life pathways are blocked. This barrier prompts investigators to seek alternative locations where prebiotic molecule production could take place.

For the synthesis of prebiotic molecules at deep-sea hydrothermal vents (and for life to originate in this environment), ammonia (NH3) must be present. Ammonia serves as a key starting material in the synthesis of amino acids and other biologically important nitrogen-containing compounds. Researchers now recognize that ammonia did not exist on the early Earth. Therefore, high-temperature origin-of-life scenarios must be discarded unless ammonia somehow formed at deep-sea hydrothermal vents as a precursor to prebiotic molecules.

Penn State and SUNY-Stony Brook researchers recently evaluated the likelihood of ammonia formation under primitive hydrothermal vent conditions.4 In principle, ammonia (NH3) could form in such an environment from nitrogen (N2) gas via two possible chemical routes involving hydrogen sulfide (H2S) and iron (2) sulfide FeS.

Laboratory experiments conducted by the two researchers demonstrate that FeS-mediated reaction occurs far too slowly to have contributed to ammonia formation. Ammonia production by hydrogen sulfide occurs rapidly enough but yields insufficient quantities to sustain prebiotic molecule formation.

This lack of adequate ammonia production makes the origin of life at deep-sea hydrothermal vents unlikely. Moreover, it eliminates another possible source of prebiotic molecules on the early Earth. Each new discovery makes the origin-of-life problem more intractable for naturalists.

  1. Karl O. Stetter, “The Lesson of Archaebacteria,” in Early Life on Earth: Nobel Symposium No. 84, Stefan Bengtson, ed. (New York: Columbia University Press, 1994), 143-51.
  2. For example, see J. P. Amend and E. L. Shock, “Energetics of Amino Acid Synthesis in Hydrothermal Ecosystems,” Science 281 (1998): 1659-62.
  3. Francois Raulin, “Atmospheric Prebiotic Synthesis,” presentation at the 12th International Conference on the Origin of Life and the 9th meeting of the International Society for the Study of the Origin of Life, San Diego, CA, 1999.
  4. Martin A. A. Schoonen and Yong Xu, “Nitrogen Reduction Under Hydrothermal Vent Conditions: Implications for the Prebiotic Synthesis of C-H-O-N Compounds,” Astrobiology 1 (2001): 133-42.