WASHINGTON — American planetary scientists have identified key ingredients that were in large concentrations on Earth nearly 4 billion years, a chaotic period when the planet’s chemistry improbably gave rise to the very first organisms.
A study, published on Monday in the journal Astrobiology, has shown that a class of molecules called sulfidic anions may have been abundant in Earth’s lakes and rivers.
The researchers from Massachusetts Institute of Technology (MIT) and the Harvard-Smithsonian Center for Astrophysics calculated that, around 3.9 billion years ago, erupting volcanoes emitted huge quantities of sulfur dioxide into the atmosphere, which eventually settled and dissolved in water as sulfidic anions, specifically, sulfites and bisulfites.
These molecules likely had a chance to accumulate in shallow waters such as lakes and rivers, according to them.
Preliminary work by MIT’s Sukrit Ranjan and his collaborators suggested that sulfidic anions would have sped up the chemical reactions required to convert very simple prebiotic molecules into RNA, a genetic building block of life.
In 2015, chemists from University of Cambridge, led by John Sutherland, who is a co-author of this study, discovered a way to synthesize the precursors to RNA using just hydrogen cyanide, hydrogen sulfide, and ultraviolet light, all ingredients that are thought to have been available on early Earth before the appearance of the first life forms.
It was previously unclear whether such ingredients would have been sufficiently abundant to jumpstart the first living organisms.
Ranjan, a postdoctoral researcher in MIT’s Department of Earth, Atmospheric and Planetary Sciences, consulted the geological record to determine the amount of volcanism that likely took place around 3.9 billion years ago, around the time the first life forms are thought to have appeared and he looked up the types and concentrations of gases that this amount of volcanism would have produced.
He created an aqueous geochemistry model to calculate how much of these gases would have been dissolved in shallow lakes and reservoirs, which are environments that would have been more conducive to concentrating life-forming reactions, versus vast oceans, where molecules could easily dissipate.
Ultimately, Ranjan found that, while volcanic eruptions would have spewed huge quantities of both sulfur dioxide and hydrogen sulfide into the atmosphere, it was the former that dissolved more easily in shallow waters, producing large concentrations of sulfidic anions, in the form of sulfites and bisulfites.
The new results point to sulfites and bisulfites as a new class of molecules that chemists can now test in the lab, to see whether they can synthesize from these molecules the precursors for life.
Early experiments led by Ranjan’s colleagues suggest that sulfites and bisulfites may have indeed encouraged biomolecules to form.
The team carried out chemical reactions to synthesize ribonucleotides with sulfites and bisulfites, versus with hydrosulfide, and found the former were able to produce ribonucleotides and related molecules 10 times faster than the latter, and at higher yields.
“Prior to this work, people had no idea what levels of sulfidic anions were present in natural waters on early Earth; now we know what they were,” Ranjan said. “This fundamentally changes our knowledge of early Earth and has had direct impact on laboratory studies of the origin of life.”