Many artists have tried to depict what Earth might have looked like billions of years ago, before life made its appearance. Many scenes trade snow-covered mountains for lava-gushing volcanoes and blue skies for lightning bolts pummeling what's below from a hazy sky. But what did early Earth actually look like? This question has been the subject of intense scientific research for decades.
Sulfur, loathed by scientists, loved by nature:
A publication led by Sukrit Ranjan, an assistant professor in the University of Arizona's Lunar and Planetary Laboratory, shines a spotlight on sulfur, a chemical element that, while all familiar, has proved surprisingly resistant to scientific efforts in probing its role in the origin of life.
"Our picture of early Earth is pretty fuzzy," said Ranjan, who explores sulfur concentrations in early Earth's waters and atmosphere. The same processes that make our planet habitable - liquid water and plate tectonics - constantly destroy the rocks that hold Earth's geologic record, he argues. "It's great for us because it recycles nutrients that would otherwise be locked up in Earth's crust, but it's terrible for geologists in the sense that it removes the messengers."
Published in the journal AGU Advances in December, Ranjan's paper was selected as an editor's highlight, in recognition of "experiments that were extremely difficult to perform but provide constraints for ongoing laboratory prebiotic chemistry experiments."
At the core of efforts to pull back the curtain on the emergence of life on Earth has been a concept known as the "RNA world," Ranjan said, referring to ribonucleic acid, a class of molecules that are present in every living cell and crucial to life as we know it.
The RNA world hypothesis is based on an interesting feature of modern biology, which is that of the four major categories of biomolecules - amino acids, carbohydrates, lipids and nucleic acids - RNA is the only one that can perform the role of an enzyme and the storage and replication of genetic information, by making copies of itself, all by itself. There's just one problem: It's really hard to make.
"For about 50 years, people have tried to figure out how to make RNA without enzymes, which is how biology does it," Ranjan said, explaining that it wasn't until the last five years that researchers figured out non-enzymatic pathways to make RNA.
"If we can get RNA, then on the far horizon we see a pathway to get everything else going," he said. "And this begs the question: Was this molecule actually available earlier in any quantities whatsoever? And this is actually a major open question."
Recently, scientists have completed a half-century quest to make RNA molecules without biological enzymes, a huge step forward to demonstrating the RNA world. However, these chemical pathways all rely on a critical sulfur molecule, called sulfite. By studying rock samples from some of Earth's oldest rocks, scientists know there was plenty of sulfur to go around on the early, prebiotic Earth.
But how much of it was in the atmosphere? How much of it ended up in water? And how much of it ended up as RNA-producing sulfite? Those are the questions Ranjan and his team set out to answer.
"Once it's in the water, what happens to it? Does it stick around for a long time, or does it go away quickly?" he said. "For modern Earth we know the answer - sulfite loves to oxidize, or react with oxygen, so it'll go away super-fast."
By contrast, as geological evidence indicates, there was very little oxygen in early Earth's atmosphere, which could have allowed sulfite to accumulate and last much longer. However, even in the absence of oxygen, sulfite is very reactive, and many reactions could have scrubbed it from the early Earth environment.
One such reaction is known as disproportionation, a process by which several sulfites react with each other, turning them into sulfate, and elemental sulfur, which are not useful for origin-of-life chemistry. But how fa...
