A recipe for the perfect, life-yielding, primordial soup has eluded science for decades. But a team of biochemists say they now have a key ingredient.
Charles Carter and Richard Wolfenden, both of the University of North Carolina, have uncovered new evidence of abiogenesis, the process by which life arises from non-living chemical matter. Their study, published Thursday in the Journal of Biological Chemistry, suggests that a single ancient gene may have used each of its opposite DNA strands to code for different chemical catalysts. Those separate catalysts would have both activated amino acids, which then formed proteins – essential to the production of living cells.
Where does life come from? Despite years of research, scientists still rack their brains over this most existential question. If the universe did begin with a rapid expansion, per the Big Bang theory, then life as we know it sprung from nonliving matter. How this process, known as abiogenesis, could have occurred is a source of much scientific debate.
In the early 20th century, the “primordial soup” model of abiogenesis started to gain traction. It proposes that in Earth’s prebiotic history, simple organic matter was exposed to energy in the form of volcanoes and electrical storms. That energy would have catalyzed chemical reactions that, in the span of a few hundred million years, could have produced self-replicating molecules.
In 1952, Stanley Miller and Harold Urey tested that hypothesis. They combined water, methane, ammonia, and hydrogen in sealed vials in attempt to replicate Earth’s original atmosphere. They bombarded the vials with heat and continuous electrode sparks to simulate volcanic activity and lightening. Eventually, the reaction produced a number of amino acids – the building blocks of proteins and, by extension, life itself.
Today, the Miller-Urey experiment is contested for a number of reasons, including the possibility that Earth’s original atmosphere may have had a different composition. Still, the production of organic compounds from inorganic “precursors” laid a strong foundation for the primordial soup hypothesis. And new findings support that hypothesis, Dr. Carter says.
“Our work furnishes a likely explanation for how nature overcame one of the main obstacles in turning the building blocks, demonstrated by Miller, into genetic coding and inheritance,” Carter explains.
The obstacle Carter refers to is the fact that certain chemical reactions, essential to spontaneous protein assembly, occur very slowly. Unless they are sped up and regulated, the prospect of life becomes all but impossible. In modern living cells, that reaction is catalyzed by enzymes called aminoacyl-tRNA synthetases. These complex molecules belong to two separate families, or classes. Class I synthetases activate 10 of the 20 amino acids that form proteins. Class II synthetases activate the other 10.
In their experiments, Carter and colleagues took modern synthetases and stripped away all but their essential and universal components. They found that the remaining structure, which they call “Urzymes,” were actually functional. These Urzymes probably resemble the ancestral molecules which eventually gave way to life, Carter says.
“We discovered Urzymes within the elaborate modern aminoacyl-tRNA synthetases by ignoring all the bells and whistles created by evolution,” Carter says. “We showed that what was left was fully capable of translating the code.”
According to Carter, the genetic code itself is strangely organized. One coding strand forms the outer surface of the protein, while the other forms the core. In other words, the two strands rely on “inside-out” interpretations of the same genetic information.
“We devised a way to show experimentally that the two families are related to each other, despite all evidence to the contrary,” Carter says. “Our experiment shows that the ancestral Class II protozyme was built from exactly the same blueprint as the ancestral Class I protozyme, only the blueprint behaved as if it were written on glass and interpreted from the opposite side. The stunning thing is that both interpretations work equally well in the test tube.”
In other words, nature solved the protein production problem by evolving a single gene to do two separate jobs. And while Carter and Dr. Wolfenden’s study leaves many questions unanswered, it does provide a “new set of tools” with which to move forward. Carter says his work could inform new experiments to “fill the gaps” in prebiotic chemistry.
Existential implications aside, there is another motivation for answering the abiogenesis question. If we fully understand which materials and conditions are necessary to the production of life, we can narrow our search for life elsewhere in the cosmos. In other words, a primordial soup recipe could revolutionize the study of astrobiology.
Carter, however, isn’t as interested in his work’s extraterrestrial applications.
“I myself am an inveterate ‘terrestrial chauvinist,’” Carter says. “I believe that life as we know it involves so many enchanting coincidences that it is both unique and inevitable, given appropriate environments. My point of view is probably an outlier, but it is based on my life trying to understand what makes biochemistry tick and discovering just how well-suited so many of nature’s choices really are.”