Our asteroid-blasted prehistoric earth, shaped by bubbling geothermal pools, does not seem to be hospitable today. But somewhere in the chemical chaos of our early planet, life was forming. As? For decades, scientists have been trying to make miniature replicas of infant Earth in the lab. There they search for the original ingredients that have created the essential building blocks for life.
It is attractive to chase our history of origin. However, this quest can do more than just thrill. Knowing how the earth built its first cells could affect our search for extraterrestrial life. If we determine the ingredients and the environment needed for a spontaneous life, we could look for planets in our entire universe for similar conditions.
Much research into the origin of life today focuses on one particular building block: RNA. While some scientists believe that life is made up of simpler molecules and later-developed RNA, others are looking for evidence to prove (or disprove) the RNA initially formed. RNA is a complex yet versatile molecule that stores and relays genetic information and helps to synthesize proteins, making it a capable candidate for the backbone of the first cells.
To test this "RNA world hypothesis," researchers face two challenges. First, they must find out which ingredients reacted to produce the four nucleotides of RNA ̵
So far, scientists have made significant progress in finding precursors for C and U. But A and G remain elusive. Jack W. Szostak, professor of chemistry and chemical biology at Harvard University, and first author and PhD student Seohyun (Chris) Kim now propose in another article published in PNAS that another could be started with RNA Set of nucleotide bases. Instead of guanine, RNA could have relied on a replacement inosine.
"Our study suggests that the earliest life forms (with A, U, C, and I) could come from a different set of nucleobases found in modern life (A, U, C, and G)," said Kim. How did he and his team come to this conclusion? Laboratory tests for the production of A and G, purine-based nucleic acids, produced too many undesirable by-products. However, researchers recently discovered a way to make versions of adenosine and inosine – 8-oxo-adenosine and 8-oxo-inosine – from materials available on earth. Kim and his colleagues began to investigate whether RNA constructed with these analogs can efficiently replicate.
However, the substitutes did not work. Like a cake baked with honey instead of sugar, the end product may look and taste similar, but it does not work so well. The honey cake burns and drowns in liquid. The 8-oxo-purine RNA is still powerful, but loses both the speed and accuracy needed for copying. If it replicates too slowly, it will fall apart before it completes. If it makes too many mistakes, it can not serve as a faithful tool for reproduction and evolution.
Despite their inadequate performance, the 8-oxo-purines brought an unexpected surprise. As part of the test, the team compared the capabilities of 8-oxo-inosine with a control, inosine. In contrast to its 8-oxo counterpart, inosine allowed RNA to replicate at high speed with few errors. "It turns out that it shows reasonable rates and accuracy in RNA copying reactions," concluded the team. "We suggest that inosine could have served as a substitute for guanosine in the early development of life."
The discovery of Szostak and Kim could help justify the RNA world hypothesis. Over time, their work could confirm the primary role of RNA in our lineage of origin. Or scientists might find that the early Earth offered many ways to grow. Armed with this knowledge, scientists could eventually identify other planets with the essential ingredients and determine if we are sharing this universe or are indeed alone.