The question of the origin of life is a chicken-and-egg problem that is perhaps best explained by the ribosome. These small intercellular structures translate genes into proteins, a process essential for life. And yet, the ribosomes themselves are made of proteins and RNA. So what came first, the ribosomes that make proteins or the proteins that make ribosomes? In the early 1960s, the RNA world hypothesis emerged as an answer to this question. RNA is the single-stranded cousin of DNA, the molecule that contains the genetic information for cell biology. Often called the “workhorse molecule” of cells, RNA can take three different forms to fulfill three different functions. Messenger RNA (mRNA) carries genetic instructions from DNA to protein-producing ribosomes, ribosomal RNA (rRNA) helps produce the ribosomes themselves, and transfer RNA (tRNA) synthesizes proteins from mRNA. The RNA world hypothesis suggests that life on Earth began with self-replicating RNA molecules. After all, RNA is much simpler than DNA, so it makes sense that it appears first. In this scenario, the first single-celled life forms would have used RNA to store genetic information and create other molecules. Today, cells divide these tasks between DNA and RNA. But when exactly did the world of RNA emerge? If this was indeed the starting point of life, answering that question is key to understanding our origins and whether life could arise in a similar way on other planets. A study published today in the journal Nature Communications suggests that conditions became optimal for an RNA world about 4.33 billion years ago, offering a more precise timeline for the origin of life. Life emerges from chaos The early Earth was a violent place. Our young planet was constantly bombarded by asteroids and other debris from planetary formation, and these impacts overheated its surface. This made Earth inhospitable to prebiotic chemistry, the chemical reactions that give rise to life. For the RNA world to emerge, the avalanche of space rocks had to slow enough for the planet to cool. To determine when that transition might have occurred, researchers led by Oleg Abramov, a senior scientist at the Planetary Science Institute in Tucson, Arizona, used a three-dimensional computer model to simulate the thermal effects of impacts on the Earth’s crust between 4.5 and 3.5 billion years ago. “We used a different approach than previous studies, which were based on geochemical models, biomolecular analyzes and early atmospheric chemistry models,” Abramov told Gizmodo in an email. “We examined both the detrimental effects of impacts, such as temperature-induced degradation of key biomolecules, and the life-supporting effects, such as the generation of hydrothermal systems.” The model simulated temperatures in the upper crust along with the thermal stability limits of RNA and other life-giving molecules. The analysis revealed that Earth’s temperature probably became suitable for the RNA world between 4.4 and 4.3 billion years ago, and that optimal conditions emerged around 4.33 billion years ago. However, this estimate assumes that other necessary environmental conditions already existed. “RNA chemistry still needs liquid water, reduced nitrogen and other key elements, and a way to concentrate them,” Abramov explained. “Local chemistry also matters: pH, salinity, water activity, mineral surfaces, and whether the environment is a wet-dry subaerial aquifer or a hydrothermal system.” The model does not take those variables into account, and the RNA chemistry could still have failed in a thermally favorable crust if any of these conditions were not present. The study therefore narrows the timeline for the emergence of life on Earth, but does not pinpoint exactly when the first RNA life forms might have appeared. The search for life in other words Geological evidence could help validate this estimated timeline. According to Abramov, his team’s prediction is already consistent with evidence from zircon crystals showing that about 4.4 billion years ago there was liquid water on Earth. Researchers could look for other clues in samples of Earth’s mantle, or in more zircon crystals that serve as records of water, reduction-oxidation reactions, or mineral changes that arise from interactions with fluids at relatively low temperatures. More dated lunar samples could also help limit the impact environment of the early Earth, Abramov said. As scientists focus on the beginning of Earth’s habitable window, he hopes this work will help answer questions about the habitability of other planets, such as Mars. “An intriguing question is the timing of the habitable window on Mars, which probably opened much earlier than on Earth due to Mars’ smaller size and lower average impact velocity,” Abramov said. “That’s a parallel question of habitability, not evidence that life began on Mars or was transferred to Earth, although it remains an intriguing possibility.”
