Scientists have identified a "sweet spot" in Earth's deep past when conditions were optimal for a proposed early stage of life to take hold — a phase known as the RNA World.

The sweet spot was 4.33 billion years ago, although Earth may have become suitable for the RNA World as early as 4.4 billion years ago, given that extreme bombardment by asteroids, comets and other rocky leftovers of planet formation had largely stopped by then, the researchers reported in a study published Tuesday (Sept. 22) in the journal Nature Communications.

The findings give a more precise estimate of the RNA World's onset than previous studies based on geochemical modeling, biomolecular analyses and simulations of early atmospheric chemistry did, study first author Oleg Abramov, a senior scientist at the Arizona-based Planetary Science Institute, told Live Science in an email.

"The timing is consistent with previous estimates of approximately 4.35 billion years ago, but our range of uncertainty is significantly narrower," Abramov explained, adding that "previous ranges included 4.46 to 4.26 billion years ago, and 4.45 to 3.9 billion years ago."

The RNA World is a hypothetical time in Earth's history before the emergence of the Last Universal Common Ancestor (LUCA), the single microbe from which all living things on Earth descend. RNA, or ribonucleic acid, is a single-stranded molecule that is structurally similar to DNA and performs vital functions in living cells. Scientists think the RNA World preceded the emergence of DNA; in that early epoch, RNA would have been the primary replicating substance, carrying genetic information across generations of basic lifeforms by copying itself independently.

RNA evolved into its more chemically stable cousin, DNA, sometime before the emergence of LUCA. Scientists think the RNA World first developed into an RNA-and-protein world, and that DNA then appeared in two distinct stages nicknamed the U-DNA and T-DNA worlds.

While researchers have suggested alternative and hybrid scenarios for the emergence of life on Earth, the RNA World is a leading hypothesis, Abramov said. To determine when it might have existed, he and his colleagues built a computer model of Earth's interior that simulated how giant impacts on the planet's surface between 4.5 billion and 3.5 billion years ago affected conditions in the crust and Earth's overall compatibility with life.

"We constructed an impact bombardment model constrained by observables such as the lunar cratering record," Abramov said. "We examined both detrimental effects of impacts, such as temperature-induced degradation of key biomolecules, and effects conducive to life, such as generation of hydrothermal systems."

The model showed that space rocks measuring hundreds of miles across — an order of magnitude larger than the asteroid that killed the dinosaurs, but far smaller than the impactor that created the moon — went on slamming into Earth and sterilizing its surface until 4.4 billion years ago. Life could not have survived such intense bombardment, which probably triggered widespread melting of the crust, ocean vaporization, extreme temperature gradients in the crust and continuous showers of burning material and rock-vapor rain at the surface, Abramov said.

However, the conditions stabilized after 4.4 billion years ago. And as a result, hydrothermal vent clusters appeared within the near-surface crust that concentrated the ingredients for RNA, including nucleotides, short amino-acid chains called peptides and fat-like compounds known as lipids, Abramov said.

"These criteria point to the Earth becoming suitable for an RNA World between 4.4 and 4.3 billion years ago, with optimal conditions at approximately 4.33 billion years ago," he said.

The results suggest there was a gap of roughly 130 million years between the heyday of the RNA World and the emergence of LUCA, which scientists think lived about 4.2 billion years ago. However, there are still large uncertainties around both events, so the interval could be as big as 240 million years, Abramov noted.

"The duration of the RNA World is highly uncertain, and our study did not explicitly constrain it," he said.