Earth May Have Reached Its “Sweet Spot” for the RNA World 4.33 Billion Years Ago

Scientists have identified a potential “sweet spot” in Earth’s deep past when conditions may have been ideal for the earliest stages of life to take hold—a proposed phase known as the RNA world.

The most favorable conditions may have occurred about 4.33 billion years ago, although Earth could have become suitable for an RNA world as early as 4.4 billion years ago. By then, the extreme impacts from asteroids, comets and other planet-forming debris had largely subsided, according to a study published Tuesday, Sept. 22, in Nature Communications.

When could the RNA world have begun?

The study provides a more precise estimate for when the RNA world may have emerged than earlier research based on geochemical modeling, biomolecular analysis and simulations of early atmospheric chemistry, lead author Oleg Abramov, a senior scientist at the Planetary Science Institute, told Live Science in an email.

“This time period is consistent with previous estimates of about 4.35 billion years ago, but the range of uncertainty is much narrower,” Abramov explained.

Earlier estimates placed the possible timing of the RNA world between 4.46 billion and 4.26 billion years ago, or between 4.45 billion and 3.9 billion years ago.

What was the RNA world?

The RNA world is a hypothetical period in Earth’s history that may have preceded the emergence of the last universal common ancestor, or LUCA—the single microorganism from which all life on Earth is thought to descend.

RNA, or ribonucleic acid, is a single-stranded molecule that can store genetic information and perform important functions inside living cells. Scientists have proposed that RNA existed before DNA became the primary genetic material. During this early phase, RNA may have acted as a replicator, copying and transmitting genetic information among primitive life forms.

RNA later may have been joined by proteins and eventually replaced as the main genetic material by the more chemically stable DNA. Researchers have also proposed alternative and hybrid scenarios in which RNA and DNA appeared around the same time or shared some building blocks.

“The RNA world is the leading hypothesis for the emergence of life on Earth,” Abramov said.

How asteroid impacts shaped early Earth

To estimate when Earth became suitable for an RNA world, Abramov and his colleagues simulated how giant impacts affected the planet’s crust and its overall habitability between 4.5 billion and 3.5 billion years ago.

“We built an impact model constrained by the moon crater record,” Abramov said. The researchers examined both the destructive effects of impacts—such as the heat-driven breakdown of important biomolecules—and their potentially life-supporting effects, including the formation of hydrothermal systems.

The model showed that space rocks hundreds of miles in diameter continued to strike Earth after its formation. These objects were far larger than the asteroid that wiped out the dinosaurs, although they were much smaller than the impactor believed to have formed the moon.

Until about 4.4 billion years ago, the impacts may have repeatedly sterilized Earth’s surface. They likely caused widespread melting of the crust, ocean evaporation, extreme temperature differences within the crust, and persistent showers of hot material and vapor.

Hydrothermal vents may have provided the right ingredients

After about 4.4 billion years ago, conditions began to stabilize. Clusters of hydrothermal vents could then have formed in the shallow crust, providing environments enriched with potential building blocks of RNA.

These ingredients included nucleotides, short chains of amino acids called peptides, and fatty compounds known as lipids, Abramov said.

“These criteria indicate that Earth became suitable for an RNA world between 4.4 billion and 4.3 billion years ago, with optimal conditions occurring about 4.33 billion years ago,” he said.

A 130-million-year gap before LUCA

The results suggest there may have been a gap of roughly 130 million years between the peak of the RNA world and the emergence of LUCA, which scientists believe existed about 4.2 billion years ago.

However, both dates remain uncertain. The gap between the two events could have been as large as 240 million years, Abramov noted.

“The longevity of the RNA world is highly uncertain, and our study did not clearly constrain it,” he said.