Scientists identify 'sweet spot' for early stage of life on Earth that preceded LUCA, the ancestor of all living things

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.

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