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 e...