Did Life Begin in an RNA World?

The RNA-world hypothesis proposes that life began with RNA molecules because RNA can both store genetic information and catalyze chemical reactions, roles now divided mainly between DNA and proteins. This could solve the chicken-and-egg problem of how DNA and proteins first arose: early RNA might have copied itself, accumulated mutations, and undergone natural selection, eventually enabling more complex chemistry and protocells. Lab work has shown that ribozymes can join nucleotides into RNA, but experiments also expose a major obstacle. In one study, mutations often destroyed the catalytic function of large ribozymes, making sustained evolution difficult. Those molecules were hundreds of nucleotides long, whereas spontaneous assembly experiments have produced only much shorter strands, leaving a daunting gap between plausible early chemistry and functional replicators.

More recent experiments offer a possible way across that gap. Philipp Holliger’s group identified a 45-nucleotide RNA, QT45, that can copy another RNA strand, along with two similarly small molecules with related abilities. Finding these catalysts in a pool of about 10 trillion sequences suggests such activity may not be extraordinarily rare. The researchers also demonstrated replication cycles driven by changing acidity and freezing and thawing, conditions that might have occurred in icy environments on early Earth. Still, the hypothesis depends on suitable habitats lasting long enough for replication to take hold, and self-copying alone may not count as life. A living system also needs metabolism: ways to obtain energy and use it to build useful molecules.