LIFE × GENETICS
The RNA World
between 4.4 and 3.8 billion years ago (if it existed)
The RNA world is a hypothetical stage in the origin of life when a single molecule, RNA, both stored instructions and carried them out, before DNA and proteins divided the two tasks.
In the RNA world hypothesis, one kind of molecule once did the two jobs that living things now divide between DNA and proteins. DNA stores the instructions of a cell and proteins carry them out. Neither can be made without the other, and this circularity makes it hard to explain how either began. RNA can do both jobs, though it excels at neither. Like DNA, it stores instructions in the order of its components, called its sequence. Like a protein, it can catalyse a chemical reaction, that is, speed it up. An RNA that does so is called a ribozyme. The evidence comes from our own cells. Metabolism, the set of chemical reactions that keep a cell alive, depends on helper molecules called coenzymes. Several coenzymes are built from the same components as RNA, as though they had been left in place when an older chemistry was replaced.
Why it matters
The RNA world is the most concrete answer proposed so far to the hardest question in biology: how ordinary chemistry became a process capable of copying itself.
Dating & uncertaintyDisputed
The RNA world left no trace in the rocks. Its date is only bracketed: it comes after liquid water appeared and before the last common ancestor of all living things. Researchers vigorously debate whether RNA genuinely preceded DNA and short proteins or appeared at the same time as them. Xu and colleagues (2020) produced components of both RNA and DNA under the same experimental conditions.
Earth history as one day
02:19:3321 h 40 min before midnight
if Earth’s 4.54 billion years were compressed into a single day