the RNA-world hypothesis
/ AR-en-ay world hy-POTH-uh-sis /
Life today runs on a partnership: DNA holds the information, proteins do the chemistry, and RNA shuttles between them. But this raises a chicken-and-egg puzzle for life's origin. You need proteins to copy DNA, but you need DNA to specify proteins. Neither can come first. The RNA-world hypothesis offers a way out: long before either, there was a time when RNA did both jobs at once.
The idea is that early life used RNA as both its genetic material and its catalyst. A single kind of molecule that can store a sequence (like DNA) and also fold up to speed reactions (like an enzyme) could, in principle, copy itself and evolve without needing anything else. RNA fits because we now know it can do both: it carries sequence, and as a ribozyme it can cut, join, and even, in the lab, copy short stretches of RNA. Over time, the hypothesis goes, RNA outsourced information storage to the more stable DNA and chemistry to the more versatile proteins, keeping for itself the messenger and adapter roles we see today.
The strongest evidence is the fossils RNA left in modern cells. The ribosome's catalytic core is RNA. The cell's central energy carrier, ATP, and key helpers like coenzyme A and NAD are built around RNA-like nucleotides. Splicing involves RNA catalysts. These look like leftovers from an age when RNA ran the show. The hypothesis is well supported and widely held, but it is not proven fact: no one has built a self-replicating RNA from scratch, and how the first RNA arose remains an open, honest question.
ATP, the molecule your cells spend as energy currency, is essentially an adenine RNA nucleotide with extra phosphates. That a core metabolite is an RNA building block is one of the clues that RNA-based chemistry came first.
RNA-like molecules at the heart of metabolism hint at an early world run by RNA.
The RNA world is a strong, popular hypothesis, not established fact. RNA is chemically fragile and hard to make under early-Earth conditions, so some researchers argue RNA had simpler precursors or co-evolved with peptides. Treat it as the best current story, not the final word.