Did Life Start with DNA or RNA?

According to the ‘RNA world’ theory, life started with RNA molecules. Then, through evolution, this RNA world gave way to the era of DNA and proteins—because DNA is more stable and durable than RNA. But a study published today in Nature challenges this theory, instead suggesting that life could have begun with a bit of both DNA and RNA.

In the study, researchers simulated the conditions on a primordial rocky Earth with shallow ponds in the lab. They dissolved RNA-forming chemicals in water, dried them out and heated them, and then simulated the early sun’s rays by exposing them to UV radiation. In this recreation of early Earth geochemistry, intermediates in the synthesis of two of the building blocks of RNA were simultaneously also converted into two of the building blocks of DNA.

“The RNA world hypothesis suggests that life began with RNA, before a genetic takeover occurred involving primitive biosynthetic machinery and natural selection to result in DNA,” says senior author John Sutherland of Cambridge. “Our work suggests that in conditions consistent with shallow primordial ponds and rivulets, there was a mixed genetic system with RNA and DNA building blocks co-existing at the dawn of life. This fulfills what many people think is a key precondition for the spontaneous emergence of life on Earth.”

The team’s experiments to simulate early Earth geochemistry showed that four of the building blocks for DNA and RNA can arise from the same reagents and conditions. They produced cytidine and uridine, two of the building blocks of RNA, as well as deoxyadenosine, which is a building block of DNA. Deoxyadenosine was partly converted to deoxyinosine, which represents another DNA building block. The researchers believe that these four building blocks may have coexisted before life evolved and that they were the beginnings of a primitive genetic alphabet.

“The nucleic acids, RNA and DNA, are clearly related and this work suggests that they both derive from a hybrid ancestor, rather than one preceding the other,” Sutherland adds. “Since genetic information always flows from nucleic acids to proteins and never in reverse—a principle called the ‘central dogma’ of molecular biology by Francis Crick—we now need to uncover how the information that can be stored and purveyed by these nucleic acids could have been first used to make to proteins.”