Scientists have been closing in on RNA (ribonucleic acid, the close companion of DNA) as being the most likely candidate for the origin of life on Earth, and now even more evidence has come to light to support this idea. Scientists have demonstrated that the key ingredients to make RNA molecules could have been present on the surface of Earth in its earliest days.

The beauty of RNA is that it can act as both a messenger and a catalyst in the human body, and because of those characteristics, scientists have long suspected that it could have helped life come about. However, the new study fills in some of the missing pieces in the puzzle - how two essential building blocks of RNA came into being.

You're no doubt pretty familiar with DNA, the chemical carrying the genetic code that determines our physical characteristics. RNA acts as a kind of messenger for DNA, carrying out its instructions in various ways and controlling the synthesis of proteins through the body.

However, RNA can also store genetic data too, and some scientists think RNA kick-started life on Earth, with DNA coming along later on the evolutionary timeline.

To test this hypothesis, scientists have been trying to replicate the same conditions found in the first stages of Earth's life, to see if RNA could develop. While two of the RNA building blocks - uracil and cytosine - could be produced, the other two - adenine and guanine - remained stubbornly absent.

Now chemists from the Ludwig Maximilian University of Munich in Germany have created conditions where adenine and guanine can be created, as Bob Yirka reports at Phys.org.

The hypothesis is that formic acid found on comets crashing into Earth's surface could have reacted with existing compounds called purines already present in the atmosphere, creating many more purines - including our friends adenine and guanine.

In other words, the research shows a way that all the key ingredients to make RNA molecules could have been present.

If the ingredients were there, then RNA might have been there too, right at the very earliest stages of Earth's formation, making it a credible candidate as the first self-replicating molecule on our planet.

"We now have a pathway that would allow us to use simple molecules that were likely present on the early Earth," lead researcher Thomas Carell told New Scientist.

There's more work to be done. The chemical conditions that give rise to adenine and guanine don't match those that can create uracil and cytosine, so we're still missing a few pieces of the puzzle, as Robert F. Service reports at Science. It's still not clear how all these bases joined up to form RNA, either.

But despite this, it's an important step forward for the RNA world hypothesis, and an amazing peek back through several billion years' worth of history.

The research has been published in Science.