Very often important science is constructed by a myriad of small advances in knowledge. This is almost certainly going to be true for answering one of the big questions in biology: “How did life on Earth originate?”
It’s been known for a long time that it probably originated where there was a concentrated mixture of organic compounds. (Just because they’re called organic, doesn’t mean all such compounds come from living things – it simply means they’re carbon-based.) Out of that mixture, which is usually labeled the primal soup came the chemical processes that eventually put together some of the available organic compounds until they became ‘self-assembling’ – a process that would automatically repeat following natural chemical reactions (or pathways). For this to happen, it was necessary that short organic compounds (‘short’ meaning just a few elements such as carbon, oxygen, and hydrogen in a simple chain) into even longer organic compounds – polymers. Eventually within the ‘primal soup’ polymers were created that at least partly resembled RNA (Ribonucleic Acid), which is now the ‘messenger’ of the DNA code but archaically almost certainly developed before DNA.
The ‘proto-RNA’ was probably a short polymer. Longer polymers don’t form easily. That’s a way of expressing the fact that the two ends of a polymer chain have a tendency to bond with each other, rather than forming longer chains with other materials. The tendency is called cyclization – forming loops. With a proto-RNA or DNA that would have been the end of development. There is a way out of this tendency to loop, which has been identified as an intercalator. This is a molecule that reacts with the strands of RNA (or DNA) making them spread their molecules. (In modern RNA, this usually means the strand expands at the point where the intercalator is present.) When that spread occurs, other organic compounds can be attached – thus creating a longer polymer.
Such an intercalator is what a team at the Georgia Institute of Technology (USA) has discovered. They found that the molecule of ethidium (traditionally an antiviral or trypanocidal agent) assisted short oligonucleotides (a nucleic acid polymer, in this case a short piece of proto-RNA or DNA) in forming longer polymers and is also involved in selecting the structure of base pairs (the four building blocks of RNA: adenosine, cytosine, guanine, and uracil).
“Our hypothesis is that before there were protein enzymes to make DNA and RNA, there were small molecules present on the pre-biotic Earth that helped make these polymers by promoting molecular self-assembly,” said Nicholas V. Hud, professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology.
“In our experiment, we found that the midwife molecules present had a direct effect on the kind of base pairs that formed. We’re not saying that ethidium was the original midwife, but we’ve shown that the principle of a small molecule working as a midwife is sound. In our lab, we’re now searching for the identity of a molecule that could have helped make the first genetic polymers, a sort of ‘unselfish’ molecule that was not part of the first genetic polymers, but was critical to their formation,” said Hud.
Ethidium or its equivalent must have been present in the mix of the ‘primal soup.’ One more small piece of the chemistry set that eventually became life.