All cellular life on Earth is based on DNA, which transfers information—about everything from hair color to personality traits—from one generation to the next. The four chemical bases that convey this information are adenine (A), cytosine (C), guanine (G), and thymine (T).
The other essential “information molecule” on Earth is RNA, in which thymine (T) is replaced by uracil (U). RNA has a one-string structure rather than a double-string structure like DNA. The first cellular life on our planet is thought to have relied exclusively on this means of transferring genetic information—in the so-called “RNA world”—and even today there are viruses (like the one that causes COVID) that only use RNA.
In a paper recently published in Science, a research group led by Dona Sleiman from the Institute Pasteur in Paris has discovered that some viruses show more variation in their genetic coding than was previously known. In the RNA of these viruses, adenine (A) is replaced with Z, where Z stands for diaminopurine.
This follows an earlier study by Zunyi Yang and colleagues at the Foundation for Applied Molecular Evolution in Gainesville, Florida, showing that an artificial genetic system could be created by adding two additional non-standard bases to ordinary DNA. Amazingly, the artificial six-base system continued to evolve rather than reverting back to the natural four-base system. This implies that the DNA we take as standard—made of A, C, G, and T—is just one of many viable solutions to the challenge of biological information transfer.
The variability does not stop here. Strings of DNA are organized in base triplets that determine which of the standard 20 amino acids are assigned to synthesize proteins. However, these triplet assignments are not universal. For example, CUG, which usually codes for the amino acid serine, instead codes for the amino acid leucine in some types of fungi. Also, some organisms naturally encode for two additional amino acids instead of the standard 20 amino acids.
What does this brief excursion into genetics have to do with alien life? While it is believed that all life on our planet derives from one common ancestor, the genetic code is much more flexible and diverse than usually appreciated. The biochemistry of information transfer in an alien species would almost certainly use different building blocks and encodings, and perhaps even a different number of bases. Our genetic code is surely highly optimized for life on Earth, but I feel certain that there are many optimal solutions—perhaps some that are even better—for transferring information chemically from one generation to the next.
We, of course, cannot say what type of genetic code an alien species would use. But given that it would most likely be biochemically different, it would mostly likely be easily distinguishable from life on Earth. It may even be more different than we expect. A fascinating out-of-the-box genetic system has been suggested by Gerald Feinberg and Robert Shapiro, based on magnetic orientations rather than chemistry. They showed how magnetized particles, when approaching a magnetic chain, will align with the chain. As a result, the chain is duplicated, and this method could in principle be used to convey information in a binary code.
So, while alien life may well transmit genetic information using structures similar to RNA and DNA, we should always be prepared to expect the unexpected.