© Adam Batchelor

On the banks of Pavilion, I am perpetually buzzed by iridescent blue dragonflies, while a loon paddles by. After two days of ROV-only operations, human divers are now on the scene. To accommodate them, the team is taking an extra boat out to the dive site. This time I’m out on the water with them, although my primary task is to stay out of the way. In fact, my view was better back in the trailer: I am limited to watching the scientists watch the monitors and steer the ROVs, unable to witness what the divers are actually doing.

Dragonflies, loons, divers and even unusual bacteria are recognisably alive – as a Sesame Street song says, they ‘breathe and eat and grow’. But do all living things?

The hardest part of finding life elsewhere in the cosmos may be recognising it when we see it. Most life on Earth is microbial, and though we often associate bacteria with disease, most species care not for humans one way or the other. A huge number of species thrive in places that would kill us, and vice versa: deep water, acid caves, bitter cold or boiling hot. Yet there is still kinship between these organisms and us, though evolution and adaptation have separated us.

Because of that kinship, all life on Earth is built from cells; it all uses liquid water as part of its essential structure; it is all built of similar molecules containing carbon, oxygen, nitrogen and a few other common elements; and it all uses DNA and RNA to code information about itself and pass that information along to future generations. Yet we must ask: does life have to be that way? If we replayed the history of our solar system, would life use the same chemistry, make cells and shape its environment in the same way?

Life is organic, which simply means ‘molecules containing carbon’. Organic molecules are pretty common in our galaxy. Astronomers have found hints of amino acids (the building blocks of proteins) in comets, and nucleobases (the genetic ‘letters’ of DNA and RNA) in clouds of gas between stars.

But although water may be necessary for life, it’s so abundant on other worlds and in interstellar space as to be unremarkable. We’ve yet to find any sign of anything out there that could be construed as ‘life’.

Paradoxical as it may sound, there might be inorganic life, too: ‘organic’ doesn’t mean ‘living’. The silicon-based life that inhabits the popular sci-fi universes of Star Trek and Terry Pratchett’s Discworld is the result of that kind of thinking. Silicon sits in the same column on the periodic table as carbon, so it is chemically similar. Ultimately the bonds it makes aren’t quite right, so we don’t see it forming the same kinds of molecules. Carbon seems uniquely able, among all the elements on the periodic table, to form structures with other atoms that are complicated enough for life.

DNA is certainly complex, which leads many researchers to wonder how it came to be in the first place. One common hypothesis is that RNA – which exists as a single chain, unlike DNA’s double chain – came first, but even RNA is complex. “Maybe life didn't start with RNA, but started with something a little bit simpler,” says John Chaput of Arizona State University. “Whatever that simpler material was, it helped produce RNA.”

The ‘D’ in DNA and the ‘R’ in RNA represent the sugars deoxyribose and ribose, respectively. Deoxyribose and ribose are the ladder struts on which the genetic letters are rungs, but they aren’t the only possible sugars for the job. Artificial genetic molecules called ‘XNA’ can be built from other sugars: X could be any one of a number of other possibilities.

Chaput is most interested in the sugar known as ‘threose’, because the resulting molecule TNA ‘recognises’ RNA and links up with it, just as DNA links up with RNA. TNA is simpler than RNA and DNA, both in chemical structure and in how easy it is to make. Chaput and like-minded researchers wonder if TNA came first on early Earth: “Because TNA was simpler to synthesise, it arose early but was quickly taken over by RNA.”

XNAs are only one possible alternative route for life. Carbon makes many more molecules than are used by life as we know it. Proteins don’t use all the types of amino acids; DNA and RNA don’t use all the nucleobase ‘letters’ that are chemically possible. It’s possible life forms elsewhere could have the same basic organic chemistry and even have genetic codes similar to ours, but use different molecules in constructing their cells.