Not only water: "Extraterrestrial life could be based on other molecules"

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Santa Fe Institute biochemist and astrobiologist Natalie Grefenstette has long been fascinated by how life can arise under different environmental conditions. As part of NASA's Agnostic Biosignatures project, the postdoctoral researcher wants to find out which organising principles and different chemical processes that are not water-based can give rise to life.

The search for extraterrestrial life has so far focused on the search for water. Why is that?

Our understanding of living systems is based on life on Earth. So it is natural to look for life as we know it. Water is very important for life on earth. Chemical reactions happen better in a liquid, a solvent. Water is an interesting liquid because it is a dipole with a charge difference. This allows it to dissolve and interact with many other molecules that are important for life as we know it, such as RNA and DNA.

You advocate broadening the search for life. Why and what should we search for besides water?

We want to think outside the box because we basically don't know what life is. We don't know if it has to be limited to the molecules on Earth. There is some evidence that life on Earth could also be different. We have added new bases to DNA and changed amino acids in synthetic biology. Also, the discoveries of extremophilic microorganisms has shown that life can exist in very different environmental conditions.

All this shows that living systems could also be based on completely different molecules. There is a lot of research into whether there are basic rules for living systems that tell us more about what spaces they can occupy. There is also research into whether living systems have basic patterns that non-living systems do not have. They are not necessarily looking for specific molecules, but for patterns in these molecules or in their distribution, or for patterns in the distribution of atoms.

What can such patterns look like?

Most living things on Earth use the same 20 amino acids. But we have also discovered amino acids with a different distribution on meteorites. Some amino acids are only found in living things, others only on meteorites. It could be that these amino acids are somehow better suited, or it could be coincidence, or it could be that life is sort of frozen in this way. Because if we change something drastically now, the rest of the system doesn't know how to use it. We don't yet know all the possibilities of what life can be and what it needs.

How can we find out?

I am interested in research that tries to understand the rules of living systems and their needs. We have this joke that if you have a meeting with 50 researchers, you get 60 definitions of life. The NASA definition "a self-sustaining chemical system capable of Darwinian evolution" is quite useful as a working definition. But much of the current discussion is about whether we need a definition of life to look for it at all. We tend to find properties that we associate with life, such as evolution, reproduction, motion or low entropy.

On the other hand, researchers are certainly considering what molecules other systems might use. In the scientific community, for example, formamide and ammonia are being discussed [as alternative solvents to water] because they are quite similar to water. Both are interesting because they are polar molecules that can dissolve molecules from living systems on Earth in particular. Also, formamide is liquid over a much wider temperature range than water, up to 210 degrees instead of 100 degrees. And you may have heard of the idea that life could also be based on silicon instead of carbon.