Even if extraterrestrial life uses completely different molecules than terrestrial organisms, it would have to stick to a set of relationships found by American scientists.

When it comes to the search for extraterrestrial life, we always focus on ‘life as we know it’. Because, well, the only planet in the universe that we know has life on it is still Earth. And that simply makes use of certain chemical elements and molecules. So what else can we do but look for signs of that?

Well, we can see if there’s anything more general to say about life. Something that should also apply to organisms that work with completely different molecules. Theoretical physicist and astrobiologist have that Sara Walker from Arizona State University and colleagues done†

Breaking up or combining molecules

Walker and her team focus on enzymes: substances that enable or accelerate a chemical reaction. It’s not about the enzymes per se, Walker explains. “After all, we don’t know whether life on other planets would use the same enzymes. We do think that the reactions they help take place are similar.”

So, in other words, it’s about the roles that enzymes can play; the functions they perform. For example, they can break up or combine molecules. Or move a group of atoms from one molecule to another.

General patterns

What Walker and colleagues have now done is look at those enzyme functions in a database of genetic material from bacteria and other organisms. And that resulted in connections between the total number enzyme functions performed in a particular biochemical system — say, a bacterium — and the number of enzyme functions a certain result has. For example, if there are 150 enzyme functions in total, then about twenty of them involve moving a group of atoms from one molecule to another.

“These are patterns that arise from large-scale statistics applied to all responses stored in the database,” Walker said. “And those patterns don’t depend on the specific chemistry that life on Earth uses.”

synthetic cell

And what can we do with such rules of thumb for living, regardless of the chemistry behind them? As for the search for extraterrestrial life, Walker thinks they could provide “a set of constraints that we might expect to apply to alien biochemistry as well.”

The rules can also come in handy when creating life in the lab. For example, say you’re designing an artificial cell, Walker says. “If you want a certain total number of reactions to take place there, these lines tell you how much of each type of reaction you need.”

The very beginning

Finally, if the rules do apply to all life, they should also apply to Earth life long ago, Walker says. “Then we can trace the properties of life from all eras, including the very beginning.”