Philip Ball writes:

Assembly theory makes the seemingly uncontroversial assumption that complex objects arise from combining many simpler objects. The theory says it’s possible to objectively measure an object’s complexity by considering how it got made. That’s done by calculating the minimum number of steps needed to make the object from its ingredients, which is quantified as the assembly index (AI).

In addition, for a complex object to be scientifically interesting, there has to be a lot of it. Very complex things can arise from random assembly processes — for example, you can make proteinlike molecules by linking any old amino acids into chains. In general, though, these random molecules won’t do anything of interest, such as behaving like an enzyme. And the chances of getting two identical molecules in this way are vanishingly small.

Functional enzymes, however, are made reliably again and again in biology, because they are assembled not at random but from genetic instructions that are inherited across generations. So while finding a single, highly complex molecule doesn’t tell you anything about how it was made, finding many identical complex molecules is improbable unless some orchestrated process — perhaps life — is at work.

Cronin and Walker figured that if a molecule is abundant enough to be detectable at all, its assembly index can indicate whether it was produced by an organized, lifelike process. The appeal of this approach is that it doesn’t assume anything about the detailed chemistry of the molecule itself, or that of the lifelike entity that made it. It’s chemically agnostic. And that makes it particularly valuable when we’re searching for life forms that might not conform to terrestrial biochemistry, said Jonathan Lunine, a planetary scientist at Cornell University and the principal investigator of a proposed mission to look for life on Saturn’s icy moon Enceladus. [Continue reading…]