The Assembly of Life

“There is no definitive signature of life.”

“In 2021, a team led by Lee Cronin of the University of Glasgow in Scotland and Sara Walker of Arizona State University proposed a very general way to identify molecules made by living systems — even those using unfamiliar chemistries.”

“Their method, they said, simply assumes that alien life forms will produce molecules with a chemical complexity similar to that of life on Earth.”

“assembly theory”

“seeks that explanation not, in the usual manner of physics, in timeless physical laws, but in a process that imbues objects with histories and memories of what came before them.”

“Assembly theory, he said, offers a way to discover the contingent histories of objects — an issue ignored by most theories of complexity, which tend to focus on the way things are but not how they got to be that way.”

“why, given the astronomical number of ways to combine different atoms, nature makes some molecules and not others”

“It’s one thing to say that an object is possible according to the laws of physics; it’s another to say there’s an actual pathway for making it from its component parts.”

““Assembly theory was developed to capture my intuition that complex molecules can’t just emerge into existence because the combinatorial space is too vast,” Cronin said.”

“the pair says, assembly theory provides a consistent and mathematically precise account of the apparent historical contingency of how things get made — why, for example, you can’t develop rockets until you first have multicellular life, then humans, and then civilization and science. There is a particular order in which objects can appear.”

““We live in a recursively structured universe,” Walker said. “Most structure has to be built on memory of the past. The information is built up over time.””

“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.”

“random molecules won’t do anything of interest, such as behaving like an enzyme.”

“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.”

“finding many identical complex molecules is improbable unless some orchestrated process — perhaps life — is at work.”

“the researchers found they could estimate AI using mass spectrometry”

“the researchers could reliably determine an AI based on the complexity of the molecule’s mass spectrum.”

“The analysis is susceptible to false negatives”

“But perhaps more importantly, the experiment produced no false positives”

“So the researchers concluded that if a sample with a high molecular AI is measured on another world, it is likely to have been made by an entity we could call living.”

“Assembly theory predicts that objects like us can’t arise in isolation — that some complex objects can only occur in conjunction with others.”

“traditional physics is only of so much use”

“the information needed to make specific objects like us wasn’t there at the outset but accumulates in the unfolding process of cosmic evolution”

“The information “is in the path,” Walker said, “not the initial conditions.””

“the keys to observed reality might not lie in universal laws but in the ways that some objects are assembled or transformed into others.”

“The theoretical physicist Chiara Marletto of the University of Oxford is developing a similar idea with the physicist David Deutsch. Their approach, which they call constructor theory and which Marletto considers “close in spirit” to assembly theory, considers which types of transformations are and are not possible.

“Constructor theory talks about the universe of tasks able to make certain transformations,” Cronin said. “It can be thought of as bounding what can happen within the laws of physics.” Assembly theory, he says, adds time and history into that equation.”

“To explain why some objects get made but others don’t, assembly theory identifies a nested hierarchy of four distinct “universes.””

“In the Assembly Universe, all permutations of the basic building blocks are allowed. In the Assembly Possible, the laws of physics constrain these combinations, so only some objects are feasible. The Assembly Contingent then prunes the vast array of physically allowed objects by picking out those that can actually be assembled along possible paths. The fourth universe is the Assembly Observed, which includes just those assembly processes that have generated the specific objects we actually see.”

“Assembly theory explores the structure of all these universes, using ideas taken from the mathematical study of graphs, or networks of interlinked nodes.”