According to assembly theory, before Darwinian evolution can proceed, something has to select for multiple copies of high-AI objects from the Assembly Possible. Chemistry alone, Cronin said, might be capable of that—by narrowing down relatively complex molecules to a small subset. Ordinary chemical reactions already “select” certain products out of all the possible permutations because they have faster reaction rates.

The specific conditions in the prebiotic environment, such as temperature or catalytic mineral surfaces, could thus have begun winnowing the pool of life’s molecular precursors from among those in the Assembly Possible. According to assembly theory, these prebiotic preferences will be “remembered” in today’s biological molecules: They encode their own history. Once Darwinian selection took over, it favored those objects that were better able to replicate themselves. In the process, this encoding of history became stronger still. That’s precisely why scientists can use the molecular structures of proteins and DNA to make deductions about the evolutionary relationships of organisms.

Thus, assembly theory “provides a framework to unify descriptions of selection across physics and biology,” Cronin, Walker, and colleagues wrote. “The ‘more assembled’ an object is, the more selection is required for it to come into existence.”

“We’re trying to make a theory that explains how life arises from chemistry,” Cronin said, “and doing it in a rigorous, empirically verifiable way.”

One Measure to Rule Them All?

Krakauer feels that both assembly theory and constructor theory offer stimulating new ways to think about how complex objects come into being. “These theories are more like telescopes than chemistry labs,” he said. “They allow us to see things, not make things. That is not at all a bad thing and could be very powerful.”

But he cautions that “like all of science, the proof will be in the pudding.”

Zenil, meanwhile, believes that, given an already considerable roster of complexity metrics such as Kolmogorov complexity, assembly theory is merely reinventing the wheel. Marletto disagrees. “There are several measures of complexity around, each capturing a different notion of complexity,” she said. But most of those measures, she said, are not related to real-world processes. For example, Kolmogorov complexity assumes a kind of device that can put together anything the laws of physics permit. It’s a measure appropriate to the Assembly Possible, Marletto said, but not necessarily to the Assembly Observed. In contrast, assembly theory is “a promising approach because it focuses on operationally defined, physical properties,” she said, “rather than abstract notions of complexity.”

What’s missing from such previous complexity measures, Cronin said, is any sense of the history of the complex object—the measures don’t distinguish between an enzyme and a random polypeptide.

Cronin and Walker hope that assembly theory will ultimately address very broad questions in physics, such as the nature of time and the origin of the second law of thermodynamics. But those goals are still distant. “The assembly-theory program is still in its infancy,” Marletto said. She hopes to see the theory put through its paces in the laboratory. But it might happen out in the wild too—in the hunt for lifelike processes happening on alien worlds.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.