Fundamental constraints

It has been argued that the historical nature of evolution makes it a highly path-dependent process. Under this view, the outcome of evolutionary dynamics could have resulted in organisms with very different forms and functions. At the same time, there is ample evidence that convergence and constraints strongly limit the domain of the potential design principles that evolution can achieve. Are these limitations relevant in shaping the fabric of the possible? We argue that fundamental constraints are associated with the logic of living matter itself — constraints that any life form, whether on Earth or elsewhere, would be unlikely to escape.

Life could, in principle, adopt very diverse configurations, but we claim that all life forms will share some inevitable features. These include the use of linear information polymers, the cellular nature of the building blocks of life, the threshold nature of computations in cognitive systems, and the discrete architecture of ecosystems. In each of these domains, deep regularities emerge that hint at some kind of universality. Remarkably, many of these regularities were predicted theoretically before empirical evidence confirmed them — a hallmark of genuine scientific laws.

Consider, for example, the problem of biological information carriers. All known life forms require physical structures that can reliably encode a large number of phenotypic states and be faithfully replicated. The widespread use of long, linear polymers for this purpose is striking. Is there anything special about one-dimensional polymers that makes them a universal solution? Three arguments — based on evolvability, computation and thermodynamics — converge to suggest that the linear polymer is the expected option. This insight connects, in a deep way, to Alan Turing's 1936 formalization of computation: the Turing machine itself operates on a one-dimensional tape, reading and writing symbols sequentially. The parallel between the abstract logic of computation and the physical logic of genetic information is, we believe, far from accidental.

At larger scales, similar constraints appear. Multicellularity, for instance, involves a limited repertoire of developmental building blocks — a toolkit that appears again and again across independent evolutionary lineages. Cognitive architectures seem to require threshold-based processing units organized in multilayer structures, whether they are biological neurons or engineered systems. And at the ecosystem level, the presence of parasites appears to be an inevitable outcome of any evolved system complex enough to sustain replication and information processing, a regularity that is enormously strong empirically even if we do not yet possess a fully universal theoretical argument.

Our approach to these problems draws from thermodynamics, information theory, computation, synthetic biology and evolutionary theory. Synthetic life, in particular, can provide profound clues on what to expect and how likely certain organizational features are under given conditions. To us, synthetic biology is a powerful way to interrogate nature about the possible. This programme fits within the broader goal of finding general theories of life that transcend the specifics of life on Earth — a road map towards understanding the fundamental logic that underwrites biological complexity across scales.