Autocatalytic networks are groups of chemical reactions in which products of the network help catalyze reactions that make more of its members. They could have organized early metabolism before cells had modern genetic machinery, but scientists have not shown that such networks existed on the early Earth or were the first step toward life. The strongest evidence so far is that RAF patterns occur in the metabolism of living microbes—not that those patterns were present before life.
What is an autocatalytic network?
In an autocatalytic network, reactions support one another: molecules made by some reactions can catalyze other reactions in the network. The term does not necessarily mean that one molecule makes an identical copy of itself. The self-support is collective, across a set of reactions.
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This idea matters to origin-of-life research because it offers a possible way for chemical organization to arise before systems based on genes and the proteins they encode. A network that helps make its own catalysts and reactants might sustain organized chemistry from a simpler supply of starting materials. That would be a possible precursor to life, not proof that the network itself was alive.
How RAF theory makes the idea precise
RAF theory provides a formal test for whether a reaction network has two properties, given a specified food set—the basic molecules available from outside the network—and a specified pattern of which molecules catalyze which reactions.
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- Reflexively autocatalytic: every reaction in the set has a catalyst that is either in the food set or produced by reactions in the set.
- Food-generated: the reactants needed by the set’s reactions can be built from the food set using reactions in the set.
Together, these conditions describe a network whose reactions can be catalyzed and whose required reactants can be generated from the stipulated starting materials. They are properties of a mathematical model, not a guarantee that the chemistry would persist in a particular real environment. Nor do they show that a network could evolve into life.
The idea traces back to early work by Stuart Kauffman. Later mathematical and computational work developed ways to identify and analyze RAFs, while reviews also describe experimental autocatalytic networks. So the concept is not only an abstract mathematical proposal—but a laboratory example still cannot establish what happened on the early Earth.
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What microbial metabolism tells us—and what it cannot
A 2020 study in Proceedings of the Royal Society B, “Autocatalytic chemical networks at the origin of metabolism,” searched metabolic networks in living microbes for RAF structures. It reported RAFs in the metabolism of ancient anaerobic autotrophs. In the study’s analysis, these networks, when supplied with small-molecule catalysts, could generate acetyl-CoA, amino acids and bases. The authors also reported that amino acids and bases without organic catalysts did not generate metabolic RAFs in their analysis.
The authors interpret the findings as consistent with an autotrophic origin of metabolism and suggest that autocatalytic chemical networks may have preceded proteins and RNA. The result makes RAF theory relevant to questions about how metabolism might have become organized; it does not directly demonstrate that the same network existed before life.
The study states the evidence boundary plainly: “Autocatalytic sets smaller than metabolic networks were proposed as transitory intermediates at the origin of life, but evidence for their role in prebiotic evolution is lacking.” Its analysis starts with metabolic structures in extant microbes and asks what those structures may imply about origins. It did not observe a prebiotic network or recreate the origin of life.
What later network analysis adds
A 2020 methods paper in the Journal of the Royal Society Interface, “The structure of autocatalytic networks, with application to early biochemistry,” developed tools to explore and visualize RAF structures and applied them to large metabolic networks from archaeal and bacterial lineages near early branches of the tree of life. This supports RAFs as a way to analyze biochemical organization. It is not independent proof that RAFs were historical intermediates before life.
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How to interpret the proposal
The distinction is between a plausible mechanism and evidence that the mechanism actually occurred. RAFs found in microbial metabolism show that the formal pattern can occur within biological networks. They can inform scenarios in which metabolism arose through self-supporting chemical organization, including proposals for an autotrophic origin. But evidence from present-day networks is not direct evidence of prebiotic chemistry.
When comparing this proposal with other origin-of-life scenarios, useful questions include what starting materials and food set each assumes, where its catalysis comes from, how its chemistry sustains itself, and what experimental or geological evidence connects the mechanism to early Earth conditions. The studies discussed here do not comprehensively adjudicate among origin-of-life hypotheses, so they do not establish autocatalytic networks as a settled winner.
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Further reading
For an overview of the development of the idea and its theoretical, computational and experimental work, see Wim Hordijk and Mike Steel’s 2017 review, “Chasing the tail: The emergence of autocatalytic networks,” in Biosystems. Their 2018 review, “Autocatalytic Networks at the Basis of Life’s Origin and Organization,” in Life, explains RAFs and distinguishes conditions for network structure from conditions sufficient for life-like behavior. Stuart Kauffman’s At Home in the Universe: The Search for the Laws of Self-Organization and Complexity (Oxford University Press, 1995) offers broader background on his ideas about self-organization and the origin of life; it predates modern RAF research and is not a current review.
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