Life has transformed the Earth, with the transduction of matter and energy through metabolism enabling cascading changes. Many of the greatest innovations in the history of life on Earth, however, are ‘evolutionary singularities’ – from the origin of life itself to eukaryogenesis to nitrogen fixation, they have appeared precisely once and gone on to change the world. Why are these innovations singular – are they very difficult to evolve with us only observing them due to anthropic selection effects, or do eco-evolutionary forces suppress the origin of newcomers in the face of incumbents? To approach this question, we study the evolution of phototrophy: the ability to use light for metabolic energy, one of the most significant metabolic innovations of all time. Unlike other singular innovations, this was invented exactly twice – chlorophototrophy and retinalophototrophy. This ‘dual singularity’ presents a unique opportunity to study the origins of evolutionary innovations: while apparently accessible enough to evolve twice, something has nonetheless limited its origins to only two. We find that these two forms cleanly ecologically partition phototrophic niche space, suggesting that each of them lays down an ‘evolutionary priority effect’ preventing the evolution of newcomers sufficiently similar to itself. This suggests that other singular innovations could be singular not due to difficulty of evolution, but due to eco-evolutionary feedbacks preventing the evolution of sufficiently similar forms. This further suggests that many major innovations taken to represent ‘hard steps’ in the evolution of a biosphere may instead be simpler to evolve than previously thought.

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