# Priority effects inhibit the repeated evolution of phototrophy

## TL;DR

This paper explains why phototrophy, a crucial energy innovation, has only evolved twice by showing how early systems block new ones from emerging.

## Contribution

The study introduces the concept of an evolutionary priority effect caused by early phototrophic systems saturating the bioenergetic landscape.

## Key findings

- Chlorophototrophy and retinalophototrophy saturate the bioenergetic space for light-harvesting systems.
- These systems block new phototrophic innovations due to biophysical trade-offs.
- Early innovations can constrain future evolutionary possibilities, making rare events seem 'easy'.

## Abstract

The emergence of phototrophy is one of the most significant innovations in the history of life, vastly increasing available metabolic energy. Phototrophy is, however, known to have arisen only twice. This raises a curious question: if phototrophy was accessible enough to evolve twice, why has it never arisen again despite billions of years of subsequent evolution? Through physiological modeling, we demonstrate that chlorophototrophy and retinalophototrophy together saturate the bioenergetic landscape available to light-harvesting systems. They represent opposite solutions to key biophysical trade-offs: maximizing efficiency per photon versus maximizing metabolic flux, specialization versus versatility, and sophistication versus simplicity. Together they create an evolutionary priority effect, blocking any newly-arising phototrophic system from succeeding. By revealing the basis of this competitive exclusion, our work sheds light on a general principle - that early innovations can saturate ecological space such that they constrain future evolutionary possibilities, making apparently ‘easy’ innovations appear as rare events.