The rise of plants and animals such as mammals are all deeply engrained in our collective imagination as some of the most significant events in the history of life on Earth. But there is an equally dramatic history of evolutionary change to be found at the microscopic scale: changes in the biochemistry of individual proteins have in some cases altered the trajectory of life as fundamentally as the evolution of any macroscopic life-form ever did. Until recently, our understanding of these transitions at the molecular level has remained vague, but technological advances now make it possible to retrace in unprecedented clarity how history-changing new protein activities came to be.

In this talk, I will discuss our work on the evolution of ribulose-1,5-bisphosphate carboxylase/oxygenases (Rubiscos) as an example of this emerging research program. The evolution of Rubiscos that discriminate strongly between their substrate CO2 and the undesired side-substrate O2 was a key step in the rise of oxygenic photosynthesis and complex aerobic life. I will discuss how we used ancestral sequence reconstruction and the biochemical characterization of ancient, resurrected Rubiscos to recapitulate this crucial event. This showed how Rubisco increased its specificity and carboxylation efficiency through the gain of a new accessory subunit before significant atmospheric oxygen was present. It also allowed us to understand biochemically why Rubisco subsequently became completely dependent on this new interaction. I will conclude by briefly discussing ongoing work on the evolution of the ‘Rubiscosome’: a set of dedicated chaperones that became necessary to fold, assemble, and activate Rubisco along the lineage to plants. Together, this work shines light on key events in the history of life and also deepens our understanding of the biochemistry of extant Rubiscos.