Abstract

The Earth was formed ~4.6 billion years ago (Ga), and traces of ancient life can be found as far back at 3.8 Ga. What was life like in the remote past? What metabolisms were used to sustain growth? When did critical metabolisms that changed our planet evolve, such as oxygenic photosynthesis (the ability to convert water to molecular oxygen)? In this lecture, I will discuss how geobiologists approach these questions, illustrating the challenges we face in making meaningful inferences about the nature of life so long ago. In particular, I will discuss my research group’s efforts to properly interpret the meaning of a specific type of biomarker called a “2-methyl-hopane”, which for many years was thought to mark the rise of oxygenic photosynthesis, but which we now have evidence to suggest represents an entirely different evolutionary process.