How and why are certain lipids biosynthesized, what regulates this activity, how are biosynthetic capacities phylogenetically distributed and can we constrain the evolution of such capacities throughout Earth history? We focus on fresh biomass and on fossil lipids, using a combination of molecular, isotopic and genomic approaches to understand the origins of modern biosynthetic pathways, and to obtain more details about isotope partitioning during these. As a new addition, we explore the utility of isotope chemistry as a new tracer tool for reading the human metabolome.