Big Questions in Evolution

Evolution refers to the historic ‘‘unfolding’’ of organismal life on Earth, which already intrigued Greek philosophers. It cannot be studied directly—just as the plot of a theatre play cannot be inferred from watching the last second. Instead; we study the fossil record; also, synthetic biology has increasing power to mimic key steps in evolution. The comparative approach, however, remains most informative. It infers ancient conditions from the comparison of extant species. Classically, evolutionary biologists compare tissues and organs—during development and in adults. More recently, comparative genomics allow tracking the increase in protein complexity in divergent lineages. Now, a new level of comparison has emerged, linking organ, tissue, and protein evolution—the cell type. Cells are the basic building blocks of life; once we understand their evolutionary diversification into types, this will solve the secrets of multicellular life. A combination of whole-organism single-cell transcriptomics, proteomics, expression atlases, and CRISPR-Cas9-based functional studies in various organisms opens up exciting new questions: What is the cell type complement in one species as compared to others? How is it specified and maintained? What are the cell type-specific molecular machines, or modules, and how did these diversify in evolution? Answering such questions will allow us to examine even more complex structurestissues and organs, ultimately learning about their evolution as an emergent property of their constituting cell types.