The so-called ‘Major Transitions’ framework is an attempt to explain the hierarchical structure of life on Earth: genes within chromosomes, chromosomes within cells, cells within cells (eukaryotic cells), individuals within sexual partnerships, cells within multicellular organisms, and organisms within societies. The best-known attempt to unify the origins of these relationships is a book by John Maynard Smith* and Eörs Szathmáry, The Major Transitions in Evolution.
First published in 1995, the book focused on the origins of these hierarchical levels, connecting them with the unifying theme that
…entities that were capable of independent replication before the transition can replicate only as part of a larger whole after it.
For example, after a transition from unicellular to multicellular organisms (there were several), cellular reproduction either contributes to the growth of the organism or to production of new multicellular organisms.