# The Major Transitions in Evolution

The Major Transitions in Evolution is a book written by John Maynard Smith and Eörs Szathmáry (Oxford University Press, 1995).
Maynard Smith and Szathmary authored a review article in Nature.
Transitions described in the book :
| Transition from: | Transition to: | Notes |
| Replicating molecules | "Populations" of molecules in compartments | Can't observe |
| Independent replicators (probably RNA) | Chromosomes | RNA world hypothesis |
| RNA as both genes and enzymes | DNA as genes; proteins as enzymes | |
| Prokaryotes | Eukaryotes | Can observe |
| Asexual clones | Sexual populations | Evolution of sex |
| Protists | Multicellular organisms — animals, plants, fungi | Evolution of multicellularity |
| Solitary individuals | Colonies with non-reproductive castes | Evolution of eusociality |
| Primate societies | Human societies with language, enabling memes | Sociocultural evolution |
Maynard Smith and Szathmáry identified several properties common to the transitions:
* Smaller entities have often come about together to form larger entities, e.g. chromosomes, eukaryotes, sex multicellular colonies.
* Smaller entities often become differentiated as part of a larger entity, e.g. DNA-protein, organelles, anisogamy, tissues, castes
* The smaller entities are often unable to replicate in the absence of the larger entity, e.g. DNA, chromosomes, organelles, tissues, castes.
* The smaller entities can sometimes disrupt the development of the larger entity, e.g. meiotic drive (selfish non-Mendelian genes), parthenogenesis, cancers, coup d’états.
* New ways of transmitting information have arisen, e.g. DNA-protein, cell heredity, epigenesis, universal grammar.
As stated by the authors, this book was aimed at professional biologists and assumes considerable prior knowledge. They have also published a summary of their arguments in Nature as well as a presentation of their ideas for a general readership under the title The Origins of Life — From the Birth of Life to the Origins of Language. Two decades later, Eörs Szathmáry published an "update" of his thesis in the original book, and this update involved demoting sex from a major transition as well as promoting new transitions, such as the origins of plastids, to the list. The major transitions generally involve the formation of new levels of units of selection, consisting of ensembles of pre-existing entities. Therefore, the evolution of the major transitions can also be seen as the framework for studying the evolution of the levels of complexity in biology.
Their work has generated substantial interest and further research into major transitions, including a devoted issue of papers to the subject in 2016 in the journal Philosophical Transactions of the Royal Society B. Additional suggestions to the transitions concept include the inclusion of viruses as playing a role as major catalysts for evolutionary transitions in two ways. One, parasite-host arms race often leads to the formation of complex structures and levels of complexity to combat the threat of viruses. Two, gene transfer from viruses and virus-like elements may contribute important genes for the emergence of higher levels of organization. Others have noted that the concept of transitions in macroevolutionary history focuses on increases in the levels of complexity, whereas macroevolutionary events can also proceed through simplifications which undo these hierarchical increases in complexity (e.g. multicellular organisms losing adherence genes and so transitioning into unicellular organisms, or the animal and plant lineages with degenerated organelles such as mitosomes). Furthermore, simplifications can also enable other macroevolutionary complexifications (e.g. the bacterial endosymbiont that simplified into the integrated mitochondrial organelle). Thus, incorporating simplification dynamics will help further elucidate the emergence of life's lineages.