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). At the time of its publication, Egbert Giles Leigh, Jr reviewing for "Evolution" commented that it "may be the most important book on evolution since R.A. Fisher's (1930) "The Genetical Theory of Natural Selection"". Maynard Smith and Szathmary authored a review article in "Nature". Maynard Smith and Szathmáry identified several properties common to the transitions: 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 mitochondria and plastid organelles). 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.