Szathmáry, Eörs, and John Maynard Smith. “The major evolutionary transitions.” Nature 374, no. 6519 (1995): 227-232.
URL1 URL2
There is no theoretical reason to expect evolutionary lineages to increase in complexity with time, and no empirical evidence that they do so. Nevertheless, eukaryotic cells are more complex than prokaryotic ones, animals and plants are more complex than protists, and so on. This increase in complexity may have been achieved as a result of a series of major evolutionary transitions. These involved changes in the way information is stored and transmitted.