During the latter part of the 20th century, several important ‘paradigm shifts' occurred in our view of cell evolution. Perhaps the most obvious of these, because it was so hotly contested, was the symbiotic origin of mitochondria and chloroplasts. It was an old idea, with roots in the late 19th century, whose successful revival was championed by Margulis (1970). Ironically, although the symbiotic origin for these organelles has been generally accepted, her additional hypothesis of a symbiotic origin of the ‘9+2’ organelles (centrioles/basal bodies and derivative structures) as an origin for the eukaryotic mitotic process (Sagan, 1967) has not been supported by convincing evidence. Never one to quit, symbiotic spirochaete involvement is still espoused in her latest publication (Margulis & Sagan, 2002). The formulation of a theory that incorporated hypotheses for the symbiotic origin of mitochondria and plastids sequentially was dubbed the ‘serial endosymbiosis theory’ (SET; Taylor, 1974) in a paper that also introduced the now widely used concepts of primary and secondary symbiosis that are particularly relevant to plastid acquisition.
A less obvious but equally important major paradigm shift was the resurrection and development of another 19th-century concept: the protistological view of the basal nexus and radiation of unicellular eukaryotes preceding and giving rise to plants, animals and fungi but also many lineages independent of these multicellular groups. This replaced the long-held subdivision of all eukaryotes, and even prokaryotes, into the classical plant and animal kingdoms.
The dualistic Plant–Animal view had its roots in Aristotle and his teacher Plato, in the Academy in Athens almost 2500 years ago (ca. 360 BC). Two living supergroups were formalized in 1735 by Carl von Linné in his monumental Systema Naturae as the Kingdoms Plantae and Animalia, into which all organisms were placed. By the 12th, and last, edition (Linnaeus, 1766), he had added a third kingdom of the natural world, Lapides (for ‘rocks’; solid bodied, not living, not ‘sentient’, i.e. not having ‘senses’, a trait shared with plants in his scheme). This division of the living world was reflected in the disciplines of Botany and Zoology, housed in their own separate academic Departments for more than 250 years. Beginning in the mid 20th century, accompanying growing realization that plant and animal cells were fundamentally very similar in biochemistry, structure and genetic systems, a more common biological approach began. This was reinforced unequivocally by the 1960s discovery that all living beings use fundamentally the same genetic code.
As a result of these developments, Life Science departments or other disciplinary units began to replace the traditional departments by fusion and reorganization. Introductory biology courses began to replace introductory botany and zoology courses in the 1960s and the influential textbook The Science of Biology by Paul Weisz became widely used in introductory courses in North America.
Microbiology had separated earlier at many universities as a result of the recognition that the presence or absence of nuclei in cells was associated with other fundamentally different cell features and because of its medical bias. The French protistologist Édouard Chatton introduced the names ‘prokaryotic’ and ‘eukaryotic’ (with a ‘c’ instead of a ‘k’; Chatton, 1925), based simply on the absence or presence of nuclei, respectively (his 1938 publication is often given as the source, but this was a summary of earlier work; Chatton, 1938). This fundamental distinction of cell types led to an alternative dualistic view of two Kingdoms or Superkingdoms, Prokaryota and Eukaryota (e.g. Whittaker & Margulis, 1978; Möhn, 1984).