Biologists have long theorized about the evolution of life cycles, meiosis, and sexual reproduction. We revisit these topics and propose that the fundamental difference between life cycles is where and when multicellularity is expressed. We develop a scenario to explain the evolutionary transition from the life cycle of a unicellular organism to one in which multicellularity is expressed in either the haploid or diploid phase, or both. We propose further that meiosis might have evolved as a mechanism to correct for (...) spontaneous whole‐genome duplication (auto‐polyploidy) and thus before the evolution of sexual reproduction sensu stricto (i.e. the formation of a diploid zygote via the fusion of haploid gametes) in the major eukaryotic clades. In addition, we propose, as others have, that sexual reproduction, which predominates in all eukaryotic clades, has many different advantages among which is that it produces variability among offspring and thus reduces sibling competition. (shrink)

Julius Sachs (1832–1897), who has been quite rightly called “the father of plant physiology,” was a German physiologist of international standing, whose research interests contributed to virtually every branch of the plant sciences, and whose work presaged plant molecular biology and systems biology. Here, we focus on one of his last publications, from 1892, wherein he argued that the term “cell” (_Zelle_) is misleading and should be replaced by “energid” (_Energide_), which he defined as “a nucleus together with the corresponding (...) protoplasm that is governed by it,” based on his observations of coenocytic algae such as _Caulerpa_ whose nuclei “can only control” so much cytoplasm. Although most of his colleagues did not accept this novel terminology for the description of the “basic, minimal living unit” (_Elementarorganismus_) of animals, plants, and microbes, we argue that the energid concept prefigured the subsequent discovery of mRNA. We also argue that the resistance to the energid concept revolved around a deep-seated philosophical debate between those adhering to cell theory versus organismal theory. The first English translation of the seminal work by Sachs, “Physiologische Notizen. II. Beiträge zur Zellentheorie. a) Energiden und Zellen,” originally published in _Flora_ (75: 57–67, 1892), is provided as a separate article in this volume as part of the journal’s “Classics in Biological Theory” collection ( https://doi.org/10.1007/s13752-022-00399-w ); the original German version is available here as supplementary material in the online version of this article. (shrink)