Abstract

The evolution of biparental sexual reproduction in animals and plants is a prominent focus in modern biology. One hundred and fifty years ago, the German biologist Julius Sachs (1832–1897) published the fourth and final edition of his influential _Textbook of Botany_. In the text, he referred to the work of Wilhelm Hofmeister (1824–1877) and proposed that it is possible to reconstruct the origins and evolution of sexuality via systematic comparisons among the life cycles of simple versus complex organisms. Sachs’s 1874 book presented the green alga _Pandorina_ as an example of ancestral life cycles, and that of the land plants as the most complex photosynthetic organisms. Herein, we describe the 150-year-old hypothesis proposed by Sachs and show how the purported “architect of modern plant physiology” provided insights into (1) recent papers implicating the role of HMG-box genes for the expression of male versus female sex determination in complex eukaryotes, such as animals, brown algae, and fungi; and (2) the continuing debate about the meaning of homology in the context of evolution. We conclude that the physiologist Sachs, along with Hofmeister, was also one of the founders of comparative sex research in animals and plants.