An evolutionary scenario for the conquest of land by streptophytes. Streptophyte algae are the only photosynthetic eukaryotes from which the macroscopic land flora evolved (red lines). That said, throughout the course of evolution, algae from various other lineages have colonized land (yellow lines)—but also streptophyte algae have continuously and independently made the wet to dry transition (convergence of red and yellow). Throughout history, numerous lineages have become extinct (‘x’ labels). Terrestrial algae of various taxonomic affiliations dwell on rock surfaces and form biological soil crusts. From the diversity of the paraphyletic streptophyte algae, however, did an organism whose descendants eventually conquered land on a global scale emerge: a likely branched filamentous—or even parenchymatous—organism that formed rhizoidal structures and experienced desiccation from time to time. From this ‘hypothetical hydro-terrestrial alga’, the lineages of Zygnematophyceae and embryophytes (land plants) arose. In its infancy, the trajectory leading to the embryophytes was represented by the—now extinct (see also Delaux et al., 2019)—earliest land plants. The earliest land plants probably interacted with beneficial substrate microbiota that aided them in obtaining nutrients from their substrate. Furthermore, the earliest land plants had to successfully overcome a barrage of terrestrial stressors (including UV and photosynthetically active irradiance, drought, drastic temperature shifts, etc.). They succeeded because they had the right set of traits—a mix of adaptations that were selected for in their hydro-terrestrial algal ancestors, exaptations, and the potential for co-option of a fortuitous set of genes and pathways. During the course of evolution, some members of the populations of the earliest land plants gained traits that are adaptive in terrestrial environments (such as some form of water conductance, stomata-like structures, embryos, etc.); eventually, the ‘hypothetical last common ancestor of land plants’ emerged. From this ancestor, the extant bryophytes and tracheophytes evolved. While the exact trait repertoire of the hypothetical last common ancestor of land plants is uncertain, it will certainly have entailed properties of vascular and non-vascular plants. What is also certain is that the last common ancestor of land plants had traits of algal ancestry.

All dating is roughly based on Morris et al. 2018.

• Morris JL, Puttick MN, Clark JW, Edwards D, Kenrick P, Pressel S, Wellman CH, Yang Z, Schneider H, Donoghue PCJ (2018). "The timescale of early land plant evolution". Proceedings of the National Academy of Sciences, 115: E2274–E2283.
• Delaux PM, Hetherington AJ, Coudert Y, et al. (2019). "Reconstructing trait evolution in plant evo–devo studies". Current Biology, 29: R1105–R1121.