THE EARLY EVOLUTIONARY HISTORY OF CHLOROPLASTIDA BASED ON THE MICROFOSSIL RECORD

Cells with chloroplasts have a common ancestry and share certain biosynthesis, reproduction and cyst formation pathways that are also phenotypically expressed and may be recognized throughout the evolutionary history of Chloroplastida. The origin and minimum divergence age of Chloroplastida (green algae and plants) are constrained by the microfossils identified as early representatives of green algae set in a geological time scale. This timing is substantially older than estimates using molecular clocks based on genomic sequencing. Certain organically preserved unicellular microfossils show patterns of cyst morphology and formation, reproductive structures, internal bodies, cell wall ultrastructure (when available), and biochemical properties of refractory macromolecules that have also been identified in extant algal lineages of Prasinophyceae and Chlorophyceae within Chlorophyta of Chloroplastida.

The somatic cell and reproductive cyst wall ultrastructure and biochemistry are important because these characters are diagnostic of major protistan clades and are used in modern biota classifications. Microfossil cysts, with multi-layered, primary and secondary walls with trilaminal sheath structure or lined with internal platelets, have been observed since the Mesoproterozoic Era. Based on these characteristics, interpreted as being algal cysts, some leiosphaerids persistently show a record of chlorophyceans since 1.8 Ga.

The internal body or bodies, defined by their own walls, demonstrate a specific developmental stage of the microorganisms and provide direct evidence for the reproduction mode and likely phylogenetic relatedness. These internal bodies are inferred to be endocysts containing zygotes and/or sacs of swarmers, if single, or offspring cells, if multiple, in sexual and asexual generations. Based on the earliest occurrence of microfossils with an internal body in the Dictyosphaera plexus, the sexual reproduction of photosynthesizing microbiota was evident at c. 1.6-1.4 Ga. Microfossils with phycoma-like cyst morphologies suggesting prasinophycean affinity were identified at a minimum age of 1.4–1.2 Ga . Thus, the divergence of Chlorophyta in the group Chloroplastida occurred at the minimum age of 1.8 Ga, and the origin of primary plastid is likely prior to this time.