In the Cryogenian era, roughly 80 Ma years of extremely cold climate, large portions of the Rodinia supercontinent were covered in glaciers and ice sheets. The surface of melting glaciers is considered the most widespread and predictable freshwater habitat on the planetary scale during this period, also called “Snowball Earth”. Importantly, this era overlaps with the confidence intervals of current estimates of the Embryophyta-Zygnematophyta split.
A glacial life style was likely the driving force of the simplification of Zygnematophyta; this included the loss of flagellated reproductive cells and an alternative way of enabling the sexual process – the conjugation. This might solve the paradox that Zygnematophyta, the sister group to land plants according to the recent state of knowledge, is a morphologically very simple group compared to the Coleochaetophyceae or Charophyceae. The common ancestor of Zygnematophyta and Embryophyta (land plants), so called ‘androphytes’, is suggested to have retained the ability to produce flagellated cells. A number of signatures of the likely glacial origin of land plants can be found in the genetic toolkits emerging with the “higher branching ZCC-grade” of Streptophyta (the groups Charophyceae, Coleochaetophyceae and Zygnematophyceae). These genes then further developed over the course of plant evolution, modified and diversified their functions. This hypothesis paper offers a new interpretation of genetic data and insight into the early evolution of land plants.
Žárský J, Žárský V, Hanáček M, Žárský V (2022) Cryogenian glacial habitats as a plant terrestrialisation cradle – the origin of the Anydrophytes and Zygnematophyceae split. Frontiers in Plant Science 12:735020 doi: 10.3389/fpls.2021.735020