Topic summary
Boring Billion
The Boring Billion, otherwise known as the Mid-Proterozoic and Earth's Middle Ages, is an informal geological time period between 1.8 and 0.8 billion years ago (Ga) during the middle Proterozoiceon spanning from the Statherian to the Tonianperiods, characterized by more or less tectonic stability, climatic stasis and slow biological evolution. Although it is bordered by two different oxygenation events (the Great Oxygenation Event and Neoproterozoic Oxygenation Event) and two global glacial events (the Huronian and Cryogenian glaciations), the Boring Billion period itself actually had very low oxygen levels and no geological evidence of glaciations.
The oceans during the Boring Billion may have been oxygen-poor, sulfidic and nutrient-deficient (i.e. euxinic), populated mainly by purple bacteria, a type of bacteriochlorophyll-based photosynthetic bacteria which uses hydrogen sulfide (H2S) for carbon fixation instead of water and produces sulfur as a byproduct instead of oxygen. This is known as a Canfield ocean, and such composition may have caused the oceans to be colored black-and-milky-turquoise instead of blue or green as later. (By contrast, during the much earlier "Purple Earth" phase of the Archean eon, primitive anoxygenic photosynthesis was performed mostly by archaeal colonies using retinal-based proton pumps that absorb green light to directly synthesize adenosine triphosphate, and the oceans would have been magenta-purple.)
Despite such adverse conditions, eukaryotes may have evolved via symbiogenesis around the beginning of the Boring Billion, developed several novel adaptations such as various organelles, multicellularity and possibly sexual reproduction, and diversified into algae, fungi, protists and the early ancestors of animals at the end of this time interval. Such advances may have been important precursors to the large-scale evolutionary radiation of macroscopic complex life later in the EdiacaranAvalon Explosion and the subsequent PhanerozoicCambrian Explosion. Nonetheless, prokaryoticcyanobacteria were the dominant autotrophic lifeforms during this time period, and likely supported an energy-poor food-web with a small number of protists at the apex level. The land was likely inhabited at the coastal and riparian margins by washed-ashore cyanobacteria and eukaryotic proto-lichens, the latter being more successful on dry land probably due to greater robustness against arid environment and lethal solar irradiation (as then the protective ozone layer had not yet formed).