TheBoring Billion, otherwise known as theMid Proterozoic andEarth's Middle Ages, is an informalgeological time period between 1.8 and 0.8 billion years ago (Ga) during the middleProterozoiceon spanning from theStatherian to theTonianperiods, characterized by more or lesstectonic stability,climatic stasis and slowbiological evolution. Although it is bordered by two differentoxygenation events (theGreat Oxygenation Event andNeoproterozoic Oxygenation Event) and twoglobal glacial events (theHuronian andCryogenian glaciations), the Boring Billion period itself actually hadvery low oxygen levels and no geological evidence of glaciations.

The oceans during the Boring Billion may have been oxygen-poor, nutrient-poor and sulfidic (euxinia), populated by mainlyanoxygenicpurple bacteria, a type ofbacteriochlorophyll-based photosynthetic bacteria which useshydrogen sulfide (H₂S) forcarbon fixation instead ofwater and producessulfur as abyproduct instead ofoxygen. This is known as aCanfield 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 earlierPurple Earth phase during the Archean, photosynthesis was performed mostly byarchaeal colonies usingretinal-basedproton pumps that absorb green light, and the oceans would bemagenta-purple.)

Despite such adverse conditions,eukaryotes may have evolved around the beginning of the Boring Billion, and adopted several novel adaptations, such as variousorganelles,multicellularity and possiblysexual reproduction, and diversified intoalgae,fungi and the ancestors ofanimals at the end of this time interval.^[1] Such advances may have been important precursors to the evolution of large, complex life later in theEdiacaranAvalon Explosion and the subsequentPhanerozoicCambrian Explosion. Nonetheless,prokaryoticcyanobacteria were the dominantautotrophic lifeforms during this time, and likely supported an energy-poorfood-web with a small number ofprotists at theapex level. The land was likely inhabited by prokaryotic cyanobacteria and eukaryotic proto-lichens, the latter more successful here probably due to the greater availability of nutrients than in offshore ocean waters.

In 1995, geologists Roger Buick, Davis Des Marais, andAndrew Knoll reviewed the apparent lack of major biological, geological, and climatic events during theMesoproterozoicera 1.6 to 1 billion years ago (Ga), and, thus, described it as "the dullest time in Earth's history".^[2] The term "Boring Billion" was coined by paleontologistMartin Brasier to refer to the time between about 2 and 1 Ga, which was characterized by geochemical stasis and glacial stagnation.^[3] In 2013, geochemist Grant Young used the term "Barren Billion" to refer to a period of apparent glacial stagnation and lack ofcarbon isotope excursions from 1.8 to 0.8 Ga.^[4] In 2014, geologists Peter Cawood and Chris Hawkesworth called the time between 1.7 and 0.75 Ga "Earth's Middle Ages" due to a lack of evidence oftectonic movement.^[5]

The Boring Billion is now largely cited as spanning about 1.8 to 0.8 Ga, contained within theProterozoiceon, mainly the Mesoproterozoic. The Boring Billion is characterized by geological, climatic, and by-and-large evolutionary stasis, with low nutrient abundance.^[6][7][8]

In the time leading up to the Boring Billion, Earth experienced theGreat Oxygenation Event due to the evolution ofoxygenic photosyntheticcyanobacteria, and the resultantHuronian glaciation (Snowball Earth), formation of theUV-blockingozone layer, and oxidation of several metals.^[9] Oxygen levels during the Boring Billion are thought to have been markedly lower than during the Great Oxidation Event, perhaps 0.1% to 10% of modern levels.^[10] It was ended by the breakup of the supercontinentRodinia during theTonian (1000–720 Ma) period, a second oxygenation event, and another Snowball Earth in theCryogenian period.^[5][11]

Reconstruction ofColumbia (1.6 Gya)

The evolution of Earth'sbiosphere, atmosphere, andhydrosphere has long been linked to thesupercontinent cycle, where the continents aggregate and then drift apart. The Boring Billion saw the evolution of two supercontinents:Columbia (or Nuna) andRodinia.^[8][12]

The supercontinent Columbia formed between 2.0 and 1.7 Ga and remained intact until at least 1.3 Ga. Geological andpaleomagnetic evidence suggest that Columbia underwent only minor changes to form the supercontinent Rodinia from 1.1 to 0.9 Ga.Paleogeographic reconstructions suggest that the supercontinent assemblage was located inequatorial andtemperate climate zones, and there is little or no evidence for continental fragments inpolar regions.^[12]

Due to the lack of evidence of sediment build-up (on passive margins) which would occur as a result ofrifting,^[13] the supercontinent probably did not break up, and rather was simply an assemblage of juxtaposed proto-continents andcratons. There is no evidence of rifting until the formation of Rodinia, 1.25 Ga in North Laurentia, and 1 Ga in EastBaltica and SouthSiberia.^[8] Breakup did not occur until 0.75 Ga, marking the end of the Boring Billion.^[5] This tectonic stasis may have been related in ocean and atmospheric chemistry.^[8][6]

It is possible theasthenosphere—the semi-solid layer of Earth'smantle that tectonic plates essentially float and move around upon—was too hot to sustain modern plate tectonics at this time. Instead of vigorous plate recycling atsubduction zones, plates were linked together for billions of years until the mantle cooled off sufficiently. The onset of this component of plate tectonics may have been aided by the cooling and thickening of thecrust that, once initiated, made plate subduction anomalously strong, occurring at the end of the Boring Billion.^[5]

Nonetheless, majormagmatic events still occurred, such as the formation (viamagma plume) of the 220,000 km² (85,000 mi²) central AustralianMusgrave Province from 1.22 to 1.12 Ga,^[14] and the 2,700,000 km² (1,000,000 mi²) CanadianMackenzie Large Igneous Province 1.27 Ga.^[15] Plate tectonics were still active enough to build mountains, with severalorogenies, including theGrenville orogeny,^[16] occurring at the time.

There is little evidence of significant climatic variability during this time period.^[4][17] Climate was likely not primarily dictated by solar luminosity because theSun was 5–18% less luminous than it is today, but there is no evidence that Earth's climate was significantly cooler.^[18][19] In fact, the Boring Billion seems to lack any evidence of prolonged glaciations, which can be observed at regular periodicity in other parts of Earth's geologic history.^[19] High CO₂ could not have been a main driver for warming because levels would have needed to be 30 to 100 times greater than pre-industrial levels^[18] and produced substantialocean acidification^[19] to prevent ice formation, which also did not occur. Mesoproterozoic CO₂ levels may have been comparable to those of thePhanerozoic eon, perhaps 7 to 10 times higher than modern levels.^[20] The first record of ice from this time period was reported in 2020 from the 1 Ga ScottishDiabaig Formation in theTorridon Group, wheredropstone formations were likely formed by debris fromice rafting; the area, then situated between35–50°S, was a (possibly upland) lake which is thought to have frozen over in the winter and melted in the summer, rafting occurring in the spring melt.^[21]

A higher abundance of other greenhouse gases, namely methane produced by prokaryotes, may have compensated for the low CO₂ levels; a largely ice-free world achieved by anatmospheric methane concentration of 140parts per million (ppm).^[20][18]Methanogenic prokaryotes could not have produced so much methane, implying some other greenhouse gas, probablynitrous oxide, was elevated, perhaps to 3 ppm (10 times today's levels). Based on presumed greenhouse gas concentrations, equatorial temperatures during the Mesoproterozoic may have been about 295–300 K (22–27 °C; 71–80 °F), in the tropics 290 K (17 °C; 62 °F), at 60° 265–280 K (−8–7 °C; 17–44 °F), and the poles 250–275 K (−23–2 °C; −10–35 °F);^[22] and the global average temperature about 19 °C (66 °F), which is 4 °C (7.2 °F) warmer than today. Temperatures at the poles dropped below freezing in winter, allowing for temporary sea ice formation and snowfall, but there were likely no permanent ice sheets.^[7]

It has also been proposed that, because the intensity ofcosmic rays has been shown to be positively correlated to cloud cover, and cloud cover reflects light into space and reduces global temperatures, lower rates of bombardment during this time due to reduced star formation in the galaxy caused less cloud cover and prevented glaciation events, maintaining a warm climate.^[19][23] Also, some combination of weathering intensity which would have reduced CO₂ levels by oxidation of exposed metals, cooling of themantle and reducedgeothermal heat and volcanism, and increasing solar intensity and solar heat may have reached an equilibrium, barring ice formation.^[4]

Conversely, glacial movements over a billion years ago may not have left many remnants today, and an apparent lack of evidence could be due to the incompleteness of the fossil record rather than absence. Further, the low oxygen and solar intensity levels may have prevented the formation of theozone layer, preventinggreenhouse gasses from being trapped in the atmosphere and heating the Earth via thegreenhouse effect, which would have caused glaciation.^[24][25][26] Though not much oxygen is necessary to sustain the ozone layer, and levels during the Boring Billion may have been high enough for it,^[27] the Earth may have been more heavily bombarded byUV radiation than today.^[28]

The oceans seem to have had low concentrations of key nutrients thought to be necessary for complex life, namelymolybdenum, iron,nitrogen, andphosphorus, in large part due to a lack of oxygen and resultantoxidation necessary for thesegeochemical cycles.^[29][30][31] Nutrients could have been more abundant in terrestrial environments, such as lakes or nearshore environments closer to continental runoff.^[32]

In general, the oceans may have had an oxygenated surface layer, asulfidic middle layer,^[33][34][35] andsuboxic bottom layer.^[36][37] The predominantly sulfidic composition may have caused the oceans to be a black-and milky-turquoise color instead of blue.^[38]

Earth's geologic record indicates two events associated with significant increases in oxygen levels on Earth, with one occurring between 2.4 and 2.1 Ga, known as theGreat Oxidation Event (GOE), and the second occurring an approximate 0.8 Ga, known as theNeoproterozoic Oxygenation Event (NOE).^[39] The intermediary period, during the Boring Billion, is thought to have had low oxygen levels (with minor fluctuations), leading to widespreadanoxic waters.^[34]

The oceans may have been distinctly stratified, with surface water being oxygenated^[33][34][35] and deep water being suboxic (less than 1μM oxygen),^[37] the latter possibly maintained by lower levels ofhydrogen (H₂) and H₂S output by deep seahydrothermal vents which otherwise would have been chemically reduced by the oxygen, i.e.,euxinic waters.^[36] Even in the shallowest waters, significant quantities of oxygen may have been restricted mainly to areas near the coast.^[40] Thedecomposition of sinking organic matter would have also leached oxygen from deep waters.^[41][34]

The sudden drop in O₂ after the Great Oxygenation Event—indicated byδ13C levels to have been a loss of 10 to 20 times the current volume of atmospheric oxygen—is known as theLomagundi-Jatuli Event, and is the most prominentcarbon isotope event in Earth's history.^[42][43][44] Oxygen levels may have been less than 0.1 to 1% of modern-day levels,^[45] which would have effectively stalled the evolution of complex life during the Boring Billion.^[39][35] However, a Mesoproterozoic Oxygenation Event (MOE), during which oxygen rose transiently to about 4% PAL at various points in time, is proposed to have occurred from 1.59 to 1.36 Ga.^[46] In particular, some evidence from the Gaoyuzhuang Formation suggests a rise in oxygen around 1.57 Ga,^[47] while the Velkerri Formation in the Roper Group of theNorthern Territory ofAustralia,^[48] the Kaltasy Formation (Russian:Калтасинская свита) ofVolgo-Uralia,Russia,^[40] and the Xiamaling Formation in the northernNorth China Craton^[49][50] indicate noticeable oxygenation around 1.4 Ga, although the degree to which this represents global oxygen levels is unclear.^[48] Estimations from gas analyses suggests the levels of oxygen in the atmosphere was 3.7% of modern levels.^[51] Oxic conditions would have become dominant at the NOE causing the proliferation ofaerobic activity overanaerobic,^[33][34][41] but widespread suboxic and anoxic conditions likely lasted until about 0.55 Ga corresponding withEdiacaran biota and theCambrian explosion.^[52][53]

In 1998, geologistDonald Canfield proposed what is now known as theCanfield ocean hypothesis.^[33] Canfield claimed that increasing levels of oxygen in the atmosphere at the Great Oxygenation Event would have reacted with and oxidized continentaliron pyrite (FeS₂) deposits, withsulfate (SO₄²⁻) as a byproduct, which was transported into the sea.^[54]Sulfate-reducing microorganisms converted this tohydrogen sulfide (H₂S), dividing the ocean into a somewhat oxic surface layer, and a sulfidic layer beneath, withanoxygenic bacteria living at the border, metabolizing the H₂S and creating sulfur as a waste product. This created widespreadeuxinic conditions in middle-waters, an anoxic state with a high sulfur concentration, which was maintained by the bacteria.^[55][41][35] Many deposits from the Boring Billion contain mercury isotopic ratios characteristic of photic zone euxinia.^[56] More systematic geochemical study of the Mid-Proterozoic indicates that the oceans were largely ferruginous with a thin surface layer of weakly oxygenated waters,^[57] and euxinia may have occurred over relatively small areas, perhaps less than 7% of the seafloor.^[58] The very low concentrations of molybdenum in the Mesoproterozoic could sufficiently be explained even with such a relatively low percentage of the seafloor being euxinic.^[34] Euxinia expanded and contracted, sometimes reaching the photic zone and at other times being relegated to deeper waters.^[59] Evidence from the McArthur Basin of northern Australia reveals that intracontinental settings in particular were low in sulphide intermittently.^[60]

Among rocks dating to the Boring Billion, there is a conspicuous lack ofbanded iron formations, which are believed to form from iron in the upper water column (sourced from the deep ocean) reacting with oxygen and precipitating out of the water. They seemingly cease around the world after 1.85 Ga. Canfield argued that oceanicSO2−4reduced all the iron in the anoxic deep sea.^[33] Iron could have been metabolized by anoxygenic bacteria.^[61] It has also been proposed that the 1.85 GaSudbury meteor impact mixed the previously stratified ocean via tsunamis, interaction between vaporized seawater and the oxygenated atmosphere, oceaniccavitation, and massive runoff of destroyedcontinental margins into the sea. Resultant suboxic deep waters (due to oxygenated surface water mixing with previously anoxic deep water) would have oxidized deep-water iron, preventing it from being transported and deposited on continental margins.^[36]

Nonetheless, iron-rich waters did exist, such as the 1.4 Ga Xiamaling Formation of Northern China, which perhaps was fed by deep water hydrothermal vents. Iron-rich conditions also indicate anoxic bottom water in this area, as oxic conditions would have oxidized all the iron.^[61]

Low nutrient abundance may have facilitatedphotosymbiosis—where one organism is capable of photosynthesis and the other metabolizes the waste product—amongprokaryotes (bacteria andarchaea), and the emergence ofeukaryotes. Bacteria, Archaea, and Eukaryota are the threedomains, the highest taxonomic ranking. Eukaryotes are distinguished from prokaryotes by anucleus and membrane-bound organelles, and almost all multicellular organisms are eukaryotes.^[62]

Prokaryotes were the dominant lifeforms throughout the Boring Billion.^[9][63][33]Microfossils indicate the presence of cyanobacteria,green andpurple sulfur bacteria, methane-producing archaea, sulfate-metabolizing bacteria,methane-metabolizing archaea or bacteria, iron-metabolizing bacteria,nitrogen-metabolizing bacteria, and anoxygenic photosynthetic bacteria.^[64]

Anoxygenic cyanobacteria are thought to have been the dominant photosynthesizers, metabolizing the abundant H₂S in the oceans. In iron-rich waters, cyanobacteria may have suffered fromiron poisoning, especially in offshore waters where iron-rich deep water mixed with surface waters, and thus were outcompeted by other bacteria which could metabolize both iron and H₂S. However, iron poisoning could have been abated bysilica-rich waters orbiomineralization of iron within the cell.^[64]

Unicellular planktonic lineages of cyanobacteria evolved in freshwater during the middle of theMesoproterozoic, and during theNeoproterozoic both benthic marine and some freshwater ancestors gave rise to marine planktonic cyanobacteria (both nitrogen-fixing and non-nitrogen fixing), contributing to the oxygenation of the Pre-Cambrian oceans.^[65][66]

Research on cyanobacteria in the laboratory has shown that the enzyme nitrogenase, which is used to fix atmospheric nitrogen, stops working when oxygen levels are higher than 10% of current atmospheric levels. The absence of nitrogen due to an increased amount of oxygen would have created anegative feedback loop where atmospheric oxygen levels stabilised at 2%, which began to change about 600 million years ago when land plants started releasing oxygen. By 408 million years ago, nitrogen fixating cyanobacteria had evolved heterocysts to protect their nitrogenase from oxygen.^[67][68]

Eukaryotes may have arisen around the beginning of the Boring Billion,^[1] coinciding with the accretion of Columbia, which could have somehow increased oceanic oxygen levels.^[11] Although there have been claimed records of eukaryotes as early as 2.1 billion years ago, these have been considered questionable, with the oldest unambiguous eukaryote remains dating to around 1.8-1.6 billion years ago in China.^[69] Following this, eukaryotic evolution was rather slow,^[9] possibly due to the euxinic conditions of the Canfield ocean and a lack of key nutrients and metals^[5][1] which prevented large, complex life with high energy requirements from evolving.^[24] Euxinic conditions would have also decreased the solubility of iron^[33] andmolybdenum,^[70] essential metals innitrogen fixation. A lack of dissolved nitrogen would have favored prokaryotes over eukaryotes, as the former can metabolize gaseous nitrogen.^[71] An alternative hypothesis for the lack of diversification among eukaryotes implicates high temperatures during the Boring Billion rather than low oxygen levels, positing that the fact that oxygenation events prior to the Late Neoproterozoic did not kickstart eukaryotic evolution suggests it was not the main limiting factor inhibiting it.^[72]

Nonetheless, the diversification ofcrown group eukaryotic macroorganisms seems to have started about 1.6–1 Ga, seemingly coinciding with an increase in key nutrient concentrations.^[1] According tomolecular clock analysis, plants diverged from animals and fungi about 1.6 Ga; animals and fungi about 1.5 Ga;Sponges from other animals diverged about 1.35 Ga;^[74]Bilaterians andcnidarians (animals respectively with and withoutbilateral symmetry) about 1.3 Ga; andAscomycota andBasidiomycota (the two divisions of the fungussubkingdomDikarya) 0.97 Ga.^[74] The paper's authors state that their time estimates disagree with the scientific consensus.

Fossils from the late Palaeoproterozoic and early Mesoproterozoic of the Vindhyan sedimentary basin of India,^[75] the Ruyang Group of North China,^[76][77][78] and the Kotuikan Formation of theAnabar Shield of Siberia,^[79] among other places, indicate high rates (by pre-Ediacaran standards) of eukaryotic diversification between 1.7 and 1.4 Ga,^[80] although much of this diversity is represented by previously unknown, no longer existing clades of eukaryotes.^[79] The earliest knownred algae mats date to 1.6 Ga.^[73] The earliest known fungus dates to 1.01–0.89 Ga from Northern Canada.^[81] Multicellular eukaryotes, thought to be the descendants of colonial unicellular aggregates, had probably evolved about 2–1.4 Ga.^[82][83] Likewise, early multicellular eukaryotes likely mainly aggregated intostromatolite mats.^[11]

The red algaBangiomorpha is the earliest known sexually reproducing andmeiotic lifeform,^[84] and evolved by 1.047 Ga.^[85] Based on this, these adaptations evolved between ca. 2–1.4 Ga.^[1] Alternatively, these may have evolved well before the last common ancestor of eukaryotes given that meiosis is performed using the same proteins in all eukaryotes, perhaps stretching to as far back as the hypothesizedRNA world.^[86]

Cellorganelles probably originated from free-livingcyanobacteria (symbiogenesis)^[87][88][1] possibly after the evolution ofphagocytosis (engulfing other cells) with the removal of the rigidcell wall which was only necessary for asexual reproduction.^[9]Mitochondria had already evolved in the Great Oxygenation Event, butplastids used inprimoplants forphotosynthesis are thought to have appeared about 1.6–1.5 Ga.^[74]Histones likely appeared during the Boring Billion to help organize and package the increasing amount of DNA in eukaryotic cells intonucleosomes.^[9]Hydrogenosomes used in anaerobic activity may have originated in this time from an archaeon.^[89][87]

Given the evolutionary landmarks achieved by eukaryotes, this time period could be considered an important precursor to the Cambrian explosion about 0.54 Ga, and the evolution of relatively large, complex life.^[9]

Due to the marginalization of large food particles, such as algae, in favor of cyanobacteria and prokaryotes which do not transmit as much energy to highertrophic levels, a complexfood web likely did not form, and large lifeforms with high energy demands could not evolve. Such a food web probably only sustained a small number ofprotists as, in a sense,apex predators.^[63]

The presumably oxygenic photosynthetic eukaryoticacritarchs, perhaps a type ofmicroalga, inhabited the Mesoproterozoic surface waters.^[90] Their population may have been largely limited by nutrient availability rather than predation because species have been reported to have survived for hundreds of millions of years, but after 1 Ga, species duration dropped to about 100 Ma, perhaps due to increased herbivory by early protists. This is consistent with species survival dropping to 10 Ma just after the Cambrian explosion and the expansion of herbivorous animals.^[91]

The relatively low concentrations of molybdenum in the ocean throughout the Boring Billion have been suggested as a major limiting factor that kept populations of open ocean nitrogen fixing microorganisms, which require molybdenum to producenitrogenases, low, although freshwater and coastal environments close to riverine sources of dissolved molybdenum may have still hosted significant communities of nitrogen fixers. The low rate of nitrogen fixation, which only ended during the Cryogenian with the evolution of planktonic nitrogen fixers, meant that free ammonium was in short supply across this time interval, severely constraining the evolution and diversification of multicellular biota.^[92]

Some of the earliest evidence of the prokaryotic colonization of land dates to before 3 Ga,^[93] possibly as early as 3.5 Ga.^[94] During the Boring Billion, land may have been inhabited mainly by cyanobacterial mats.^{[95][96][97][98]} Dust would have supplied an abundance of nutrients and a means of dispersal for surface-dwelling microbes, though microbial communities could have also formed in caves and freshwater lakes and rivers.^[28][99] By 1.2 Ga, microbial communities may have been abundant enough to have affected weathering,erosion, sedimentation, and various geochemical cycles,^[96] and expansive microbial mats could indicatebiological soil crust was abundant.^[28]

The earliest terrestrial eukaryotes may have been lichen fungi about 1.3 Ga,^[100] which grazed on the microbial mats.^[28] Abundant eukaryotic microfossils from the freshwater ScottishTorridon Group seems to indicate eukaryotic dominance in non-marine habitats by 1 Ga,^[101] probably due to increased nutrient availability in areas closer to the continents and continental runoff.^[32] These lichen may have later facilitated plant colonization 0.75 Ga in some manner.^[100] A massive increase in terrestrial photosynthetic biomass seems to have occurred about 0.85 Ga, indicated by a flux in terrestrially-sourced carbon, which may have increased oxygen levels enough to support an expansion of multicellular eukaryotes.^[102]

• Precambrian – History of Earth 4600–539 million years ago
• Ediacaran biota – Life of the Ediacaran period
• Francevillian biota – Possible Palaeoproterozoic multicellular fossils from Gabon
• L'Atalante basin – Anoxic hypersaline brine basin at the bottom of the Mediterranean Sea
• Snowball Earth – Worldwide glaciation episodes during the Proterozoic eon
• Thenatural nuclear fission reactors at what is nowOklo, Gabon were active in this era

• Astrobiology
• Proterozoic
• Plate tectonics
• Geological periods
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