Despite such adverse conditions,
eukaryotes may have evolved around the beginning of the Boring Billion, and adopted several novel adaptations, such as various
organelles,
multicellularity and possibly
sexual reproduction, and diversified into
algae,
fungi and early
animals at the end of this time interval.[1] Such advances may have been important precursors to the evolution of large, complex life later in the
EdiacaranAvalon Explosion and the subsequent
PhanerozoicCambrian Explosion. Nonetheless,
prokaryoticcyanobacteria were the dominant
autotrophic lifeforms during this time, and likely supported an energy-poor
food-web with a small number of
protists at the
apex 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.
Description
In 1995, geologists Roger Buick, Davis Des Marais, and
Andrew Knoll reviewed the apparent lack of major biological, geological, and climatic events during the
Mesoproterozoicera 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 paleontologist
Martin 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 of
carbon 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 of
tectonic movement.[5]
The Boring Billion is now largely cited as spanning about 1.8 to 0.8 Ga, contained within the
Proterozoiceon, 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 the
Great Oxygenation Event due to the evolution of
oxygenic photosyntheticcyanobacteria, and the resultant
Huronian glaciation (
Snowball Earth), formation of the
UV-blocking
ozone 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 supercontinent
Rodinia during the
Tonian (1000–720 Ma) period, a second oxygenation event, and another Snowball Earth in the
Cryogenian period.[5][11]
The evolution of Earth's
biosphere, atmosphere, and
hydrosphere has long been linked to the
supercontinent cycle, where the continents aggregate and then drift apart. The Boring Billion saw the evolution of two supercontinents:
Columbia (or Nuna) and
Rodinia.[8][12]
The supercontinent Columbia formed between 2.0 and 1.7 Ga and remained intact until at least 1.3 Ga. Geological and
paleomagnetic 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 in
equatorial and
temperate climate zones, and there is little or no evidence for continental fragments in
polar regions.[12]
Due to the lack of evidence of sediment build-up (on passive margins) which would occur as a result of
rifting,[13] the supercontinent probably did not break up, and rather was simply an assemblage of juxtaposed proto-continents and
cratons. There is no evidence of rifting until the formation of Rodinia, 1.25 Ga in North Laurentia, and 1 Ga in East
Baltica and South
Siberia.[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 the
asthenosphere—the molten layer of Earth's
mantle 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 at
subduction 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 the
crust that, once initiated, made plate subduction anomalously strong, occurring at the end of the Boring Billion.[5]
Nonetheless, major
magmatic events still occurred, such as the formation (via
magma plume) of the 220,000 km2 (85,000 sq mi) central Australian
Musgrave Province from 1.22 to 1.12 Ga,[14] and the 2,700,000 km2 (1,000,000 sq mi) Canadian
Mackenzie Large Igneous Province 1.27 Ga.[15] Plate tectonics were still active enough to build mountains, with several
orogenies, including the
Grenville orogeny,[16] occurring at the time.
Climatic stability
There is little evidence of significant climatic variability during this time period.[4][17] Climate was likely not primarily dictated by solar luminosity because the
Sun 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 substantial
ocean acidification[19] to prevent ice formation, which also did not occur. Mesoproterozoic CO2 levels may have been comparable to those of the
Phanerozoic 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 Scottish
Diabaig Formation in the
Torridon Group, where
dropstone formations were likely formed by debris from
ice rafting; the area, then situated between
35–
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 an
atmospheric methane concentration of 140
parts per million (ppm).[20][18]Methanogenic prokaryotes could not have produced so much methane, implying some other greenhouse gas, probably
nitrous 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 of
cosmic 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 the
mantle and reduced
geothermal 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 the
ozone layer, preventing
greenhouse gasses from being trapped in the atmosphere and heating the Earth via the
greenhouse 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 by
UV radiation than today.[28]
Oceanic composition
The oceans seem to have had low concentrations of key nutrients thought to be necessary for complex life, namely
molybdenum, iron,
nitrogen, and
phosphorus, in large part due to a lack of oxygen and resultant
oxidation necessary for these
geochemical 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, a
sulfidic middle layer,[33][34][35] and
suboxic 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]
Oxygen
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 the
Great Oxidation Event (GOE), and the second occurring an approximate 0.8 Ga, known as the
Neoproterozoic Oxygenation Event (NOE).[39] The intermediary period, during the Boring Billion, is thought have had low oxygen levels (with minor fluctuations), leading to widespread
anoxic waters.[34]
The oceans may have been distinctly stratified, with surface water being oxygenated[33][34][35] and deep water being suboxic (less than 1
μM oxygen),[37] the latter possibly maintained by lower levels of
hydrogen (H2) and H2S output by deep sea
hydrothermal 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] The
decomposition 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 by
δ13C levels to have been a loss of 10 to 20 times the current volume of atmospheric oxygen—is known as the
Lomagundi-Jatuli Event, and is the most prominent
carbon 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 the
Northern Territory of
Australia,[48] the Kaltasy Formation (
Russian: Калтасинская свита) of
Volgo-Uralia,
Russia,[40] and the Xiamaling Formation in the northern
North China Craton[49][50] indicate noticeable oxygenation around 1.4 Ga, although the degree to which this represents global oxygen levels is unclear.[48] Oxic conditions would have become dominant at the NOE causing the proliferation of
aerobic activity over
anaerobic,[33][34][41] but widespread suboxic and anoxic conditions likely lasted until about 0.55 Ga corresponding with
Ediacaran biota and the
Cambrian explosion.[51][52]
Sulfur
In 1998, geologist
Donald Canfield proposed what is now known as the
Canfield ocean hypothesis.[33] Canfield claimed that increasing levels of oxygen in the atmosphere at the Great Oxygenation Event would have reacted with and oxidized continental
iron pyrite (FeS2) deposits, with
sulfate (SO42−) as a byproduct, which was transported into the sea.[53]Sulfate-reducing microorganisms converted this to
hydrogen sulfide (H2S), dividing the ocean into a somewhat oxic surface layer, and a sulfidic layer beneath, with
anoxygenic bacteria living at the border, metabolizing the H2S and creating sulfur as a waste product. This created widespread
euxinic conditions in middle-waters, an anoxic state with a high sulfur concentration, which was maintained by the bacteria.[54][41][35] Many deposits from the Boring Billion contain mercury isotopic ratios characteristic of photic zone euxinia [55] More systematic geochemical study of the Mid-Proterozoic indicates that the oceans were largely ferruginous with a thin surface layer of weakly oxygenated waters,[56] and euxinia may have occurred over relatively small areas, perhaps less than 7% of the seafloor.[57] 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.[58] Evidence from the McArthur Basin of northern Australia reveals that intracontinental settings in particular were low in sulphide intermittently.[59]
Iron
Among rocks dating to the Boring Billion, there is a conspicuous lack of
banded iron formations, which 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 oceanic SO2−4reduced all the iron in the anoxic deep sea.[33] Iron could have been metabolized by anoxygenic bacteria.[60] It has also been proposed that the 1.85 Ga
Sudbury meteor impact mixed the previously stratified ocean via tsunamis, interaction between vaporized seawater and the oxygenated atmosphere, oceanic
cavitation, and massive runoff of destroyed
continental 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.[60]
Lifeforms
Low nutrient abundance may have facilitated
photosymbiosis—where one organism is capable of photosynthesis and the other metabolizes the waste product—among
prokaryotes (
bacteria and
archaea), and the emergence of
eukaryotes. Bacteria, Archaea, and Eukaryota are the three
domains, the highest taxonomic ranking. Eukaryotes are distinguished from prokaryotes by a
nucleus and membrane-bound organelles, and almost all multicellular organisms are eukaryotes.[61]
Prokaryotes were the dominant lifeforms throughout the Boring Billion.[9][62][33]Microfossils indicate the presence of cyanobacteria,
green and
purple sulfur bacteria, methane-producing archaea, sulfate-metabolizing bacteria,
methane-metabolizing archaea or bacteria, iron-metabolizing bacteria,
nitrogen-metabolizing bacteria, and anoxygenic photosynthetic bacteria.[63]
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 from
iron 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 by
silica-rich waters or
biomineralization of iron within the cell.[63]
Unicellular planktonic lineages of cyanobacteria evolved in freshwater during the middle of the
Mesoproterozoic, and during the
Neoproterozoic 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.[64][65]
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 a negative feedback loop where atmospheric oxygen levels stabilised at 2%, which began to change about 600 million years ago when landplants started releasing oxygen. By 408 million years ago, nitrogen fixating cyanobacteria had evolved heterocysts to protect their nitrogenase from oxygen.[66][67]
Eukaryotes
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.[68] 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] and
molybdenum,[69] essential metals in
nitrogen fixation. A lack of dissolved nitrogen would have favored prokaryotes over eukaryotes, as the former can metabolize gaseous nitrogen.[70] 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.[71]
Nonetheless, the diversification of
crown group eukaryotic macroorganisms seems to have started about 1.6–1 Ga, seemingly coinciding with an increase in key nutrient concentrations.[1] According to
molecular clock analysis, plants diverged from animals and fungi about 1.6 Ga; animals and fungi about 1.5 Ga;
Bilaterians and
cnidarians (animals respectively with and without
bilateral symmetry) about 1.3 Ga;
sponges 1.35 Ga;[73] and
Ascomycota and
Basidiomycota (the two divisions of the fungus
subkingdomDikarya) 0.97 Ga.[73] 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,[74] the Ruyang Group of North China,[75][76][77] and the Kotuikan Formation of the
Anabar Shield of Siberia,[78] among other places, indicate high rates (by pre-Ediacaran standards) of eukaryotic diversification between 1.7 and 1.4 Ga,[79] although much of this diversity is represented by previously unknown, no longer existing clades of eukaryotes.[78] The earliest known
red algae mats date to 1.6 Ga.[72] The earliest known fungus dates to 1.01–0.89 Ga from Northern Canada.[80] Multicellular eukaryotes, thought to be the descendants of colonial unicellular aggregates, had probably evolved about 2–1.4 Ga.[81][82] Likewise, early multicellular eukaryotes likely mainly aggregated into
stromatolite mats.[11]
The red alga Bangiomorpha is the earliest known sexually reproducing and
meiotic lifeform,[83] and evolved by 1.047 Ga.[84] 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 hypothesized
RNA world.[85]
Cell
organelles probably originated from free-living
cyanobacteria (
symbiogenesis)[86][87][1] possibly after the evolution of
phagocytosis (engulfing other cells) with the removal of the rigid
cell wall which was only necessary for asexual reproduction.[9]Mitochondria had already evolved in the Great Oxygenation Event, but
plastids used in
primoplants for
photosynthesis are thought to have appeared about 1.6–1.5 Ga.[73]Histones likely appeared during the Boring Billion to help organize and package the increasing amount of DNA in eukaryotic cells into
nucleosomes.[9]Hydrogenosomes used in anaerobic activity may have originated in this time from an archaeon.[88][86]
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]
Ecology
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 higher
trophic levels, a complex
food 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 of
protists as, in a sense,
apex predators.[62]
The presumably oxygenic photosynthetic eukaryotic
acritarchs, perhaps a type of
microalga, inhabited the Mesoproterozoic surface waters.[89] 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.[90]
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 produce
nitrogenases, 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.[91]
Life on land
Some of the earliest evidence of the prokaryotic colonization of land dates to before 3 Ga,[92] possibly as early as 3.5 Ga.[93] During the Boring Billion, land may have been inhabited mainly by cyanobacterial mats.[94][95][96][97] 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][98] By 1.2 Ga, microbial communities may have been abundant enough to have affected weathering,
erosion, sedimentation, and various geochemical cycles,[95] and expansive microbial mats could indicate
biological soil crust was abundant.[28]
The earliest terrestrial eukaryotes may have been lichen fungi about 1.3 Ga,[99] which grazed on the microbial mats.[28] Abundant eukaryotic microfossils from the freshwater Scottish
Torridon Group seems to indicate eukaryotic dominance in non-marine habitats by 1 Ga,[100] 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.[99] 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.[101]
See also
Precambrian – History of Earth 4600–539 million years ago
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