[Widdekind]

1. Terminal Archaean Ice Age (~2.3 billion years ago)

The earliest Eukaryotic Cells lived "not less than 2.7 billion years ago*". This coincides with "a small rise in Oxygen levels 2.7 billion years ago**". Indeed, "the appearance of Oxygen seems to have driven the evolution of new forms of Metabolism, and new branches in the Tree of Life***". By about 2.6 billion years ago, (big) Banded Iron Formations began being deposited#, by the periodic Oxidization of soluble Ferrous Iron, probably brought about by periodic "blooms" of Photosynthesizers##.

* Nick Lane. Oxygen, pg. 36.
** ibid., pg. 43.
*** ibid., pg. 47.
# ibid., pg. 45.
## ibid., pp. 38-41.

But, this Oxygenation:

was finally shattered by an apocalyptic climate change about 2.2 to 2.3 billion years ago. The Earth plummeted into the first ever Ice Age. This was no trivial Ice Age... but a global freeze that may have covered the tropics in glaciers a kilometre thick -- in Joseph Kirschvink's memorable phrase, a 'Snowball Earth'... It was to last for 35 million years*.

Indeed, James Kasting has argued that "the appearance of free Oxygen itself brought about the freeze**", linking that rise in Oxygen to the removal of Greenhouse Gases:

As the Greenhouse Effect was undermined, temperatures plummeted and the Earth succumbed to the grip of an Ice Age***.

Indeed again,

By about 2 billion years ago, toxic corrosive Oxygen was pouring into the air, in the most catastrophic air pollution event ever. It may have even triggered the first recorded Ice Age, at the end of the Archaean, because so much Carbon Dioxide was withdrawn from the Atmosphere#.

Surely, by their Photosynthesis, primitive microbes consumed so much Carbon Dioxide, that they plunged the planet into a deep, global freeze.

* ibid.
** ibid.
*** ibid.
# Stephen L. Gillett. World-Building, pg. 107.

2. Protozeroic Ice Ages (~900 to 600 million years ago)

The Terminal Archaic Ice Age ended as

the planet was racked by a period of heightened Tectonic Activity, leading to major Continental Rifting and the uplift of mountain belts on a scale comparable with the Andes*.

Trapped beneath the global glacial 'cocoon', heat had accumulated in the Mantle, until Magmas melted through both the Crust, and the overlaying layer of Ice, erupting upwards powerfully**.

* Nick Lane, ibid., pg. 48.
** History Channel How the Earth was Made (DVD)

Such large-scale Volcanism emanated massive amounts of Carbon Dioxide back into the Atmosphere, and the microbial Photosynthesizers returned:

2.2 billion years ago, Oxygen levels rose agin, following hard on the heels of the Snowball Earth. In the period of geological unrest that followed, huge Banded Iron Formations precipitated from the oceans all around the world... By 2 billion years ago... Oxygen levels reached around 5 to 18% of present atmospheric concentration. In the rocks, we see a sudden explosion of diversity in fossil Eukaryotes. Many have Mitochondria. All the elements of the modern world, bar true multicellular organisms, are in place*.

Indeed, by 1 billion years ago, early Algae took "the first tentative steps towards Multicellular Life"**. And, by perhaps 0.8 billion years ago, early Animals (Multicellular Metazoans) had developed***.

* Nick Lane, ibid., pp. 52-53.
** ibid.
*** ibid., pg. 57.

Then, this steady Evolutionary progress "was shattered for a second time by another series of Snowball Earths"*, from roughly 900 to 590 million years ago**. During these global Ice Ages, Carbon Dioxide was "scrubbed from the skies", and Carbon-13 Isotopic Analysis implies "the near-extinction of all living things"***.

* ibid., pg. 60.
** ibid., pg. 64 ; http://andvari.vedur.is/~halldor/HB/205Bold/Lecture7.html
*** ibid., pg. 63.

But, as in the Terminal Archaean period, pent-up Geologic Forces eventually broke these icy impasses:

Deep beneath the surface of the Earth, however, the forces of Volcanism were oblivious to the icy crust. Active Volcanoes burst through the ice, spewing Volcanic Gases into the air, among them Carbon Dioxide, rewarming the Earth. Finally the glaciers began to melt*.

Then, world weather would wax warmer, fostering "searing temperatures, tropical storms, and torrential rain[s]"**. Soon, the primitive Photosynthesizers bloomed once more:

Now the survivors had a whole planet to themselves. They must have multiplied like mad. In this they were aided by high levels of minerals and nutrients, eroded by glaciers worldwide and swept into the oceans by the deluge. All these nutrients, all this empty space, must have stimulated the greatest blooming of Cyanobacteria & Algae the world has ever seen: a world of blue-green ocean. These blooms must have produced prodigious amounts of free Oxygen in a short period, Oxygenating the surface oceans and air between each of the Ice Ages***.

Surely, by their Photosynthesis, primitive microbes consumed so much Carbon Dioxide, that they repeatedly plunged the planet into deep, global freezes.

* ibid., pg. 62.
** ibid., pg. 62.
*** ibid., pg. 64-65.

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3. Phanerozoic Era Ice Ages (~450 to 250 million years ago)

A. Terminal Ordovician Period Ice Age (~450 to 440 million years ago)

During the tropical Ordovician Period (~488 to 444 million years ago), early Land Plants developed, like Liverworts & Mosses*. The spread of primitive Plants across the Continents coincides with the Terminal Ordovician Glaciation, and an ensuing Mass Extinction:

The Ordovician-Silurian Extinction Events may have been caused by an Ice Age that occurred at the end of the Ordovician Period, as the end of the Late Ordovician was one of the coldest times in the last 600 million years**.

Indeed, these Extinction Events "were triggered by the onset of a long Ice Age, perhaps the most severe Glaciation of the Phanerozoic", and were "preceded by a fall in atmospheric Carbon Dioxide"***.

* David Burnie. (DK/Google E.guides) Plant, pg. 8.
** http://en.wikipedia.org/wiki/Ordovi...ilurian_extinction_events
*** http://en.wikipedia.org/wiki/Ordovician

Plausibly, by their Photosynthesis, primitive Land Plants consumed so much Carbon Dioxide, that they again plunged the planet into a deep, global freeze.


B. Terminal Devonian Period Ice Age (~364 million years ago)

During the Devonian Period (~416 to 359 million years ago), primitive Land Plants developed into the first true Trees, and colonized Continents:

Early Devonian plants did not have roots or leaves... and many had no Vascular Tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall. But, by the Middle Devonian, shrub-like forests of primitive plants existed... Most of these plants had true roots and leaves, and many were quite tall. The earliest known trees... appeared in the Late Devonian around 380 million years ago. In the Late Devonian, the Tree-like ancestral fern Archaeopteris and the giant Cladoxylopsid trees grew with true wood. These are the oldest known trees of the world's first forests. Prototaxites was the fruiting body of an enormous Fungus that stood more than 8 meter tall. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion"...

The 'greening' of the Continents acted as a Carbon Dioxide sink, and atmospheric levels of this Greenhouse Gas may have dropped. This may have cooled the climate and led to a Mass Extinction Event*.

Indeed, by the Late Devonian, Land Plants grew as high as 30 meters, and:

the development of Seeds permitted reproduction and dispersal in areas which were not waterlogged, allowing plants to colonise previously inhospitable inland and upland areas. The two factors combined to greatly magnify the role of plants on the global scale. In particular, Archaeopteris forests expanded rapidly during the closing stages of the Devonian**.

Indeed too, Archaeopteris "quickly became the dominant tree all over the Earth", inhabiting "all of the land areas that were habitable"***.

* http://en.wikipedia.org/wiki/Devonian
** http://en.wikipedia.org/wiki/Late_Devonian_extinction
*** http://en.wikipedia.org/wiki/Archaeopteris. Archaeopteris perished in the Late Devonian Mass Extinction.

Surely, by their Photosynthesis, developing Trees consumed so much Carbon Dioxide, that they, too, plunged the planet into another deep, global freeze.


C. Carboniferous Period Ice Age (~320 to 250 million years ago)

The Carboniferous Period also combines rapid Plant growth, high Oxygen concentrations, and ensuing Ice Ages & Mass Extinctions:

The early stages of the Carboniferous Period (354-290 million years ago) were characterized by warm and damp conditions -- caused by landmasses shifting closer to the equator. Swamp forests released huge amounts of oxygen into the atmosphere... The Carboniferous Period ended in a cold period, known as the Karoo Ice Age, as the level of insulating gases in the atmosphere fell*.

This Permo-Carboniferous Glaciation lasted for 70 million years (~320 to 250 million years ago**). Its onset coincides with the Mid-Carboniferous Mass Extinction Event (~320 million years ago***), brought about by dropping Sea Levels#, consistent with a Global Glaciation.

* (DK Eyewitness Compansions) Weather, pp. 24-25.
** http://andvari.vedur.is/~halldor/HB/205Bold/Lecture7.html
*** http://en.wikipedia.org/wiki/File:P...biodiversity_blank_01.png
# http://en.wikipedia.org/wiki/Carboniferous#Paleogeography

Surely, by their Photosynthesis, Carboniferous Swamp Forests consumed so much Carbon Dioxide, that they, too, plunged the planet into a deep, global freeze.


D. Quaternary Period Ice Ages (~2 million years ago until today)

Recent Glaciations are characterized by the retreat of forests from Europe*, while Inter-Glacial warm-periods witness the advance of forests as far as Scandinavia & Siberia**.

* Ted Oakes. Land of Lost Monsters, pg. 89.
** Philip Brooks. Prehistoric People, pg. 31.

Now, recent Glaciations coincide with the Milankovich Variations, "small, periodic variations in the Earth's Orbit & Axial Tilt" which "cause small but consistent changes in the average amount of Sunlight -- 'insulation' -- that high latitudes receive over a year", although "it's still not clear how [these cycles] trigger an Ice Age"*.

* Stephen L. Gillett, ibid., pg. 102.

Surely, by affecting the available Light Budget of Land Flora (like Trees), the Milankovich Variations modulate Plant Growth. In turn, by their Photosynthesis, Plants consume so much Carbon Dioxide, that they plunge the planet into repeated, deep, global freezes.

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CONCLUSION (???):

Primitive Organisms "have only two speeds -- stop & go"*. This causes cyclical "Boom-Bust" growth patterns, as rapid divisions and Exponential Growth ultimately collides against the Carrying Capacity of their Environment**.

* (DK Megabites) Microlife, pg. 45.
** ibid., pg. 38.

All Photosynthesizers, from Cyanobacteria to Algae & Land Plants, seemingly maintain this primitive Boom-Bust behavior. As such, they expand until they suck the Carbon Dioxide from the skies. Then, the whole world plunges into a Global Glaciation.

As Earth has aged, and Volcanic Fluxes have declined, these Boom-Bust Cycles, swinging from Greenhouse Conditions to Global Glaciations, have increased in frequency.

[Widdekind]

The Gaia Hypothesis holds that "living organisms regulate the Climate and the chemistry of the Atmosphere in their own interest"*.

* Robert Henson. The Rough Guide to Climate Change (2nd. Ed.), pg. 209. The Gaia Hypothesis was developed by James Lovelock in the 1960s AD.

Now, Plants' Photorespiration supports such claims:

A biological feedback mechanism, for regulating Oxygen, is a curious phenomenon, that affects plants, suppressing their growth and productivity. In some circumstances, plant growth can be halted completely. The phenomenon is known as Photo-Respiration, and, unlike the plant's normal Mitochondrial Respiration, takes place only in Sunlight... The net effect, is that the plant takes up Oxygen and releasees Carbon Dioxide, which parallels normal Respiration (hence the name), but fails to generate any Energy. Also unlike normal Respiration, Photo-Respiration competes with Photo-Synthesis for the use of an Enzyme [called] Rubisco... This contest for the affections of Rubisco undermines the efficiency of Photo-Synthesis, and so reduces plant growth... Photo-Respiration stunts plant growth when Atmospheric Oxygen levels are high*.

Moreover:

Despite its apparent futility, Photo-Respiration is almost universal among plants... It has been maintained by Evolution for some reason... It must be useful for something, otherwise it would have been unceremoniously dumped in the struggle for survival**.

Moreover still:

The potential magnitude of Photo-Respiration fits it as a plausible mechanism for stabilizing Atmospheric Oxygen. If Oxygen levels were to rise, the rate of Photo-Respiration would rise in tandem, and plant growth would falter. Stunted plants produce less Oxygen by Photo-Synthesis, thereby adjusting Atmospheric Oxygen back to previous levels***.

Also, more ancient types of plants (~350 Ma) are less prone to Photo-Respiration, while more modern types of plants (~150 Ma) are more prone to the process.

* Nick Lane. Oxygen, pp. 80-82.
** ibid.
*** ibid.

CONCLUSION: The development of plants' Photo-Respiration strongly supports the Gaia Hypothesis. Furthermore, Gaia appears to have progressively developed over time, with plants Photo-Respiration feedback mechanism increasing in effectiveness.

[Widdekind]

Schematic diagram of Earth's Atmospheric Oxygen levels over the last billion years (ibid., pg. 64). Oscillating Oxygen Levels correspond to "boom-bust" cycles of Snowball Earths succeeded by warmer eras. Note that the last two (2) such cycles show signs consistent with the development of primitive Photo-Respiration (indicated on the chart as "Gaia ?").

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[Widdekind]

Article on the Gaia Hypothesis of NASA scientists James Lovelock & Dian Hitchcock:

http://www.newscientist.com/article...n-worst-enemy.html?page=1