From Hothouse to Icehouse The Increase in Diversity of Habitats for Life

The progressive increase in the BEW as a product of biotic and biospheric evolution intensified the carbon sink with respect to the atmosphere/ocean system, leading to the transition of climate from a 'hothouse' in the Early Precambrian to an 'icehouse' in the Phanerozoic. By the Late Proterozoic, the rise of atmospheric oxygen may have resulted in a substantial increase in terrestrial biotic productivity, which along with the onset of frost wedging substantially increased the BEW. Climate and life coe-volved as a tightly coupled system, constrained by abiotic factors (varying solar luminosity, and the crust's tectonic and impact history). Self-regulation of this coupled system is a property of geophysiology.

With the long-term drop of surface temperature over the last 3.8 billion years, a progressive expansion of the diversity of habitats occurred (i.e., first hyperthermophiles, then thermophiles, mesophiles, finally psychophiles, the organisms living near 0 °C). Since the first appearance of ice on high mountains, likely by the Proterozoic, the diversification of habitats opened up new ecological niches, with a cumulative retention of older habitats (e.g., there are still hot springs with thermophiles and the 'deep hot biosphere' persisted since the Hadean). As global temperatures dropped, particularly in the Mid- to Late Proterozoic, latitudinal differences in temperature and therefore a zonal differentiation of ecosystems followed roughly parallel to the equator. Of course, the position and configuration of the continents also changed over time, affecting then, as now, the distribution of ecosystems.

As the atmosphere changed its composition in the Mid-Proterozoic, becoming aerobic with the rise of oxygen, the area of surface aerobic environments expanded from the microenvironments (e.g., cyanobacterial mats) previously present. Anaerobic environments of course persisted in soils, deep in the ocean and in stratified bodies of freshwater, and finally in the gut of nearly emerged animals. If biotic productivity increased with the rise of macroeukaryotic algae in the oceans and on land, then the deposition of organic carbon in stratified water bodies could have maintained anaerobic conditions.

Further, new pCO2 (high, then low) and total pressure (first possibly >10 bars, then low) environments progressively emerged since the origin of the biosphere at no later than 3.8 Ga. Again, the diversity increased as new environments were added, while old conditions persisted (e.g., high pCO 2 and total pressure in the deep hot biosphere). In the case of pH, both very acid and basic and intermediate habitats were likely present even before the origin of life at hydrothermal vents. In Michael Russell's brilliant scenario for biogenesis, hot alkaline springs, generated in the early ultramafic crust by interaction with circulating seawater, react with acidic seawater at the seafloor to produce iron sulfide and clay membranes, the postulated sites for catalyzed reactions producing protocells. A primordial pH diversity is consistent with the present wide range of optimal pH for growth of thermophilic prokaryotes. The coupling of the full spectrum of pH habitats with lower temperatures and higher oxygen levels occurred with long-term cooling and the rise of atmospheric oxygen.

With the progressive increase in diversity of habitats, the evolution of life naturally quickly filled these habitats with novel varieties. These new habitats of course included those within newly emerging organisms and ecosystems (e.g., within the digestive tracts of Metazoa, the forest canopy, and soil). The evolution of the biosphere does not optimize conditions for existing biota, unlike the original Gaia hypothesis. At least two catastrophes for existing life occurred; the well-known oxygen catastrophe and an earlier temperature catastrophe for thermophilic bacteria, once the likely colonizers of the ocean and land, now restricted to living in hot springs, hydrothermal vents on the ocean floor, porosity in the first few kilometers of the crust.

Surface temperature is one parameter which is regulated by the biosphere, since progressive cooling arises from the circularity of the feedbacks where BEW intensifies from both the direct effects of biotic evolution and regional/global effects (e.g., frost wedging in mountains), the latter themselves being a product of the evolving biosphere as a whole. The main abiotic constraints on the feedback of biospheric evolution to surface temperature are solar history, and the Earth's tectonic and impact histories. The main factors of tectonics relevant to the first-order surface temperature history are the variation over time of volcanic/metamorphic outgassing of carbon dioxide and land area. The feedbacks from biospheric to biotic evolution include this long-term decrease in surface temperature, constraining microbial evolution, and the process of endosymbiogenesis, including the history of emergence of cell types (e.g., eukaryotes). Endosymbiogenesis begins with parasitic relations between prokaryotes, progresses to symbiotic, and finally culminates in the intimacy of new cells and organisms.

Besides changes in productivity and oxygen-producing capacity, these changes in biotic evolution feed back into biospheric evolution, which likely includes the progressive increase in biomass on the planet's surface, particularly with the rise of higher plant forests.

A continuing debate centers around a challenge to a long-held view of evolutionary biology, namely, that if the 'tape' of life's history were to be played again, the results would be radically different owing to the stochastic nature of evolutionary emergence. The alternative is that the evolution of the biosphere is roughly deterministic, that is, its history and the general pattern of biotic evolution and the tightly coupled evolution of biota and climate are very probable, given the same initial conditions. Major events in biotic evolution were likely forced by environmental physics and chemistry, including photosynthesis as well as the merging of complementary metabolisms that resulted in new types of cells (such as eukaryotes) and multicellularity. Determinism likely breaks down at finer levels. Critical constraints in this deterministic aspect of biotic evolution have likely included surface temperature, along with oxygen and carbon dioxide levels. In addition to events that were probably inevitable, it is likely there is a role for randomness in both abiotic and biotic evolution even on the coarsest scale, for example, including the influence of large impacts on the history of life, the possible multiple attractor states in mantle convection and therefore plate tectonic history, even multiple attractors for steady-state climatic regimes.

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