S269 Earth and Life
"Evolving Life and the Earth"
SUMMARY
While some conditions at the Earth's surface (e.g. mean temperature may have remained within modest limits for most of its history, others (e.g. atmospheric composition) have undergone radical changes. Yet life has apparently been present almost throughout, though the main diversification of eukaryotes occurred only over the last 1000 Ma.
Eukaryote cell nuclei contain orders of magnitude more DNA, in chromosomes, than the simple loop attached to the inside of the rigid outer wall of prokaryote cells. The eukaryote cell also possesses an internal framework of protein rods, which can alter cell shape and control its internal structure. These differences have vastly expanded the relative scope of eukaryote evolution, to include multicellularity with cell differentiation, allowing them to build up multi-tiered trophic pyramids. Prokaryotes, meanwhile, have been confined to production and decomposition through under a wide range of conditions.
There is an increased probability of mutation associated with the increased amounts of DNA in eukaryotes. Compensation for this is provided by doubling of the chromosomes, together with the mixing of genes from different individuals through sexual reporduction.
Sexual reproduction has in turn increased the efficacy of natural selection, potentially allowing all possible permutations of the genes available in a population to be tested in the struggle for existence. Hence even complex multicellular organisms with slow rates of reproduction can evolve rapidly.
Adaptations that evolve through natural selection nevertheless remain of selfish benefit, in terms of fitness, to the individuals (or genetic entities) possessing them. They may influence higher levels, e.g. populations and ecosystems, but only by way of incidental effect, which may be good or bad. Hence, while natural selection can explain adaptations for homeostasis in individuals, the theory does not predict the emergence of analogous systems at higher levels, contrary to the claims of the 'Gaia hypothesis'.
Stromatolites dominate all but the last part of the Cryptozoic fossil record, reaching a peak around 1000 Ma ago and declining thereafter. Exceptionally preserved early examples contain microfossils of tiny filamentous and spheroidal prokaryotes. My mid- to late Proterozoic times, there was a clear differentiation of communities in different habitats, including both prokaryotes and simple eukaryotes. Planktonic forms supplied organic material to the deep sea floor.
Fossils of probable eukaryotes show that they had arisen at least by the mid-Proterozoic (around 2100 Ma ago). Organelles such as mitochondria and chloroplasts probably evolved from endosymbiotic prokaryotes that took up residence in the ancestral eukaryotic cells. The nucleus, by contrast, probably evolved within the original cell structure. The acquisition of mitochondria may have been in response to the first appearance of molecular oxygen.
Both fossils and molecular data point to an evolutionary explosion of eukaryotes commencing between about 1200 Ma and 1000 Ma ago. This may reflect the evolution of sexual reproduction: no paricular environmental trigger has been implicated.
The last 70 Ma of the Proterozoic (the Vendian Period), was marked by the appearance of assemblages of large enigmatic fossils collectively referred to as Ediacaran faunas. They have been subject to a variety of alternative interpretations, but most are likely to have been primitive animals. Molecular data confirm the existence of animals from at least before the Vendian (from about 1000 Ma)
A supercontinent ('Rodinia') existed from about 1000 Ma ago until a series of rifting and collision events, which ensued over the late Proterozoic, led to its break-up and reorganization. During this period, a number of widespread glacial episodes were separated by longer, warmer, periods.
Relatively high δ13Ccarb values in marine sediments from around 850 Ma ago imply high rates of sedimentary burial of organic material. This may have been assisted by high rates of hydrothermal effusion of reduced chemicals, especially Fe2+, promoting oceanic anoxia. The glaciations may then have been causesd by particularly intense episodes of drawdown of atmospheric CO2. Negative excursions of the isotope ratio follow sharply, possibly because of weathering of the newly exposed organic-rich sediments as the sea-level fell.
Low 87Sr.86Sr ratios prior to the Vendian confirm the hydrothermal influence mentioned in (2), as also do the banded iron formations associated with the earlier ice ages. A sharp increase in the strontium isotope ratio during the last, Varanger, ice age suggests a lessening of hydrothermal influence. That would have meant less scavenging of the surplus oxygen generated when sedimentary burial of organic material resumed, allowing oxygen levels in the Vendian atmosphere to rise. A higher level of atmospheric oxygen has been mooted as a possible explanation for the evolutionary diversification then of larger animals, though other hypotheses have also been proposed.
The geological record shows that, far from being progressively stabilized in partnership with life, the Earth has changed episodically, under the influence both of changes in the Earth itself and of feedbacks from life, and life has evolved in response.
The geological record for the late Vendian suggests a number of environmental upheavals, involving, for example, a global fall in sea-level, and marked fluctuations in the rate of burial of organic material (hence climatic instability) followed by lowered rates in the Cambrian. Nevertheless, Ediacaran faunas persisted to the end of the Vendian, reaching maximum diversity in the final 6 Ma, and a few forms even survived into the Cambrian. It is unclear whether the majority of Ediacaran animals suffered a mass extinction at the close of the Vendian, or whether they were ecologically displaced by newly-evolving animals.
Exceptionally preserved fossil assemblages of soft-bodied animals from the Cambrian reveal their anatomical advances over the earlier Ediacara animals. Many of the Cambrian forms show greater differentiation of body parts, including the concentration of food-trapping organs around a 'head' end, the appearance of limbs and of discrete tubular, two-ended, guts. Most significan was the associated appearance of fluid-filled spaces within the body, which allowed the gut to be decoupled from external body movements, and which could serve as hydraulic sacs giving new scope for finely regulated movement.
Accompanying these anatomical changes seems to have been a revolution in feeding relationships. In particular, the rapid proliferation of skeletal hard parts may reflect the rise of predation, and hece of multi-tiered trophic pyramids.
The proliferation of shells from early Cambrian times impinged upon biogeochemical cycles, leading, for example, to increased deposition of limestones in offshore open marine environments. Phanerozoic seas thus saw a shift in emphasis from the burial of carbon in organic material (with the decreasing frequency and extent of oceanic anoxia) to that of carbon in carbonate rocks. Increased burrowing in offshore sediments also helped to reduce the extent of organic carbon burial there.
The fossil record for families of marine animals with hard parts provides a synoptic guide to Phanerozoic mass extinctions and radiations. The early Paleozoic radiations are consistent with the idea of there having been a relative ecological 'vacuum' then. Subsequent diversification was interrupted by a succession of mass extinctions, of which five were notably severe, coming in the late Ordovician, the late Devonian, the later Permian (the most devastating), the late Triassic and the late Cretaceous. (Oh Dear Poor Tiny Creatures).
Although periodic extraterrestrial impacts have been proposed for these, and other smaller mass extinctions, differences in the relative durations and the effects of the extinctions, together with other geological data, point instead to a mixture of Earth-bound and extraterrestrial causes.
Each mass extinction was followed by a relatively rapid rebound in family numbers. In the longer term, however, there was an overall increase in diversity levels, though whether this involved successively higher equlibrium levels, or merely the effects of a dynamic interplay between ever rising diversity and numerous extinction events, remains unresolved.
Within the pattern of diversification of marine animals, three'evolutionary faunas' may be distinguised: the Cambrian Fauna dominated the initial radiations, but then tailed away thereafter; the Paleozoic Fauna continued diversifying through the Ordovician, but then commenced a long slow decline, which the Modern Fauna more slowly, but relentlessly, expanded, eventually rising to dominance after the Paleozoic. Each successive fauna seems to have been less drastically affected by mass extinctions than its predecessor. The extinctions thus left somewhat biased line-ups of survivors.
There was an intensification of predation, especially upon shelly prey, and most notably expressed among post-Paleozoic faunas. One major defensive adaptation appearing among the prey animals was deep burrowing into the sediment, which led to yet further churning of surface layers. These, and other linked changes, are collectively referred to as teh Mesozoic Marine Revolution. The associated rise of various planktonic groups with calcareous skeletons in the Mesozoic augmented carbonate sedimantation indeeper water, so further expanding the oceanic carbonate sink for carbon.
Most of the Earth's non-bacterial biomass is in the form of green land plants. The evolution of terrestrial vegetation must therefore have had profound effects on the Earth's surface systems.
Before the advent of land vegetation, the continental environment was desert-like and extremely harsh, with large daily variations in temperature. There would have been less cloud cover than at present, higher than current levels of ultraviolet radiation, erratic rainfall, sudden river discharges causing flash flooding, and a mobile mantle of sediment instead of organic-rich soils.
Plants adapted to this envrionment initially by growing close to the substrate surface and so staying within the boundary layer where they were not subject to the more intense desiccation that might occur in the faster-moving free air above.
The selective advantages of spore dispersal by wind resulted in vertical growth, which in turn caused the boundary layer to increase in thickness. This is an example of a positive feedback loop.
The vertical growth of plants and their consequent greater exposure to desiccation resulted in selection for the following evolutionary innovations:
- an external cuticle that was largely impervious to water passing through it to the outside air;
- controllable gas-exchange pores (stomata);
- a plumbing (vascular) system for the movement of wter, nutrients and the products of photosynthesis;
- structural support tissues;
- an anchoring system adapted for absorbing water and mineral nutrients from the substrate (usually the ground).
Increases in plant height were accompanied by a modified branching system that produced a single main stem with either lateral outgrowths or flattened, webbed, branching systems (leaves) specialized for photosynthesis.
Increases in surface area (leaf-area indices) increased the wtaer flux through the air and the carbon drawdown from the atmosphere; both these factors, plus changes in the reflectivity (albedo) of the Earth's surface, affected the global climate significantly.
The periods in the Earth's history when the mean global temperature was colder than now are designated 'icehouse' times, and periods which were warmer than now are regarded as 'greenhouse' times.
Atmospheric ciruclation is driven by heat from the Sun. Zonal circulation is what would occur if the Earth were perfectly smooth and there were no land'sea thermal constant. Zonal circulation is only well developed in the real world over large areas of ocean.
Monsoonal circulation tends to be seasonal and is driven by strong thermal contrasts. From simple generalizations, it is possible to predict pressure-cell distributions for any geographic configuration. Ocean waters take longer to heat up and longer to cool down than does land, so in summer low-pressure cells tend to be shifted landward and high-pressure cells oceanward. Similarly a high plateau will be the focus of a low-pressure cell in summer and a high-pressure cell in winter. Such generalizations are the basis of qualitative paleoclimate model predictions.
Qualitative models cannot predict temperature regimes: for these, quantitative models are needed. The most sophisticated of these are atmospheric general circulation models (AGCMs) that typically operate on a set of contiguous 'boxes' of atmosphere. The oceans are treated simply; their currents are not modelled. However, this situation is rapidly changing as computers become more powerful. The simplification of the real world means these models may give unreliable results, particularly when dealing with past or future climates when the models are provided with boundary conditions that differ from those of the present.
The Carboniferous icehouse world comprised one main continent, Pangea, part of which lay over the South Pole giving a 'cap world' configuration with extensive ice sheets in high sourthern latitudes. The main kinds of plants an animals were also different to today, although Carboniferous vegetational distributions were similar with plant productivity highest in low latitudes. There was extensive CO2 sequestering and coal formation in these equatorial regions.
Other factors, such as extensive mountain building, relatively fewer mid-ocean ridges, marine regression, and even lowered levels of solar radiation, also contributed to the onset and persistence of globally cool conditions, either directly or by reducing atmospheric CO2 levels.
The onset of globally warm conditions in the Permian led to a loss of the ice sheets. Patterns of vegetation also changed, with the loss of the equatorial swamp forests of Europe and North America and the development of forests in high southern latitudes. The movement of Pangea away from the South Pole and a developing monsoonal system contributed to the overall warming. Increasingly arid conditions at low latitudes led to (a) the exposure and oxidation of oreviously-sequestered carbon, releasing CO2 back to the atmosphere, as well as (b) less vegetation and consequently reduced rates of atmospheric CO2 drawdown.
The late Permian extinction had a devastating effect, not least on marine life. It was not a 'sudden' event, since it occurred over some 3 to 8 million years although recent work suggests two discrete episodes of extinction. Sessile shallow marine organisms suffered most, due to a reduction in shelf area caused by marine regression. This also affected other forms of life, since regression led to increased coal weathering, release of CO2, ocean anoxia and further global warming.
It is the relative importance of different factors, as well as the interplay and feedbacks between different processes which must be considered. For example, marine regressions occurred both in the late Carboniferous and latest Permian, but with two different effects. The first enabled vegetation to expand and colonize the newly-exposed land, enabling further CO2 sequestering and cooling, whereas the second led to weathering of previously-buried coal, release of CO2 and enhanced global warming. This can be explained by changes in the relative importance and role of other factors, such as the development of a monsoonal system and changes in the kinds of plants able to colonize these environments.
Interpretations are further complicated by uncertainties concerning the timing of events and processes. A complex interplay of interactions and feedbacks is suspected to have been responsible for both the icehouse-greenhouse transition and the late Permian mass extinction.
The Equator-to-pole temperature gradient was much shallower in the Cretaceous than the present - the differenc ebeing largely due to much warmer poles.
Low latitudes were seasonally arid and, compared to today, there were few areas where rainforest could develop. Instead, the low-latitude plants display special adaptations to conserve water.
The polar light regime was similar to that of the present with prolonged periods of winter darkness.
The Arctic was devoid of permanent ice and supported luxuriant forests dominated by deciduous conifers, but with ferns, ginkgos, cycads and some angiosperms also present. These forests were effective carbon-sequestering systems.
Delta flood plain accumulations of peat were buried as a result of subsidence and sediment shedding from nearby mountains.
Paleoclimatic determinations cannot be based reliably on relictual species such as the cycads whose modern representatives only poorly reflect their past biololgical diversity and climatic tolerances. Instead, more time-stable techniques based on physiognomy have to be used.
Analysis of the physiognomy of the total vegetation, as well as that of the leaves of woody 'broadleaved' flowering plants, can yield quantitative climate data. Both techniques provide similar results suggesting they are reliable. Such studies have shown that mean annual temperatures at between 75 and 85 deg N were much warmer than now at such high latitudes.
Paleoclimate can be studied using atmospheric general circulation models. Model results are generally in good agreement with data obtained from the geological record.
Flooding of the continents provided broad shallow seas around the equatorial Tethys Ocean, flanked by predominantly arid lands of low relief. These seas proved favourable sites for the development of extensive carbonate platforms.
The carbonate platforms frequently expanded laterally (prograded) through excess carbonate production being swept off the tops of the platforms, onto their flanks. Episodically, basinal sediments crept back up the flanks, with brief phases of deepening.
The Cretaceous platforms differ in structure from tropical platforms today because of the effects only on the latter of significant glacio-eustatic fluctuations. Unlike today's platforms, the Cretaceous examples typically lacked marginal reefs, and their outer zones were instead dominated by migrating banks of current-swept shell sand and debris. The prolific growth of shelly organisms on these surfaces fuelled the massive carbonate sediment production of the platforms.
The rate of burial of carbonate carbon on the Tethyan platforms alone at times approximated to half that in all carbonate sediments - of deep, and shallow, water origin - today. However, to the Cretaceous total must also be added that buried in Tethyan basins and mid-latitude chalks as well as any oceanic deposits. Given the high rates of burial of organic carbon at that time, too, carbon was being buried in Cretaceous sediments at a significantly higher rate than it is today.
The only likely source for the excess carbon in the Cretaceous is increased volcanism, most plausibly associated with the 'Pacific superplume'. Rising levels of CO2 in the Cretaceous atmosphere can be attributed to lags between increases in volcanic emissions and compensatory growth in the carbonate platforms. Episodic extinction and drowning of the latter may have been asosciated with climatic cooling brought about by temporary net excesses of CO2 drawdown.
The icehouse and greenhouse case studies of Chapters 6 and 7 relect the complex process of continuous re-equilibration between the feedbacks of evolving life and the changing Earth. No consistent stable state is evident, each age finding its own unique balance of interacting influences.
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