Evolution of the biosphere a brief history

The origin of the biosphere and of earth's ecology occurred between 3.8 and 3.5 billion years ago. Both autotrophic and heterotrophic origins have been proposed. Previously the heterotrophic use of organic molecules synthesized in the pre-biotic broth was a popular idea (see Lazcano and Miller 1999). More recently, autotrophic theories have re-emerged, for two reasons: first, early cell membranes would probably have lacked sufficient permeability to transport large molecules. Second, it is now realized that metabolic cycles that grow and reinforce themselves can emerge spontaneously. If such metabolic cycles occurred in the pre-biotic world, an autotrophic ancestral metabolism would by definition result (Wachtershauser 1988, 1990). Because photosynthesis today requires a more complex biochemistry, an ecosystem consisting initially of only chemo-autotrophs is likely. It is also likely that a second trophic level would quickly be added, consuming waste products (and dead remains) of these producers. There is evidence also that photosynthesis may nonetheless have been a very early acquisition. Early life probably lacked protein enzymes to catalyse reactions, using instead RNA. It turns out that chlorophyll synthesis involves (non-intuitively) molecules bound to RNA, a likely relic of the ancient involvement of RNA in catalysis (Benner et al. 1989). The origin of photosynthesis provided a new energy source, light, for the world's ecosystems that would have massively increased its potential productivity.

The early photosynthesizers probably used hydrogen sulphide or molecular hydrogen as their source of electrons, rather than water (Xiong et al. 2000). Using water as an electron source releases molecular oxygen. We know that cyanobacteria, which today are the predominant oxygenic prokaryotes, were among the earliest prokaryotes, at least 3.5 billion years old. However, it was not until about 2.5-2 billion years ago that a great increase in atmospheric oxygen occurred, marking the end of the Archean era (Figure 3.1). Until then, some process must have removed the oxygen produced by cyanobacteria. One possible process is aerobic respiration (Towe 1990). Aerobic respiration is many times more energy-yielding than any of its anaerobic alternatives, and although at this stage there were certainly no food chains as we recognize them today, aerobic respiration made possible the future advent of the higher trophic levels with substantial biomass (Fenchel and Finlay 1995). By this very early time, 3.5 billion years ago, all basic bioenergetic processes had probably evolved, many of them several times, and the biogeochemical cycling of carbon, nitrogen, and sulphur was established as we know it today.

Although the earliest recognizable eukaryote fossils date from the time of transition to oxic atmosphere, 2100 Ma, the lineage from which modern eukaryotes are derived is a very deep evolutionary branch, and probably dates right back to the time of the earliest evidence for life, 3.5 Ga. Somewhere in this interval, one of our ancestral bacterial lineages (biomole-cular evidence suggests it resembled an archaebacterium) developed a cytoskeleton and lost its cell wall. With this came the ability to engulf large organic particles. The first eukaryote lineages were doubtless anaerobic, a fact indicated by the many extant anaerobic eukayotes belonging to basal branches of the eukaryotic tree. These early predators represented a trophic interaction quite different to anything in the prokaryotic world, in which organic material had to pass through cell walls and membranes. However, only with the advent of mitochondria, probably at the Proterozoic boundary

(Figure 3.1), could these predators fully reap photosynthetic productivity by integrating with the aerobic world.

There existed now a period of about 1 Ga that is relatively featureless in terms of historical evidence, and indeed may have been relatively stable in ecological terms, until 535 Ma. The next 100 My period, known as the Cambrian explosion and subsequent Ordovican radiation, is one of enduring interest for biologists (Figure 3.1). Over the next 100 My appeared the major animal lineages, including the first benthic and pelagic macropreda-tors and the first animals capable of burrowing more than a few millimetres into sediments.

The consequences were various and significant. The Earth became much more species rich. Use of skeletal materials based on calcium, phosphorus, and silica led to greater control of these minerals by organisms, as opposed to by inorganic processes. Disturbance of sediments by burrowers recovered carbon and other nutrients from sediments for recycling rather than burial. All these new animals produced masses of faeces. Faeces dropped to the ocean floor rather than remaining in the water column, and in doing so consumed less dissolved oxygen. Flow of oxygen from the surface waters to the ocean floor would have been facilitated, as suggested by geological evidence (Logan et al. 1995). This may itself have contributed to the Cambrian radiation by facilitating skeletal formation, large bodies, and active metabolisms. Macropredators might have stimulated novel defences in prey, which themselves would be a cause of selection on predators. Such 'co-evolution' may have been a stimulus for diversification in lifestyle and structure.

By the end Ordovician, marine ecosystems would have looked pretty modern. Soon after, however, multicellular organisms then began to form complex terrestrial ecosystems. At the time, colonization of the land was notable primarily for an expansion of earth ecospace. However, eventually, the progressive evolution of terrestrial communities led to major alterations of biogeochemical cycles, and terrestrial domination of global biodiversity and production. The earliest land plants were relatively small in stature. About 380 Ma the earliest trees appeared and by 350 Ma, forests composed of horsetails, clubmosses, ferns, progymnosperms, and seed plants had a widespread global distribution and covered a number of clearly distinguishable biomes. The consequences for the biosphere appear to have been immense. Global productivity probably soured to unprecedented levels. Coal was deposited in massive amounts never again attained. Global carbon dioxide levels dropped to 10% of their previous levels in about 50 My, eventually resting about their present level. This set the scene for subsequent periods of significant global cooling.

It is possible that the earliest terrestrial plants were relatively free from natural enemies, such as herbivores. By mid-Carboniferous there was abundant evidence that the onslaught had begun. Insects with characteristic mouthparts, and fossil leaves with evidence of bite marks, along with vertebrates with teeth designed for chewing all suggest that plants had begun their war against animal attack that continues today. This was a significant new trophic level, for now some of the plant productivity was available to other organisms. Terrestrial ecosystems had come of age.

Flight has probably evolved four times in the history of life: in insects, pterosaurs, birds, and bats. Most biologists would agree that flight has had major ecological repercussions. First, the atmosphere could at last be properly utilized. While flying species are variably adapted to an aerial existence, a few birds, such as the swifts, live the vast majority of their (often considerable) lives on the wing. Flight may also have contributed significantly to global diversity. Bats, birds, and winged insects are all species rich (see de Queiroz 1998). These organisms are likely to have contributed to diversification of other species,such as the plants they pollinate and disperse. The evolution of flowering plants is our final major transition. Angiosperms are the most species-rich division of plants today, they dominate global productivity, and their origins coincided with a rise in plant diversification that shows no signs of abating. Effects on insect diversification are also detectable and non-trivial (Farrell 1998). We have now arrived at an essentially modern ecology. How and why did these changes occur and why have they been retained? Let's look at an example, the evolution of flight in birds, insects, bats, and pterosaurs.

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