Evolutionary Processes That Create and Sustain Biodiversity

Any comprehensive definition of biodiversity also includes references to the processes that create and maintain biodiversity. The diversity of species, ecosystems, and landscapes that surround us today are the product of at least 3.8 billion years of evolution of life on earth (Mojzsis et al., 1996). Life may have first evolved under rather harsh conditions, perhaps comparable to those of the deep-sea thermal vents where chemo-autotrophic bacteria (which obtain their energy only from inorganic, chemical sources) are currently found. A subterranean evolution of life has also been suggested.

Rock layers deep below the continents and ocean floors, previously thought to be too poor in nutrients to sustain life, have now been found to support thousands of strains of micro-organisms. Bacteria have been collected from rock samples almost 2 miles below the sur face, at temperatures up to 75 degrees centigrade. These chemo-autotrophic micro-organisms derive their nutrients from chemicals such as carbon, hydrogen, iron, and sulfur. Deep subterranean communities could have evolved in situ or originated on the surface and become buried or otherwise transported down into subsurface rock strata, where they have subsequently evolved in isolation. Either way, these appear to be very old communities, and it is possible that these subterranean bacteria may have been responsible for shaping many geological processes over the history of the earth (for example, the conversion of minerals from one form to another, and the erosion of rocks [Fredrickson and Onstott, 1996]).

As early as 3.5 billion years ago, the first photosynthetic bacteria evolved and started releasing oxygen into the atmosphere. Prior to that, the atmosphere was mainly composed of carbon dioxide, with other gases such as nitrogen, carbon monoxide, methane, hydrogen, and sulfur gases present in smaller quantities. Initially the oxygen produced by photosynthesis was absorbed by the oceans, where it reacted with dissolved iron to form iron oxide.

About 1.8 billion years ago, the oceans ran out of dissolved oxygen and the levels of oxygen in the atmosphere started increasing significantly (Mojzsis, 2001). Some of the early species probably became extinct, and others probably became restricted to habitats that remained free of oxygen. Some took up residence inside other, aerobic cells. The anaerobic cells might, initially, have been incorporated into the aerobic cells after those aerobes had engulfed them as food. Alternatively, the anaerobes might have invaded the aerobic hosts and become parasites within them. Either way, a more intimate symbiotic relationship subsequently evolved between these aerobic and anaerobic cells. In these cases the sur vival of each cell was dependent on the function of the other.

The evolution of this symbiotic relationship was an extremely important step in the evolution of more complex cells—the eucaryotes. Recent studies of rocks from western Australia have suggested that the earliest forms of single-celled eucaryotes are at least 2.7 billion years old (Anon., 2001). There has, subsequently, been plenty of time for some of the genes of the invading anaerobes to have been lost, or even transferred to the nucleus of the host aerobe cell. As a result, the genomes of the ancestral invader and ancestral host have become mingled, and the two entities can now be considered as one, from a genetic standpoint.

Complete accounts of the probable evolutionary history of eucaryote organisms on earth can be found in various standard references. The important thing to note is that evolutionary his tory has physically and biologically shaped our contemporary environment. Many existing landscapes are the remains of earlier life forms. For example, some existing large rock formations are the remains of ancient reefs, formed 360 to 440 million years ago by communities of algae and invertebrates (Veron, 2000).

The flora and fauna that form today's biodiversity are a snapshot of the earth's 3.8-bil-lion-year history of life, representing just 0.1 percent of all the species that have lived on earth. Thus 99.9 percent—or virtually all of life that has existed on earth—has gone extinct (Raup, 1991). Extinction, an important part of evolution, does not occur at a constant pace. There have been at least five periods when large numbers of different species have disappeared from around the world. These are termed mass extinctions, and their timing is shown in Table 1.

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