9: Biodiversity over Time
The history of life on Earth is described in various publications and web sites (e.g., Speer, B.R. and A.G. Collins. 2000; Tudge, 2000; Lecointre and Guyader, 2001; Maddison, 2001 Eldredge, 2002); it is also discussed in the module on Macroevolution: essentials of systematics and taxonomy. For the current purpose of understanding what is biodiversity, it is only necessary to note that that the diversity of species, ecosystems and landscapes that surround us today are the product of perhaps 3.7 billion (i.e., 3.7×1093.7×109) to 3.85 billion years of evolution of life on Earth (Mojzsis et al., 1996; Fedo and Whitehouse, 2002).
Thus, the evolutionary history of Earth has physically and biologically shaped our contemporary environment. As noted in the section on Biogeography, plate tectonics and the evolution of continents and ocean basins have been instrumental in directing the evolution and distribution of the Earth's biota. However, the physical environment has also been extensively modified by these biota. Many existing landscapes are based on 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). Very old communities of subterranean bacteria may have been responsible for shaping many geological processes during the history of the Earth, such as the conversion of minerals from one form to another, and the erosion of rocks (Fredrickson and Onstott, 1996). The evolution of photosynthetic bacteria, sometime between 3.5 and 2.75 million years ago Schopf, 1993; Brasier et al., 2002; Hayes, 2002), played an important role in the evolution of the Earth's atmosphere. These bacteria released oxygen into the atmosphere, changing it's composition from the former composition of mainly carbon dioxide, with other gases such as nitrogen, carbon monoxide, methane, hydrogen and sulphur gases present in smaller quantities. It probably took over 2 billion years for the oxygen concentration to reach the level it is today (Hayes, 2002), but the process of oxygenation of the atmosphere led to important evolutionary changes in organisms so that they could utilize oxygen for metabolism. The rise of animal and plant life on land was associated with the development of an oxygen rich atmosphere.