0.8 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 andtaxonomy. For the current purpose of understanding what is biodiversity, it is only necessary to note that that thediversity of species, ecosystems and landscapes that surround us today are the product of perhaps 3.7 billion( i.e. , $3.7E9$ ) 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 inthe section on Biogeography , plate tectonics and the evolution of continents and ocean basinshave been instrumental in directing the evolution and distribution of the Earth's biota. However, the physicalenvironment has also been extensively modified by these biota. Many existing landscapes are based on the remains ofearlier life forms. For example, some existing large rock formations are the remains of ancient reefs formed 360 to 440million years ago by communities of algae and invertebrates ( Veron, 2000 ). Very old communities of subterranean bacteria may have been responsible for shaping manygeological processes during the history of the Earth, such as the conversion of minerals from one form to another, and theerosion 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. Thesebacteria released oxygen into the atmosphere, changing it's composition from the former composition of mainly carbondioxide, with other gases such as nitrogen, carbon monoxide, methane, hydrogen and sulphur gases present in smallerquantities. 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 inorganisms so that they could utilize oxygen for metabolism. The rise of animal and plant life on land was associated with thedevelopment of an oxygen rich atmosphere.