A tolerable environment

The essence of understanding Gaia is to recognize the environmental constraints that shape the form and behaviour of organisms. The constraints within which a living cell can survive include factors such as temperature, humidity, acidity (pH), salinity (see variation 34), ionic strength, and redox potential. A diagram of the window for life defined by all environmental constraints would form a parabolic hypervolume —a space that cannot he mapped on a page. The diagram above maps just three critical constraints: temperature, ionic strength, and pH (acidity). Organisms require a narrow range of conditions for these factors— temperatures from 0° to 50°C, pH from 3 to 9, and ionic strength from 0.1 to 0.9. The ability of a cell to tolerate fluctuations in its environment depends upon the ablity of its membranes to withstand disruption, and the forces that hold a cell membrane together are only weak. Organisms that live in extreme environments must have evolved special strategies to enable their cells to survive. For example, plants and animals living in the desert generally have good water conservation: organisms such as marine algae have devised means of converting salts to harmless substances; and so on. Remarkably, however, conditions in the Earth’s environment do generally meet the narrow constraints for cell survival. This fact is the strongest evidence for Gaia, the system of life and its environment.