community ecology Evolution of the biosphere

Life is characteristic of the Earth. The biosphere—which in relation to the diameter of the Earth is an extremely thin, life-supporting layer between the upper troposphere and the superficial layers of porous rocks and sediments—is clearly visible from space; it is responsible for the blue and green colours seen in satellite photographs of the Earth.

All known forms of life are based on nucleic acid–protein systems, although life systems involving different chemical components are theoretically possible. Life appears to have developed on the Earth as soon as conditions permitted. Cooling of the hot, primordial Earth was an important factor. In a universe in which extremes of temperature are the norm, most life-forms are restricted to a relatively narrow range of about 0° to 100° C.

The abiotic elements of the biosphere have been profoundly shaped by life, just as life has been molded by the environmental conditions that surround it. The biosphere has grown over time. Seven hundred million years ago it was a narrow and possibly discontinuous band encompassing only the shallower parts of the ocean. Today it reaches high into the atmosphere and deep into the ocean, invading even the tiny spaces in porous rocks. Thus, from the troposphere, which extends from 10 to 17 kilometres (6.2 to 9.9 miles) above sea level, to the deepest parts of the ocean (11 kilometres below the sea), to many hundreds of metres into the rocks of the Earth’s crust, life thrives.

Even in the most hostile of the Earth’s environments—the frozen and parched south polar desert—algae find refuge in tiny spaces in translucent rocks. The rocks provide shelter from the wind and focus the rays of the Sun, acting as a greenhouse and allowing biological activity to take place for a few weeks each year. At the other extreme, there are thermophilic (heat-loving) bacteria inhabiting deep-sea volcanic vents in which the water is heated under immense pressure to extremely high temperatures. Some researchers believe that some hyperthermophilic organisms existing in these vents can survive at temperatures above 300° C. If the temperature drops much below the boiling point, they die.

Life is changed through the process of evolution. Evolution is an inevitable consequence of inheritance, genetic variation, and competition arising from the number of individuals exceeding available resources. The result—natural selection—permits the perpetuation of some traits over others. Through billions of years this process has resulted in a great diversification of life-forms.

The history of life is characterized by an acceleration of evolutionary change and unpredictable periods of extinction, often followed by rapid diversification. There is still much debate over the causes—and even the importance—of some of these trends and events. Perhaps the most hotly debated issues at present concern theories of extinction and diversification. In the early 1970s the evolutionary biologists Stephen Jay Gould and Niles Eldredge developed a model called “punctuated equilibrium,” which describes and explains some aspects of speciation (see evolution: Patterns and rates of species evolution: Reconstruction of evolutionary history: Gradual and punctuational evolution). This theory postulates that evolution does not progress at a steady rate but rather in bursts, as brief periods of rapid evolutionary change are followed by long periods of relative evolutionary stasis.

The degree of interdependence between organic and inorganic elements of the biosphere and the importance of both negative and positive feedback mechanisms in the maintenance of life increasingly are being recognized. At one extreme the British physicist James Lovelock and the American microbiologist Lynn Margulis have argued that, because the elements of the biosphere are so interdependent and interrelated, the biosphere can be viewed as a single, self-regulating organism, which they call Gaia.

The Gaia hypothesis postulates that the physical conditions of the Earth’s surface, oceans, and atmosphere have been made fit and comfortable for life and have been maintained in this state by the biota themselves. Evidence includes the relatively constant temperature of the Earth’s surface that has been maintained for the past 3.5 billion years despite a 25 percent increase in energy coming from the Sun during that period. The remarkable constancy of the Earth’s oceanic and atmospheric chemistry for the past 500 million years also is invoked to support this theory.

Also integral to the Gaia hypothesis is the crucial involvement of the biota in the cycling of various elements vital to life. The role that living things play in both the carbon and sulfur cycles is a good example of the importance of biological activity and the complex interrelationship of organic and inorganic elements in the biosphere (see biosphere: The organism and the environment: Resources of the biosphere: Nutrient cycling: The carbon cycle and The sulfur cycle).

Although the Gaia concept has provided intriguing models of the biosphere, many researchers do not believe the biosphere to be as fully integrated as the Gaia hypothesis suggests.