Known as the biosphere to scientists and as the creation to theologians, all of life together consists of a membrane around earth so thin that it cannot be seen edgewise from a satellite yet so prodigiously diverse that only a tiny fraction of species have been discovered and named. The products of billions of years of evolution, organisms occupy virtually every square centimeter of the planet's surfaces and fill nearly every imaginable niche.
Biologists estimate that more than half the species occur in the tropical rain forests. From these natural greenhouses, many world records of biodiversity have been reported--425 kinds of trees in 2.5 acres (1 hectare) of Brazil's Atlantic forest and 1,300 butterfly species from a corner of Peru's Manu National Park, both more than 10 times the number from comparable sites in Europe and North America. At the other extreme, the McMurdo Dry Valleys of Antarctica, with the poorest and coldest soils in the world, still harbor sparse communities of bacteria, fungi and microscopic invertebrate animals.
A few remarkable species, the "extremophiles," have achieved astonishing feats of physiological adaptation at the ends of habitable Earth. In the most frigid polar waters, fish and other animals flourish, their blood kept fluid by biochemical antifreezes. Populations of bacteria live in the spumes of volcanic thermal vents on the ocean floor, multiplying in water above the boiling point. And far beneath Earth's surface, to a depth of 2 miles (3.2 km) or more, dwell the SLIMES (subsurface lithoautotrophic microbial ecosystems), unique assemblages of bacteria and fungi that occupy pores in the interlocking mineral grains of igneous rock and derive their energy from inorganic chemicals. The SLIMES are independent of the world above, so even if all of it were burned to a cinder, they would carry on and, given enough time, probably evolve new life-forms able to re-enter the world of air and sunlight.
Earth's biodiversity (short for biological diversity) is organized into three levels. At the top are the ecosystems, such as rain forests, coral reefs and lakes. Next down are the species that compose the ecosystems: swallowtail butterflies, moray eels, people. At the bottom are the variety of genes making up the heredity of each species. How much biodiversity is there? Biologists have described a total of between 1.5 million and 1.8 million species. Yet this impressive achievement is only a small beginning. Estimates of the true number of living species range, according to the method employed, from 3.6 million to more than 100 million.
Least known are the smallest organisms. By repeated sampling, biologists estimate that as few as 10% of the different kinds of insects, nematode worms and fungi have been discovered. For bacteria and other microorganisms, the number could be well below 1%. Even the largest and most intensively studied organisms are incompletely cataloged. Four species of mammals, for example, have recently been discovered in the remote Annamite Mountains along the Vietnam-Laos border. One of them, the saola or spindlehorn, is a large cowlike animal distinct enough to be classified in a genus of its own. Earth, as far as life is concerned, is still a little-known planet.