Archaea are microbes that appear similar to bacteria—like bacteria, they have a primitive structure, with no membrane around the nucleus—but are genetically distinct. Along with bacteria and eukarya (which include single-celled organisms, fungi, and all animals), archaea are one of the three domains of life, as recognized by biologists. In Chapter 1, Wilson describes how microbes, including archaea, can be found in some of the most extreme environments on Earth, such as geothermal waters where the temperature can exceed 400 degrees Fahrenheit. Such extremophiles, as organisms that can withstand such harsh conditions are called, are proof of the inestimable variety of forms life can take on the planet. 

Short for biological diversity, biodiversity is organized into three levels: environments, species, and genes. Biodiversity is greatest in environments where there’s the most solar energy, ice-free terrain, and climactic stability—like the world’s tropical forests, where half the world’s plants and animal species are thought to exist. While biodiversity is a function of the interplay between ecosystems, species, and genes, many biologists favor species as the unit by which to measure biodiversity, as in theory, they can be counted. Even if the exact number of species is unknown, there could be anywhere between 3.