Rare Earth - 1.2 THE ARCHAEANS [39 to 44]

Page 39 of 368.
Rare Earth, Peter D. Ward and Donald Brownlee, 2003.
Why Complex Life is Uncommon in the Universe.
Chapter 1. Why Life Might Be Widespread in the Universe.
1.2. THE ARCHAEANS.
Biologists have long recognized that species can be grouped into hierarchical assemblages. These units are linked by lines of descent; that is, all species that make up a higher category share a common ancestor. Species are grouped into genera. (Our species is grouped, along with the extinct human forms, into the genus Homo. This means that all species of Homo, including Homo sapiens, Homo erectus, and Homo habilis, among others, have a common ancestor.) Genera are grouped into families, families into orders, orders into classes, classes into phyla, and phyla into kingdoms. The kingdoms have always been defined as the highest level, so they are not grouped into any higher unit. The earliest practitioners of this system, which was developed by the great Swedish naturalist Carl Linnaeus in the eighteenth century, first recognized only two kingdoms: animals and plants.