SECTION 1 Prokaryotes SECTION 2 Biology of Prokaryotes SECTION 3 Bacteria and Humans
Prokaryotes are the most numerous organisms on Earth. They are found almost everywhere, from the skin of a fingertip to the waters of a thermal geyser to the freezing landscape of the Antarctic. The earliest fossils of prokaryotes, which are about 3.5 billion years old, indicate that prokaryotes lived long before other forms of life evolved.
TWO MAJOR DOMAINS: ARCHAEA AND BACTERIA
Prokaryotes are single-celled organisms that do not have a membrane-bound nucleus. They live in nearly every environment on Earth. Many live in places where no other organisms can live, such as scalding hot water and inside solid rock. Prokaryotes are a major source of food for many organisms. They also help many organisms, including humans, cattle, and insects, digest food. Prokaryotes also play an important role as decomposers of dead organic matter in the environment.
Even though most prokaryotes are tiny organisms, they differ greatly in their genetic traits, in their sources of energy and nutrients, and in their habitats. Through DNA technology, scientist Carl Woese and his colleagues found in the late 1970s that there are two major branches, or domains, of prokaryotes. One branch, formerly called Eubacteria (YOO-bak-TIR-ee-uh), is called Bacteria. The other branch is called Archaea (ahr-KEE-uh), formerly called Archaebacteria. The name archaea means "archaic" or "ancient." Most known species of archaea live in extreme environments thought to resemble harsh environments present millions of years ago.
Figure 23-1 shows the phylogenetic relationship between living organisms based on comparisons of ribosomal RNA (rRNA) sequences. The rRNAs are well suited for such studies because they make up part of ribosomes, which are organelles that all organisms have. Also, the structure of rRNA changes very slowly over time. Surprisingly, rRNA analysis has shown that archaea are more closely related to eukaryotes than they are to bacteria. In addition, archaea have some genes that are the same as bacterial genes, but most archaeal genes more closely resemble genes found in eukaryotes. This similarity of genes suggests that archaea and eukarya likely share a more recent common ancestor than eukarya and bacteria do.
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