This approach has shown that the organisms that were traditionally classed together as “bacteria” can be as widely divergent in their evolutionary origins as is any prokaryote from any eukaryote. It is now clear that the prokaryotes comprise two distinct groups that diverged early in the history of life on Earth, before the eukaryotes diverged as a separate group. The two groups of prokaryotes are called the bacteria (or eubacteria) and the archaea (or archaebacteria). Detailed genome analyses have recently revealed that the first eukayotic cell formed after a
Figure 1–15 The phototrophic bacterium Anabaena cylindrica viewed in the light microscope. The cells of this species form long, multicellular filaments. most of the cells (labeled v) perform photosynthesis, while others become specialized for nitrogen fixation (labeled h) or develop into resistant spores (labeled s). (courtesy of dave G. Adams.)
Figure 1–16 A lithotrophic bacterium.
Beggiatoa, which lives in sulfurous environments, gets its energy by oxidizing h2s and can fix carbon even in the dark. note the yellow deposits of sulfur inside the cells. (courtesy of Ralph W. Wolfe.)
ARCHAEA BACTERIA EUKARYOTES common ancestor cellAquifex Thermotoga cyanobacteria Bacillus E. coli Aeropyrum Sulfolobus Haloferax Methano-thermobacter Methanococcus Paramecium Dictyostelium Euglena Trypanosoma maize yeast human 1 change/10 nucleotides frst eukaryote Giardia Trichomonas particular type of ancient archaeal cell engulfed an ancient bacterium (see Figure 12–3). Thus, the living world today is considered to consist of three major divisions or domains: bacteria, archaea, and eukaryotes (Figure 1–17).
Archaea are often found inhabiting environments that we humans avoid, such as bogs, sewage treatment plants, ocean depths, salt brines, and hot acid springs, although they are also widespread in less extreme and more homely environments, from soils and lakes to the stomachs of cattle. In outward appearance they are not easily distinguished from bacteria. At a molecular level, archaea seem to resemble eukaryotes more closely in their machinery for handling genetic information (replication, transcription, and translation), but bacteria more closely in their apparatus for metabolism and energy conversion. We discuss below how this might be explained.
Both in the storage and in the copying of genetic information, random accidents and errors occur, altering the nucleotide sequence—that is, creating mutations. Therefore, when a cell divides, its two daughters are often not quite identical to one another or to their parent. On rare occasions, the error may represent a change for the better; more probably, it will cause no significant difference in the cell’s prospects. But in many cases, the error will cause serious damage—for example, by disrupting the coding sequence for a key protein. Changes due to mistakes of the first type will tend to be perpetuated, because the altered cell has an increased likelihood of reproducing itself. Changes due to mistakes of the second type—selectively neutral changes—may be perpetuated or not: in the competition for limited resources, it is a matter of chance whether the altered cell or its cousins will succeed. But changes that cause serious damage lead nowhere: the cell that suffers them dies, leaving no progeny. Through endless repetition of this cycle of error and trial—of mutation and natural selection—organisms evolve: their genetic specifications change, giving them new ways to exploit the environment more effectively, to survive in competition with others, and to reproduce successfully.