文档介绍：236 domain
ent from those of either bacteria or eukaryotic cells. DOMAIN ARCHAEA
Biology now uses the three-kingdom scheme: domain The Archaea contain two phyla. Phylum Crenar-
Archaea, domain Bacteria, and domain Eukarya. chaeota contains only three genera. The larger phy-
The new classification made news in biology. lum Euryarchaeota contains seven classes, each of
The New York Times reported in 1977, “Scientists which has at least one genus. Both phyla are home
studying the evolution of anisms today to a very diverse group of anisms that rep-
reported the existence of a separate form of life that resent all types of cell morphologies—rods, i,
is hard to find in nature. They described it as a ‘third helixes, and other—and gram-negative as well as
kingdom’ of living material, composed of ancestral gram-positive members. Although Woese originally
cells that abhor oxygen, digest carbon dioxide and described the archaea as mainly anaerobic, methane-
produce methane.” But many of Woese’s peers found producing anisms, microbiologists have
it difficult to reconcile the new three kingdoms since discovered aerobic and facultative anaerobic
with the previous five kingdoms. In fact, the three species.
domains lie above the kingdom level, and prokary- Many archaea live in the earth’s harshest environ-
otes and eukaryotes no longer hold as much meaning ments. For this reason, they are also extremophiles,
as they once did in defining anisms. anisms that thrive in extreme environments.
Biologists continue to study the ic makeup Archaea have been recovered from environments of
anisms to determine biotas’ common ancestor. extremely hot, cold, acidic, salt, or high-pressure con-
mon ancestor is not yet agreed upon in sci- ditions. Some archaea have adopted a less extreme
ence, but biologists do know that bacterial evolution lifestyle by living inside the digestive tract of animals.
may have split off from the evolution of archaea and These intesti